ISSN 0704-3716

CANADIAN TRANSLATION OF FISHERIES AND AQUATIC SCIENCES

No. 4741

Studies on the fisheries biology of common octopus of the northwest coast of Africa

by

H. Hatanaka

411, Original Title; Afurika hokusei gansuiiki ni okeru madako no gyogyo seibutsugakuteki kenkyu

From: Enyo Suisan Kenkyusho Hokoku 17: 13-94, 1979.

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Translated by the Translation Bureau (ÉLC/PS) Multilingual Services Division

Department of the Secretary of State of Canada

Department of Fisheries and Oceans Pacific Biological Station

Nanaimo, B.C.

1981

145 pages typescript

DEPARTMENTOFTHESECRETARYOFSTATE

TRANSLATION BUREAU

SECRÉTARIAT D'ÉTAT

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DIVISION DES SERVICES MULTILINGUAL SERVICES CANADA

INTO EN TRANSLATED FROM TRADUCTION DE

Japanese English

DATE OF PUBLICATION DATE DE PUBLICATION

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L'ORIGINAL

13 - 94

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ISSUE NO. NUMÉRO

'Dec.

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DACTYLOGRAPHI ÉES

145 ( incl. F, a)

YEAR ANNÉE

VOLUME

Unidentified 1979 17

PUBLISHER ÉDITEUR

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TF/ 71q/

DIVISION MULTILINGUES

AUTHOR - AUTEUR

Hiroshi HATANAKA

TITLE IN ENGLISH - TITRE ANGLAIS

Studies on the fisheries biology of common octopus of the

northwest coast of Africa

TITLE IN FOREIGN LANGUAGE (TRANSLITERATE FOREIGN CHARACTERS) TITRE EN LANGUE ÉTRANGÉRE (TRANSCRIRE EN CARACTÈRES ROMAINS)

Afurika hokusei gansuiiki ni okeru madako no gyogyo

seibutsugakuteki kenkyu REFERENCE IN FOREIGN LANGUAGE (NAME OF BOOK OR PUBLICATION) IN FULL. TRANSLITERATE FOREIGN CHARACTERS. RÉFÉRENCE EN LANGUE ÉTRANGÉRE (NOM DU LIVRE OU PUBLICATION), AU COMPLET, TRANSCRIRE EN CARACTÈRES ROMAINS.

Emo Suisan Kénkyumho Hokoku

REFERENCE IN ENGLISH - RÉFÉRENCE EN ANGLAIS

Bulletin of the Far Seas Fisheries Research Laboratory

REQUESTING DEPARTMENT Fisheries and Oceans MINISTÈRE-CLIENT NOTRE DOSSIER NCI

SIPB DIRECTION OU DIVISION TRADUCTEUR (INITIALES)

PERSON REQUESTING DEMANDÉ PAR

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TRANSLATION BUREAU NO, 732089

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DEPARTMENT OF THE SECRETARY OF STATE

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CLIENTS NO. DEPARTMENT DIVISION/BRANCH CITY NO DU CLIENT MINISTiRE DIVISION/DIRECTION VILLE ,

Fisheries and Oceans SIPB Nanaimo -

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732089 Japanese E. L. C. / PS

Bulletin of the Far Seas Fisheries Research Laboratory, No. 17, December 1979

Studies on the fisheries biology of common octopus

off the northwest coast of Africa

Hiroshi HATANAKA*

Common octopus, Octopus vulgaris LAmAscR, is widely distributed over most of warm and

temperate coastal waters in the Northern and Southern Hemispheres. The nominal catch of all oc-

topus species in the world amounted to 217 thousand tons in 1975 (FAO, 1976b), while the catch

of common octopus in the area off the northveest coast of Africa attained 121 thousand tons, ac-

counting for around 569c; of total octopus catch in the world. The greater part of octopus caught

in the waters off the northwest coast of Africa by vessels from Japan. Spain and Norea were car-

ried or- exported to Japan, and the amount of these octopus reached about two-thirds of Japanese

octopus consumption toward the end of 1960s.

Japanese trawlers operating off the northwest coast of Africa increased sharpl)in number from

two vessels in 1959 to 68 vessels in 1968 when the catch of octopus showed a peak of 73 thousand

tons. However, the octopus catch by Japanese vessels in this area ceased to increase thereafter,

and dropped rapidly to a low level of about 30 thousand tons in recent years. This low level of

catch in recent years might indicate that the status of octopus resources has already been in over-

exploited phase.

Taking into account the importance of octopus resources not only as a main target species for

Japanese trawl fishery in this area but also as a supplier of octopus to Jdpanese domestic market,

a research project on octopus under consideration had been started in the Far Seas Fisheries Re •

search Laboratory in 1967. The author has participated in this project from the beginning and made

a series of study on the octopus resources. The results obtained are abstracted as follows.

1. Historical review of octopus fishery

Vessels from more than twenty countries are fishing in the northwest coast of Africa (FAO.

1976a). Octopus is one of the most important target species for trawlers from the southern Europe

and Korea as veell as Japan. Total nominal catch of octopus in this area has remained on a level

of about 100 thousand tons since 1964, and more than 90° (; of the catch has been taken by Japa-

nese and Spanish trawlers (Table 3).

Three main fishing grounds for octopus are observed off the coast of Spanish Sahara and Nlau•

Received 30 August 1979. Far Seas Fisheries Research laboratory Contribution No. 201

* Far Seas Fisheries Research Laboratory

SPS-200-10-31

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7530-211-025.5332

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,eimnia (Figs. 1 and 2). Japanese fishermen call them by the names of Villa Cisneros, Cap Blanc

and Nouakchott Regions, respectively. In the Villa Cisneros Region, northernmost grounds, fishing

seasons for octopus continue from autumn to spring with two peaks. The Cap Blanc Region, cen-

tral grounds, has two distinct fishing seasons for octopus; summer season from July to Septembe r

and veinter season from December to April. In the Nouakchott Region, southernmost grounds, most

of octopus catches are taken in summer from July to September (Fig. 3.

The Japanese octopus fishery was established firstly in the Villa Cisneros Region in early 1960%,

and since then annual catches of octopus occupied from 10 to 30% of the total catches by Japanese

trawlers in this region. From 1969 onward, however, the octopus catch per unit of effort decreased

year by year, and Japanese fishing efforts expanded in this region shifted gradually to the Cap

Blanc Region. Consequently, octopus catch in this regi .on sharply decreased from 26 thousand tons

in 1967 68 fishing year to only one thousand tons in 1974 '75 (Table 2 and Fig. 4).

The Cap Blanc Region vas exploited virtually in and after 1965, and has become the btst fish-

ing grounds for Japanese octopus fishery since late 1960's, where octopus catch accounted for about

65"u of the total catch by Japanese trawlers. Thereafter, fishing effort increased steadily every year

but annual octopus catches remainded a level of 30 thousand tons except 1968 69 Table 2 and

Fig. -IL

The Nouakchott Region was exploited lately as a subsidiary fishing grounds for Japanese trawl-

ers when the octopus fishery in the Cap Blanc Region \ 'as off season. Cuttlenshes and sea bream›

were mainly caught in the early stage of exploitation in this region, but from 1970 71 onward. oc-

topus %%as also fished seasonally and its catch by Japanese trawlers amounied th three thousand tons

in recent two years (Table 2 and Fig. 41.

2. Spawning season and spawning ground

According to the observations on the Needham's sac by the naked eyes, almost all of the male

specimens larger than 50 cm in total length had spermatophores every season (Fig. 5). Meanwhile,

the minimum female specimen with mature ovary was 37.9 cm in total length, which was collected

in September 1972 in' the Cap Blanc Region. Modal length of matured female specimens was about

75 cm during the months when the)' had been dominated in numbers (Fig. 6),

Although matured females appeared throughout the year, one or two seasonal peaks of appear-

ance were found distinctly in each fishing ground. Spring peak (May June),was observed at least

in Cap Blanc and Nouakchott Regions, and autumn peak (September or September October I vas

found in all of three regions (Fig. 6).

Gonad index, in terms of a ratio of gonad weight to cubic total length, showed no de fi nite sea-

sonal change in males, but its mean values by month increased fairly in April and May, and also

slightly in August and September (Fig. 7). On the other hand, female specimtns with higher value

of the index were observed exclusively in May to June and also in September (Fig. 8). Therefore,

it is estimated that the octopus stocks off the coast of Spanish Sahara and Nlauritania have two

spawning seasons in spring (May and June) and in Autumn (September and October . ).

Twenty-seven egg masses were collected by Japanese scientists on board the commercial trawl-

ers during the periods from 1972 to 1975 (Table 5). All of them were obtained from the bottom

within a depth range from 15 to 94 m, but no depth-dependent concentration was observed at leztst

in their distributions. Twenty-fouLegg masses were collected during khe months from September

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to November, which might he laid in autumn spawning season. Localities of egg masses collected

were more or less consistent with geographical distributions of octopus catch (Fig, 9).

3. Age and growth

Age and growth of octopus were determined by the Pi SEN'S method, namely, by seasonal

shifts of modal length of length-group. The length frequency distributions used were estimated from

quartetly size-category compositions of octopus catch during three years from the end of If-i(is tu the

beginning of 1972 ( Table 6 ) by applying length-category keys ...fable The length frequenc y

distributions were divided into some elemental length-groups by application of the Gaussian Curve

(Appendix Table 2, Figs. 13 and 1.1).

The mean lengths of these elemental groups were categorized into four or five groups in each

quarter of the year (Fig. 16 ). Possible linkages between the successive groups in adjoining quar-

ters of the year were made mainly by the following three criteria, t 1 ) as will be ahle to find out

shortly, there are two different spawning stocks which are characterized by different spawning and

birth seasons, (2) modal length of matured females in respective spawning stocks have a size range

from 75 tu 85 cm in total length, and (3) none of the group is iemained from the lining. Conse-

quently, only one case of lining was obtained (Fig. 16 t.

According to a result obtained from laboratory-reared specimens. newly. Witched octopus reached

a mean length of 144 mm within 90 days in Japanese waters ; hAsti ti id., 19631, If this result can

be applied to the waters concerned , the smallest length-groups of about -10 eni. in total length. found

in May and November, are assumed to be about half a year old. Incubation period of .octopus egg),

depends on the water temperature (MANotn.n-Wnez, 1963: Ii asti. 1975 and taking into account the

temperature in this area, about one and a half months might be required for incubation. There

fore, the smallest groups found in May and November might be spa%vned ut autumn and spring

spawning seasons, respectively.. Generally speaking, the life-eycle of octopus in the northwest coast

of Africa can be expressed that the eggs spawned in Nlety and June September and ( tctobei .

hatch in early summer (early winter newly hatched youngs appear in the catch in November

May f, and they spawn their eggs in spring (autumn; when they reach a length of about 8() cm

(75 (S m) after two years.

In order to examine the sexual difference in growth rate, length frequencies in typical months

were divided into male and female, using the sex ratio of each length-class ITable 15 ■ . The length

frequencies by sex obtained were, at the next stage of calculation, separated into elemental length-

groups by the method mentioned in the early paragraph (Fig. 17). The groups larger than 90 cm

in mean length were composed only of males. The groups smaller than 90 cm appeared in both

sexes, and mean length of corresponding groups between the sexes were nearly equal in every case.

Moreover, sex ratios of these smaller groups were relativel y equal between the sexes. Therefore.

differences in growtI; rate between male and female were not significant at least in octopus smaller

than 90 cm in total length. It assumes that almost all .of the females end their life after the tiret

spawning, and larger groups consisted only of males are one y-ear older than groups with modal

length from 75 to 81 cm which are the largest ones among the groups consisted of both sexes.

The von BEtcTAL.AtvFv growth equation was applied to mean lengths at quarterly age

(Table 11), and growth equations by spawner and by fishing ground were obtained as Wows:

w.

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Villa Cisneros Region

Spring spawner . .: L 1 =123. 6 {1 —e - o-eot' 40 - 33 )}

Autumn spawner: L, =121.6 {1 —e -0.481 ‘ 4 0.27))

Cap Blanc Region

Spring spawner : L1 =111. 7 {1 —e -°- 75"°-°6) )

Autumn spawner: L, =105.0 {1 —e -0 . 69 (t+ 0 .°5 )}

where, t denotes age in years and L, denotes total length in cm at age t.

4. Stock identification and movement

Existence of some stocks of octopus was suggested through the processing of data on fishing

ground and season. Namely, three fishing grounds were virtually isolated each other (Fig. 2 », and

monthly changes of catch per unit of effort by half-a-degree square block also suggested that oc.

lupus in transitional areas between the regions were less abundant throughout the year I Fig. 2o .

Moreover, fishing seasons and long-term fluctuations of CPUE in those grounds were different eaci.

other. For these reasons, it is assumed that octopus in each fishing ground is coMposed of an in-

dependent local stock.

Within each local stock, two spawning seasons were found, and the two spawners had a

dependent life-cycle each other, so that each of them is assumed to be a separate spawning

Bathymetric movements of octopus in the Cap Blanc Region were eS:amined. Monthly

frequency distributions by depth zone were obtained from size-category • .i..ompositions of octu:.

catch (Table 12 ;. The)' were divided into some elemental length groups by the application of

sian Curve, and the ages of these elemental groups were estimated as aforementioned. Then ;»,

CPUE in number of each elemental group vas calculated using the fishing effort correspondinr

the catch in size-category composition (Table 13). NIonthly values of these CPUE's were shou n

a figure by year-class (Figs. 21 and 22).

In the ease of spring spawner, the highest CPUE appeared in shallower waters during y

months from July to September when the spawner grew to one year old. This high value of Ci

shifted successively to deeper waters, and a-t the same time, the CPUE in shallower water de

gradually as time passed. Then the CPUE's became nearly equal in every depth up to April T:e

the spawner grew to 1. 8 years old and declined greatly at its spawning season. On the othe

in the case of autumn spavener, similar changes of CPUE were observed successively at a

few months. Assuming that the CPUE in number depends on the density of octopus, it is te

that young octopuses at about one year old recruit to the offshore fishing grounds from

coastal waters, move and disperse gradually to the whole area of fishing grounds, and the ;: e»,:.

after settle in their own depositing area throughout their life without significant migraticn

while, octopus larvae born in offshore area may be transported to the shallower coastal

geostrophic 'current in their planktonic stage.

5. Other biological information

Monthly sex ratios of octopus in the Cap Blanc Region were obtained as the percentage »tu .

in each length-class (Table 15 and Fig. 26). Sex ratio in all seasons combined was abc_.

octopus smaller than 50 cm in total length, but the ratios declined ‘yith the increase of

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trend accelerated for the specimens larger than 85 cm in total length, and the specimens beyond

115 cm were composed only of males. This rapid decrease of females might be caused by death

after their breeding period. Seasonal fluctuations' of sex ratio were observed in length-classes ranging

from 50 to 90 cm. Usually, males exceeded slightly females in these classes, but males were inferior

in Nlarch and September.

Since stomach contents of octopus veere masticated into small pieces. identification of the con-

tents was difficult and might make some uncertaintly. On the other hand, as for the stomach ob-

servation on board Japanese trawlers, the contents could be compared directly with benthic animals

caught together. Stomach contents from four samples observed on board in the Cap Blanc and Nouak-

chott Regions in 1975 were classified into five groups on the taxonomie level of the class or phy-

lum. The most dominant group was shellfishes which occupied •15 tu (ii),' of the total content, in

weight. Fishes, crustaceans and cephalopoda accounted for 19 tu 31, 7 to 16 and 4 tu 1:i",, of the

total contents weight. respectively tFig. 27). The proportion of food item varied considerably with

fishing ground and timit of capture, thus it is suggested that food-preference of octopus is rallier

weak and octopus preys on whatever animals accessible.

The degree of fullness for stomach and average weight of the contents per body weight were

examined by specimens taken in the Cap Blanc Region. Winter and summer feeding seasons were

suggested (Fig. 28. These seasons correspond to the fishing season, and feeding activity declines

in spawning seasons. The high feeding activity will be necessary for the development of gonad as

well as the accumulation of nutrient for the spawning, so that the octopus in 'pre-spawning season

stays out more often from the sheltering places. For that reason, the increase. of feeding activity

leads to increase of vulnerability by trawl gear and to form the fishing season. In spawning.season,

however, the feeding activity weakens and matured female is mostly away from trawling site and

shuts herself in shelter for the spawning and the breeding. As the result, fishing season will be

terminated in accordance with the coming of spawning season. Thus, close relations among

spawning, feeding and fishing season are suggested.

6. Population dynamics and exploitation of stocks

Octopus stocks in the Cap Blanc Region have been exploited exclusively by . Japanese trawlers,

and the catch statistics are available since 1967. But in the Villa Cisneros Region, octopus has been

caught mainly by Spanish vessels, and the datailed statistics are not available. Therefore, datailed

analysis was made on stocks in the Cap Blanc Region.

Monthly stock sizes in number during three years from 1969 to 1971 were estimated Table

17 ), using following four data, (1) catch in number per one hour hauled by depth zone; 2:

average area swept in one hour hauled by trawl gear (0.41 mile 2 ) which calculated from the width

between tips of wing net and hauling distance; (3) areas of five successive depth zones. namely

10 30, 30-50, 50 70 and 70 100 m; and (4) an assumption that the gear caught all octopuses in the

area swept. In the case of spring spawner, each year class had a peak of stock size at 1.1 or 1.2

years old. However, apparent stock size of autumn spawner reached the maximum at about 1.8

years old in every year class. It is recognizable.that autumn spawner dose not complete the recruit-

ment up to about 1.8 years *old.

On the other hand, monthly actual catches in number by year class during the three years

were estimated t Table 181. using monthly catch in tons and year-class composition of the catch

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obtained from occurrence of each age-group (Table 13).

Monthly total deaths in each year class veere calculated as the difference between the stock

sizes of successive two months. These total deaths obtained were compared with the actual catch

in number in corresponding month (Fig. 30). Values of catch in number were greater alum

that of estimated total deaths younger than 1.5 years old, therefore, these stock sizes might be

underestimated. Moreover, monthly natural deaths, residuals by subtracting the catch in number

from the total deaths, sharply increased during the months from February to April (1.6 to 1, 8 years

olci t, It was suggested that the decline of vulnerability at the end of fishing season led to the

pretended increase of natural deaths.

Taking into account the underestimations of stock sizes in younger age and seasonal changes

of vulnerability. total mortality coefficients (Z) excluding the deaths after breeding were estimated

from each pair of stock sizes of octopus older than 1.8 years in the same months of the succes-

sive two years, where a half of the stock size was used as that of octopus younger than 2, 0 years

uld for the estimation by means that the older stock consisted only of males as aforementioned. Ex-

cluding some negative values. the average of total mortality coefficients vas calculated to be 1.2

t Table 19). Fishing efforts during the years from 1969 to 1971. ' hen the materials were available,

were about the same level as those in 1974 75 fishing year as estimated in the later paragraph

:Table 231, therefore, the total mortality vas assumed to be unchanged up to recent ■ cars. Nleanwhile,

stock sizes at the end of fishing season ‘vere nearly equal to the sizes at the beginning of following

fishing season in almost all of year classes, so that natural mortality set ois 6) 1x rather small.

The CPUE's by four vessel types, i. e.. ice-holcl 100 20o GRT trawlers„ freezer trawlers al 300

550 GRT, 550 1000 GRT and 1000 1500 vvere calculated by tishing bl .nch and by month . I n

the most center of fishing season in the Cap Blanc Region. Setting out the Boo 1500 Gin trawl

ers as the standard vessel type, standardizations of fishing effort for other three vessel types were

made by the ratios of CPUE between standard and each of other vessel types Table 20 and Fig.

3:( Then the total standardized fishing efforts and CPUE's by fishing season were obtained Table

, and the total standardized fishing efforts by fishing year were calculated as the soin of the

efiorts in summer and the following winter seasons (Table 23).

General Production Model (Gt IAA», 1961 ) was applied to the series of fishing effort and oc-

topus CPUE. by fi shing year. Taking into account that the octopus stocks under consideration will

be exposed to fishery for more than two years at least for males, a regression line was drown based

on the plot of CPUE in the current year against the mean fishing effort of the current and the pre-

vious year. A strong interrelation was found between them with a correlation coefficient of 0.8

t Fig. 35). According to equilibrium yield curve estimated from the regression between effort and

CPUE, the maximum equilibrium yield and optimum effort were estimated to be 28 thousand tons

and 108 thousand hours in. ' standard effort, respectively. Therefore, it is pointed out that the fishing

efforts expended in recent'years veere on the optimum level and the catches were rather stable on

the level at the maximum yield in the Cap Blanc Region.

Yield per Recruit Nlodel (Bevtarrox and Hour, 1957) was appbed to estimate the optimum age

at first capture and the optimum level of fishing effort. Taking into account the difference of life-

span between the sexes, fishable maximum age (to of four years old for male and two years old for

female, and therefore, asymptotic. weight (W,..) corresponding to each u, 4277 g for male and 2232 g

for female, were adopted. As mentioned above, natural mortality coefficient M . was assumed to be

so small that the values of 0.1 and 0, 2 were used for both sexes. ._

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Two figures of yield per recruit calculated by sex were extremely different each other (Fig.

Nam?.ly, the tigures for male were always larger than twice of corresponding figures for female.

Nloreuver, diagram on female tended to increase with the increase of fishing mortality (

in spite of that on male decreased with the increase of F in the range of age at f irai capture .t, ,

from 1.0 to 1.8 years old. The isopleth diagram drawn by the sum of both yields per recruit showed

two maximums (Fig. 37 I, The one at smaller t ; consists of male and female, and the another at

larger t. consists only of male octopus. In the case of latter maximum, large quantity of fishing

effort will be required to anal n.

The total mortalities ( Z were estimated to be about 1.2, as aforementioned. According to month: 1 y

catches in number by year class (Table )8 the value of t, in these years were assumed to ht..

about 1.2 years old on the average. Therefore, the level of recent utilization of octopus stocks in die

Cap Blanc Region is not reached yet the full y exploited stage. If the t, is shifted from 1.2 tu I. G

years olcl and the F is increased by 30 0. of those in recent years, the yield por rucruit tvill increasc

by 10% of the recent value.

Mauritania. one of the c.oastal countries, ha s limited the number of vessels operated and adapted

a license system by means of expansion of the territorial waters since 1972. Nloreover, Nlauritania

prohibited the use of trawl gear with cod-end mesh smaller than 60 mm, and closed shallower coast-

al areas on the Banc d'Arguin to foreign vessels concerned. In connection with (he understanding

that the level of exploitation is in a suitable condition in recent years, it \\ill no i be necessary to

introduce urgently ans' adclitonal regulations for the protection of octopus stocks in the Cap Blanc

Region.

On the other hand. no regulatory measure has been enforced in the Villa Cisneros Region.

Although no sufficient data are available for analyzing, it is noticeable that octopus CPUE by Japanese

trawlers dropped sharply since ) 971. and the size of octopus at first capture is considerably smaller

than that in the Cap Blanc Region. Therefore, as to the stocks in the Villa Cisneros Region, prepa-

ration of the datailed catch and effort statistics as well as full information on CI ) 1 . 1:: of octopus is

essential as a tentative measure, and some reasonable regulations should be introduced as soon as

possible.

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Contents

Text Translation page page

English Language Summary 13 1

Preface 21 11

Chapter 1 The growth and present state of • 23 14 the,octopus fishery common

Section 1 The sources of the statistics used 23 15

Section 2 Octopus catch by Japanese vessels 23 16

2.1 Summary of fishery development 23 16 and expansion

2.2 Octopus fishing grounds and 24 19 fishing seasons

2.3 The present condition of the 26 22 octopus fishery

Section 3 Summary of the octopus fisheries 29 27 of other countries

Chapter 2 Information on spawning seasons 30 29 and locations

Section 1 Materials and methods 30 29

Section 2 Seasonal changes of the gonads 32 32

2.1 Seasonal variations of maturity 32 32

2.2 Seasonal variations of the 34 35 gonad index

Section 3 The collection of egg masses 36 38

Section 4 Discussion and consideration 36 40

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Text Translation page page

Chapter 3 Age and growth 38 42

Section 1 Materials and methods 38 42

Section 2 Estimation of body length 42 50

composition in the catch and search for elemental length groups

2.1 The body length composition by 42 50

size category and estimation of of the body length composition in the catch

2.2 Search for elemental body length 43 50

groups

Section 3 Estimates of age and growth 48 58

3.1 Seasonal shifts of the body length 48 58

modes

3.2 Investigation of sex differences- 48 59

in growth

3.3 Estimation of the date of hatching 51 61

3.4 Estimation of age 51 62

Section 4 Discussion and review 54 66

Chapter 4 Stock identification and migration 54 69

Section 1 Materials and methods 55 69

Section 2 Identification of the stocks . 57 72

Section 3 Stock migration 58 74

3.1 South-North migration 58 74

3.2 Deep-Shallow migration 58 75

Section 4 The relation between migration 64 84

and ocean environment

4.1 Outline of the ocean environment 64 84

4.2 The relation between migration 65 86

and environmental changes

Section 5 Discussion and consideration 65 88

Chapter 5

Section 1

Section 2

Section 3

Other biological information

Materials and methods

The sex ratio in the catch

The stomach contents

3.1 Species composition of the stomach contents

3.2 Seasonal variations of the stomach contents, weight and state of diet

Section 4

Chapter 6

Section 1

Section 2

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Discussions and reviews

Resource fluctuations and optimum catch

Estimation of mortality coefficients

Maximum equilibrium catch and optimum fishing effort

2.1 Standardization of fishing effort

2.2 Estimation of CPUE and fishing effort

2.3 Maximum equilibrium yield and optimum fishing effort

Maximum yield per recruit

3.1 Determination of parameters

3.2 Analysis by means of yield isopleths

Section 4

Summary

References

Text page

67 90

.67 90

67 90

69 91

69 91

69 95

71 96

72 98

72 99

78 107

78 107

83 115

85 119

86 121

86 121

86 123

87 126

89 128

91 133

Section 3

Discussion and review

Translation page

• 11

Preface

The common octopus, Octopus vulgaris LAMARCK, is widely

distributed throughout the warm and tropical regions of all the oceans,

and has long been highly valued by the people of Japan and of the

Mediterranean Sea who largely rely on the oceans for food.

According to FAO statistics (FAO 1976b) the total octopus catch

in all oceans was 217 000 tons, and most of this is believed to be common

octopus. A large part of the octopus catch in the coastal regions of North

West Africa comes from the coasts of the Spanish Sahara and Mauritania

(Figure 1), and in 1975 the catch along the shores of these two countries

Fig. 1. Chart of the northwest coast of Africa and breakdown

of fishing grounds.

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amounted to 121 000 tons or 56% of the total catch of all types of octopus.

In that year the octopus catch in Japanese coastal waters was 49 000 tons,

less than half as much as in this region. Spanish, Korean or Japanese

vessels made 99% of the octopus catch off the North West coast of Africa,

and since most of the Spanish and Korean catch is exported to Japan, the

catch on the North West coast of Africa amounts to almost two thirds of

the octopus consumed in Japan.

Thus the octopus stocks in this region form an extremely important

resource not only for the Japanese Southern Trawling fishery, but also for

the whole population of Japan.

Japanese trawlers began to fish experimentally off the North West

coast of Africa in 1959. Since then the Southern Trawler fishery has

become one of Japan's most flourishing far seas fisheries, and the coastal

waters of North West Africa play a role as an important fishing ground

with a catch which in 1968 reached 150 000 tons. Meanwhile the octopus

catch increased rapidly, and in 1968 reached 73 000 tons or 48%. The

operations of Japaneœvessels have since then centred on octopus, but the

catch peaked in that year and began to diminish, dropping to 41 000 tons

in 1971.

At the beginning of the 1970's the shore countries became concerned

about the worsening conditions of the stocks and started to exclude foreign

fishing vessels and to strengthen controls. The FAO, through CECAF, also

commence international discussions in relation to fisheries control.

In 1967 Japan initiated a systematic investigation of the bottom

fish resources of the fishing grounds off the North West Coast of Africa

in order to have a basis for the evaluacion of this fishery. This

investigation was at first devoted to the preparation of fisheries p22

•

statistics and to obtaining a clear idea of the relation between the

species of fish and their commercial value. The next year the fishery

statistics based on catch reports from the fisheries representatives

were entered into a large-scale electronic computer, and thereafter the

effort was concentrated entirely on the biological data.

The author has had a part in this systematic investigation by

the Far Seas Fisheries Research Laboratory since its inception, and has

principally been in charge of the investigation and research on the stocks

of octopus. The objectives of the investigation have included the under- common

standing of the present condition of thevoctopus fishery on the North West

coast of Africa, the biological characteristics of the common octopus, and

an analysis of the variations of the stocks on whibh to base plans for their

proper management. The present report has been compiled from the results

of a series of studies.

I am deeply grateful to Professor Takao IGARASHI of Hokkaido

University for his encouragement during the preparation of the study and

for revision of the manuscript, and to Professor -

Kenichiro kUNH also

•

of Hokkaido University for his kindness, guidance and revision. I am also

grateful to Professor Dr Kunio AMAOKA of Hokkaido University for guidance

and for assistance.

From the inception of this study to the present day I have received

encouragement and guidance from Dr Yoshio FUKUDA, the Director of the Far

Seas Fisheries Research Laboratory, and from Dr Ikuo IKEDA, the Chief of

the Bottom fishing marine animal stock division of the Laboratory, and to

* translator's note : other readings possible

14

both I am very grateful. I have also received detailed guidance and advice

from Dr Norichika SATO, the Chief of the Far Seas Trawling Resources study,

and many colleagues in the Laboratory have given unstinted assistance and

much information. To them too I am deeply grateful.

Finally, I wish to express my thanks to all the members of all

ranks of the trawler crews for their cooperation during my period of

investigation at sea and to the Fishery Association for the valuable

information which they offered and for the many materials which were to be

found on the fishing grounds.

Furthermore, this report is also a thesis for a degree from

Hokkaido University.

Chapter 1 COMMOY1

The growth and present state of thevoctopus fishery

This chapter deals with the development and present state of the

Japanese Southern Trawl Fishery, with special reference to the octopus

fishery in the coastal regions of north West Africa, and also summarizes

the fisheries of other countries in these regions.

The Southern Trawl Fishery is part of the Far Seas Bottom Drag

Net Fishery, and is authorized to operate in all ocean areas other than

those in the Pacific Ocean north of 10o N latitude. It is thus a strong

fishery whose operational region is not restricted to the North West Coast

of Africa and may export to or import from other fishing grounds. •

15

Section 1

The sources of the statistics used

The fishery statistics used in this chapter are derived from

the following sources.

(1) An investigation of data from the Far Seas Bottom Dragnet Fishery

(Far Seas Trawl Fishery).

The Production Department of the Fisheries Agency has published

data on the individual quantities of fish caught and the number of

trawls for each year from 1959 to 1969. The data refer to the

whole African region and are not broken down by type of vessel.

(2) Statistical Tables from the Far Seas Bottom Dragnet (Southern

Trawl) Fishery.

These reports are based by the Ministry of Agriculture and

Forests on the fishermen's reports and the statistical lists drawn up by

the Far Seas Research Laboratory since 1967 can be used. Statistical

results can also be obtained from censuses in the three years 1964 to 1966.

The statistics are classified by the month, by the fishing ground

(divided into squares of 30 minutes of latitude and. longitude),by the

method of fishing (side trawl, standing trawl) and by type (size) of Lc) indicate- the. fishi,15 cilarb

vessel, and two further headings usedâre the number of hauls made and

the total length of the haul periods.

(3) CECAF Statistical Bulletins

The annual statistical report produced by CECAF, one of the

Regional Fisheries agencies of the Food and Agricultural Organization

(FAO) can be used for 1964 to 1974. The fishery effort is not recorded

in this annual report,and moreover that catch, by species in the largest

catch in the

•

area, that of the Soviet Union is missing. In addition the catches for

some of the coastal countries are estimates.

(4) Other statistical bOurces

For the operating conditions of the Japanese fishing vessels at

the start of the fishery, reference was made to u 5tkai suisan soran"

(World Fisheries Survey) published by Suisancho Kanshu 1965 (Edited by the

Fisheries Agency) and to "Yakushin suru enyo tororu 9yogyo" (Rapid progress

of the Far Seas Trawler Fishery) published by Suisan Hyoronsha (Fishery

News Co. 1966).

Section 2

Octopus catch by japanese vessels

2.1 Summary of Fishery development and expansion'

In the early 1950's, stagnation of production from the bottom

drag-net fishing grounds in the East China Seas and the Yellow Sea led

to the development of anxiety about the future, and as demand for the

fish caught continued to increase, expansion into the Far Seas fishing

grounds was started (Sato 1973).

Vessels on trial operations reached the North West coast of

Africa in 1959. In that year the two 500-ton trawlers operated along the

coast from Morocco to Mauritania, and discovered a good fishing ground for

sea bream on the coast of the former Spanish Sahara (Fisheries Agency 1965).

Circumstances that led to the rapid increase in operations by

Japanese trawlers included the almost virgin condition of the stock in

this region which up to then had not been developed, the good domestic

•

17

conditions, and the proximity at Las Palmas of an excellent base. Ten years

later in 1968 with 68 ships operating the catch was about 150 000 tons,

and the North West coast of Africa became a central point of the Japanese

Southern Trawler Fishery. p24

At first the main species taken was the sea bream Pagellus bellotti

STEINDACHNER, but catching effort was later transferred to the commercially

more valuable European squid (cuttlefish) Sepia officinalis LINNAEUS and

to the common octopus. The octopus fishing ground off Cape Blanco was

developed in 1965 and its proportion of the octopus fishery increased with

great rapidity so that by 1967 it attained the largest portion of the

octopus catch. However from 1969 onwards the number of large 1500 ton

vessels operating in this area rapidly diminished and the catch also started

to diminish. These large vessels moved away to thé North-West and South-

East Atlantic because of the enlargement of Mauritanian territorial waters

and the deterioration in operational expense resulting from a reduction in

the catch per unit effort (CPUE).

In 1967 Mauritania drew a straight base line from Cape Blanco to

Cape Timiris, and in 1972 again widened its territorial waters to 30 nautical

miles. Some of the Japanese fleet continued to enter this region and to

operate after 1970 by paying a fishing permit charge, but since the permit

charge was reckoned per ton on the total tonnage of the vessel, operations

were confined to small and medium sized vessels with great operating

efficiency in licensed activities. One part of the agreement with Mauritania

led to operations after 1971 by small ice-hold vessels based on the port

Nouadhibou. This has caused the number of vessels to increase in recent years,

but the catch continues to decrease and in 1975 it had dropped to 65 000 tons

(Table 1).

Fishing effort

Vessels Hauls Hours operated made fi shed

Catch in tons

Sea Cuttle- Total breams fishes Squids Octopus Year

Table 1. Fishing effort and catch of Japanese trawlers in the northwest coast of

Africa (9`.-30'N).

18

1959 2 831 - -

1960 8 4533 - -

1961 14* .• ..

1962 19* ' 22561 - -

1963 24* 38230 --

1964 29 48223 91342 1965 41 85736 142238 1966 48 128277 193127 1967 66 164494 235696

1968 68 204020 274352

1969 1 63 209780 267355 1970 59 207301 254911 1 1971** , 58 213369 241400

1972 i 83 294935 290922

1973 73 297886 280648 1974 79 311027 , 281077 1975 76 267761 231064

802 748 10

6380 4325 851

26768 10884 12485

37234 15279 11381 4121

63829 20723 18036 9969 6999

66343 18821 183113 3193 8658

109853 19393 33300 - - 30210

101877 24535 27140 - - 222211

149349 24058 31672 11524 53837

152407 16199 29374 6650 72914

131115 21060 24054 5617 46154

101816 14018 12324 43111 39744

88019 9360 15914 5102 3806:;

99560 16731 21446 4649 33003

93656 13436 21196 4441 29834

79023 10967 11670 5923 28145

64549 8484 7302 1152 24526 - . . .

--, Unknown; —, No landings * Estimated by the total number of entering into the port of Las Palmas, Gran Canaria

** Not including the effort and catch made by the ice-holcl trawlers based on Notuulhibou, Mauritania

•

2.2 Octopus fishing grounds and fishing seasons

A number of large fishing grounds are known on the coasts of the

former Spanish Sahara and of Mauritania. Their size and naines vary with

different fishermen. p25

The geographical extent of the octopus fishing grounds was investigated

19

•

by making use of the Bottom Drag-net Fisheries statistics, and determining

the annual octopus catch in each square of 30 minutes latitude and longitude.

(Figure 2).

These show that octopus were caught in all coastal waters of both

countries, but that three principal fishing grounds can be distinguished.

The central fishing ground for Japanese vessels is around Cape Blanco from

20oN to 21

oN. This has consistently been the cenire of the Japanese octopus

fishery since 1965. The second fishing ground, Villa Cisneros, extends

from 22oN to 25

o30'N. Up to 1969 it was half the size of the Cape Blanco

p26 fishery, but since 1970 the relative importance of this ground has declined.

The third fishery is centred off Nouakchott from 18°N to 19°N and has been

developed relatively recently.

As seen from the point of view of the catch distribution, the

fishing grounds can be divided into the three following regions. From

21o30'N to 26°00'N there is the fishing ground off Villa Cisneros, from

19o30'N to 21

o30'N the Cape Blanco ground, and from 16°00'N to 19

o30'N the

Nouakchott ground. -

Next the monthly catch of octopus and the mean CPUE were found

for three-year periods from the Far East Bottom Drag-net Fishery

statistics, and the octopus fishing season was studied. (Figure 3).

1.!

2_.; • :•ci

r--+

—le • . 16;

•

• '0

311.1 1111111 11111

/I BMW

I° I LT. ill

CIC

111111 CIV

0 • NO

11 010

."1 IV (1967) 19671

• • ro

te • I -

(1971

o

o

net

CI ID 111

• I.

o

o .0 •

0i0,0

(1975)

o:0 -1D tons. • :10- 100 • 100-500, •:500-1000, • :1000-2000, 9:2000-5000,0 um_

Fig. 2. Annual octopus 'catch by Japanese trawlers in each statistical block. The N'alues in 1964-1966 are virtually censused.

• -• •

RIC

CCI COB

DOD COCCI non cirri

mui

ono • • •

aD • le 1:1 aD

rn o o o

Clo_t*, oboe *le to •

I o 75'N

• i•

-4

•• • O 0.0 • lh

:j

!la!". • 91• ,rij

—1

• e • VIJ .

IV Ill

IV

oks

MCI

• 1• '1.01

'0 ist • •-; 're - o

ou.

010i • •

o o

(1964) 19(4:i 1966) 1965

--run

'1— 1°4

, •

BIC

COO arxr QaQa

CICI CICI CDC

DIM CID11

CUM MIR ° DD .

moan 130013

DODD

CCID CIDIC

o

CDC anon 0 10 Of 10 010

-r- iTiiii7-0—(7, •

4- r-11Z] el'ele

"i i-• L- * Pi :. • i _

••• (_) • le • r., ..:61, .. e'é 4 e .0

0 _,_____e_i ____, ,

0 0 Or 0:0 • a • ...,01

• • • 1. •ro-' CHM 1 • • ji 11,..

o 0 25 ° N

• itb •

ire CIC

CI • C . CDC

• •

010i • • •

;13 20 h I:IL • T

• 9

ri 1:1 CCI CCI DC

11100

Va Ca

0101 lo;O olo 10,)- • -79-9 0 le e)

aD

II

o ; I.; • o 0 1

(1972) (1973) 1974) 1970)

21

E

[ ,

,

Ni' -. 4

!'e . .,,.. ,,-- - - , . • . 'C. ---e----,3=45-:--- '-'a -el - 3. -': .

r \ •

Mon t h

Fig. 3. Mean value of monthly octopus catch (dot and solid l lue; and catch per unit of effort

(circle and broken line) in every three years in each tishing ground. Catch per unit

of effort was obtained from trawlers of 55u 15ttn

This showed that the Cape Blanco ground has a short season from

August to September, but that there are also two good seasons with high

CPUE a summer season and a fairly long winter season from December to

April. These two seasons are known as "summer octopus" and "winter

octopus". The winter season has been thoroughly developed since 1965, and

the summer season was discovered in 1967.

Up to about 1970 the flourishing fishery on the fishing ground

off Villa Cisneros showed two peaks in the fishing season, from November

to December and from February to March. However since 1970 the quantity

of octopus caught by Japanese vessels has greatly declined, and there is

no longer any season of abundance.

•

•

22

The fishing ground off Nouakchott has had a good season in July

to September since 1970, but it differs from the other fishing grounds in

having no winter season. The catch from April to July has also increased

every year, but this is because the fishing vessels move to this ground

during the lull in the Cape Blanco fishery, and having regard to the

value of the CPUE it cannot be considered as a good fishery.

2.3 The present condition of the octopus fishery

Use was made of the effort put into the fishery during the

twelve year period 1964 to 1975, of the quantity of octopus caught and of

the year-to-year variations of CPUE to investigate the development and

the present condition of the Japanese octopus fishery (Table 2, Figure 4).

If the calendar year were used the winter fishing eason would be split

in half, so to avoid this the year used as the fishery year is taken from

July to the next June.

Except in normal years, the catch in the region of largest catch,

that off Cape Blanco, is from 55% to 76% of the total catch so that the

common octopus is the principal objective of the fishery. Since the

effective start in the year 64/65 (this year is frOm July 1964 to June 1965,

and later years are taken similarly) the fishing effort has quietly but

continuously increased and reached 157 000 hours in 74/75. This increase

is due to fishing vessels newly entering the area and also to the transfer

of vessels from the ground off Villa Cisneros. It surpassed the effort

off Villa Cisneros in 70/71 and in 74/75 it reached 61% of the total effort

off the coasts of the former Spanish Sahara and of Mauritania.

•

in 200

• \ .0

40 •••

••

,Cap Blanc • - -

\ V i 1 Li 's Cisneros

X

— X — - — 0— -0

20 / • •

re - ,

•

1 _ •- _ 400

200 e Vi lla Cisneros

U.1

L.)

-

65/66 6% 7 67/68 6:k,/

Fishing Year

e- • _ Cap Blain_

Nouakchot!

79/71 71/2 7 313 (July - June)

6%5

Fig. 4. Annual changes of fishing effort, octopus catch and catch per unit of effort

in each fishing ground. Catch per unit of effort was obtained from the •

trawlers of 550-15(1(1 GRT.

•_ 0

.c

23

Cdp Blanc . . .-- .....x ,

/ . c Vil la x Cisneros \ - x .— ---

100- x --. ..--- \

.... / \ • x 7 ‘, ,.. . .

\ •--- \ o/

x --- 'X .- .

,' . _• --... --0

, Nouakch o t.

1.■

•

Hours Catch fished in tons

CPUE in kg/hour

Catch „ in tuns

CP1.11.2 in kg hour

Year 1%)

Hours fished

Year Hours fished

Hours CPUE in Ca) ch in tons fished kg hour

Table 2. Fishing effort, octopus catch and catch per unit of effort of Japanese trawlers

by fishing year (July-June) and by region. Numbers in parentheses are per-

centage of octopus in total ca,tch. CPUE's are shown for 550-1500 tonnage class.

Villa Cisneros Cap Blanc

24

•

_ 64*:65* ' 53899 6215 (14) 158 27135 14983 165. 529

65*/66* 101188 13414 (17) 147 23316 6163 136 169

66*,67 112622 15121 (23) 132 56535 23422 (64. 418

67/68 180868 26389 (26) 158 67522 25615 i61. 400

6869 141966 20705 (29) 145 106479 54751 (76. 522

697 )) 123944 7736 (17) 61 100663 24479 155. 953

70 71 76402 3930 (13) 49 131943 35916 7-1, 275

71** 72 90029 431 1; ■ 14. 50 134086 32-133 I 68. 970

72,73 113876 4929 (13 -12 131796 24080 .60. 219

73.'7 1 730-12 1759 ( 8) 20 15.10-17 23446 i 58 182

74/75 93743 902 i 9 ) 17 156858 26371 73. 182

75!*** 7301 212 (15) 99 68438 8572 .70 16 ))

Nouakchott Total'

Catch in tons "

64* 65* 1155 10 t 1, (t 82189 21208 31 •

65* . 66 2872 28 t 11 13 127370 19005 20

66' 67 3418 34 ( I; 6 172575 38577 ;36

67:68 4189 66 ( I) 20 959579 52070 36;

68 69 21299 381 ; .-P 18 269711 75837 -19

69 . 70 939 39 448 ( 4 : 21 247839 32663 32

7071 32889 1411 (11. 48 241234 4 ) 287 .45

71**,!72 . 33133 1271 (13; 62 257248 38014 ;43;

72/73 , 39472 1594 (10) 65 285144 30( 03 ;33.

73/74 43644 2818 (20) SI . 270733 28023 (37;

74/75 : 58634 3316 (16) 63 259235 30589 (46)

75/*** : 22964 1518 (20) 66 98703 10302 )49 ,

* * * Virtually censused

** Not included the ice-hold trawlers Figures from the first half of the year

•

25

The octopus catch has increased from year to year in response

to the increase in effort. The summer and winter seasons of 68/69 were

extremely good, 55 000 tons being recorded. However, except for this,

the octopus catch has remained relatively stable between 23 000 and 36 000

tons since 66/77.

The CPUE on this fishing ground was 530 kg per hour when serious

fishing operations by vessels in the 550 to 1500 ton class commenced in

64/65. However apart from 65/66 when operations were concentrated on

cuttlefish and sea bream, and from the abnormally good year 68/69, the

CPUE has declined while the amount of effort has increased, and in recent

years has reached values somewhat less than 200 kg.

The operations by Japanese vessels off Villa Cisneros, where the

total production of cuttlefish, octopus, squid an à sea bream is the

largest, are rather biased towards the autumn because of the productive

autumn season for cuttlefish and the winter season for octopus. In these

circumstances no single species ever forms more than one third of the

month's catch.

This fishing ground was at first the centre of the fishery, and

the expansion of the Southern Trawler Fishery was accompanied by a rapid

increase in fishing effort which resulted in 181 000 tons in 67/68, or

72% of the total for all three fishing grounds. However since then the

fishing conditions have worsened and the number of Spanish and Korean

vessels has increased, and the Japanese effort has begun to shift to other

areas such as the North West Atlantic and to the Cape Blanco ground, and

in 74/75 it was 44 000 hours, less than that off Nouakchott.

O

•

26

The octopus catch parallels the increase or decrease of effort,

with a peak of 26 000 tons in 67/68 and a continuous decline thereafter,

dropping to 900 tons in 74/75.

Up to 68/69 the CPUE remained stable at about 150 kg per hour

but the next year it dropped to around one third and since 73/74 it has

gone down by half to around 20kg. Also the increase in the CPUE found in

autumn and winter has since 71/72 disappeared, and Japanese octopus fishing

has in fact stopped. However, as will be mentioned later, Spanish fishing

vessels which are able to operate within 12 nautical miles have even in

recent years caught 30 000 to 90 000 tons of octopus.

The fishing area off Nouakchott has fairly recently been developed

to supplement the Cape Blanco area. The original pperations there were

for sea bream and squid during the off season in May and June at the Cape

Blanco area, but since about 68/69 there have been year-round operations

which since 70/71 have been based on octopus as the target from July to

September and the catch has increased as the effort has increased. In

recent years about 3 000 tons of octopus have been caught, which is about

10% of the total for all three areas. Originally the CPUE was about

20 kg, but it increased to 50 kg with the start of the summer season, and

has since levelled off at 60 to 80 kg.

•

•

•

27

Section 3

Summary of the octopus fisheries of other countries

A summary of fisheries of other countries is obtained from the sta-

tis4ics for the Cape Verde Coastal Division (9°N to 19oN) and the Sahara

Coastal Division (19oN to 26

oN) in the CECAF Statistical Bulletin.

However the quantities of the individual species caught by the largest

fishing fleet in the area, that of the Soviet Union, are not recorded in

these statistics.

There are six coastal states on the North West coast of Africa

oN to 26

oN) from the former Spani (9 sh Sahara to Guinea. Fishing vessels

from about 15 European, African and Asian countries operate there, and

the total catch during recent years has reached 2-300 000 tons.

From 1965 to 1968 the Japanese catch was'about 20% of the total.

The catch has decreased in recent years and it has dropped to about 3%.

Surface fish such as mackcrel form about a half of the total

caught in these regions, and cephalopods about 9% to 15%. The Mediterranean

countries, especially Spain, are active as well as Japan and Korea in

fisheries for bottom fish and cephalopods.

In 1965 the octopus catch was about 70 000 tons, but since 1967

it has increased and levelled off at 100 000 tons. The Japanese catch

reached a peak in 1968 when 127 000 tons were recorded. Since 1965, Japan

and Spain have annually taken 91% to 96% of the octopus catch, divided

equally between them. However in recent years as Japan has declined and

Spain has increased, Spain has taken about two thirds and Japan one third.

Korea,Italy and Greece also fish for octopus in this region (Table 3).

•

•

28

Table 3. Octopus catch in tons by country in the northwest coast of Africa ( . 9 —26

Country 1964 1965 1966 1967 1968 1969 1970 1971 1972 1973 197.1

Republic

1154 213-1 1773 1230 3708 650u ,

Mauritania 100 130 180 187 217 260 410 440 814 400 •

Portugal -28 •- -

Spa in 35700 3930(1 41100 49300 37800 30100 67300 79300 54700 62 131) 0

Total 12938 69363 66213 99645 126986 91507 74621 113925 117273 905 7,3 98s;

As to their modes of operation, the Spanish vessels based on

Las Palmas and on Teneriffe in the Canary Islands traditionally operate

off the coast of the former Spanish Sahara (Cabrera 1970), the main fishing

ground extending from Cape Garnet to Cape Barbas (Navarro et al 1950, 1953).

According to Japanese fleet reports, Spanish vessels are almost never

seen to operate on the Cape Blanco and Nouakchott grounds.

Greece has a fishing agreement with Mauritania and in buS, years, upLo

20 vessels are reported to operate under the agreement, but in recent p30

years the number of vessels has declined. Their main targets are sea bream

and crustaceans, and there are good opportunities to catch these by

operating off shore from Nouakchott to Bissau in Guinea. Italy has operated

two vessels by agreement with Mauritania but since the main target is sea

bream there are good opportunities other than on the Cape Blanco ground. As

to octopus fishing by other countries , we may conclude that the Cape

Blanco ground is used almarbentirely by Japanese vessels, the Villa

Cisnero ground by Spain, Korea, Japan and Italy, and the Nouakchott

ground by Greece and Italy as well as by Japan.

Bulgaria • 69 42

Greece 235 1208 656 1060 1394 2767 688 1073 749 880

Italy 3945 2115 3857 3461 3168 3372 1456 1875 2190 1033 2567

Japan 8658 30210 22220 53837 72907 46154 3971.1 -11422 32990 29832 28111

Korea

•

29

Chapter 2

Information on spawning seasons and locations

The common octopus is widely distributed throughout the warm

and tropical coastal regions of all the oceans, and much is known about

its spawning seasons. In summary, mature individuals are found throughout

the year but either one or two spawning seasons are present (and there

may be a prolific spawning season). In the Mediterranean, the spawning

season extends from March to September or October, and the prolific season

is in April to June (Mangold-Wirz 1963, Guerra 1975), and there is reported

to be a hatching season in May to July in the English Channel (Rees and

Lumby 1954). On the coast of Japan two octopus spawning seasons in spring

and autumn are known (Tanaka 1958, 1967, Itami 1915). From the seasonal

changes of the gonads, Hatanaka (1979) found two spawning seasons, in

spring and autumn, off the North West coast of Africa. Further material

including data on the collection of egg masses spawned by octopus are

presented in this chapter, and the spawning seasons and locations are

discussed.

Section 1

Materials and methods

This chapter makes use of the results of biological measurement

of about 1600 individuals collected by commercial fishing vessels and of

the results of measurements made on about 5 400 individuals during ship-

board investigations on commercial fishing vessels (Table 4).

Table 4. Number of specimens used for the estimation of spawning season.

Region Year Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Total

1967 - - - - - .--- — 15 — 15

1969 — -- 65 . 56 -- -- -- 121 Villa Cisneros 1971 - - 77 - 71 -- -- - --- 148

1972 , -. . - 19 20 — - - — 39

2P30`-2600'N 1974 - _ 4 85 -- 117 10 99 315 ° 1975 26 — 105 . - 12 - 80 - _ -- 223

Total , 26 247 16 71 56 104 100 117 25 99 861 -

. 1967 . -- _• . 53 — — _ - 53

1968 --. --- 16 13 107 20 99 - 11 12 201 •

Cap Blanc 1969 9 9 54 25 61 -- •-- 158

1970 64 -- - -- --- 64

1972 26 514 467 227 --- 1234

19 °30'-2130 N 1974 -- 7 7 14 4 83 _ _ 11 - 126

1975 . 84 369 72 40 69 480 400 1 ._ --- 1514

Total 93 -14 9 95 20 54 69 698 554 597 467 249 12 3350

.. . • -- 1971 - - 285 -146 731

Nouakchott 1972 59 103 -- 162 ,- 1974 3S -: • - " 10 75 83 -- 206

16 '00'-19"30'N 1975 - 529 1123 -- 1645 . •

•

Grand Total

Total 38 285 968 1133 75 83 59 103 -- 2744

119 412 3.1'2 58 355 110S 1887 733 780 643 377 111 6955

GW ---

TL3 Maturity index x 10

6

31

The seasonal variations in the maturity of the gonads and in a

gonad maturity index mêre obtained from these materials, and on these

were based estimates of the spawning seasons. The maturity of the

gonads was in all cases assessed by naked eye observation, using the

following criteria.

Male Immature -- No spermatophores visible in Needham's sac.

Mature -- Spermatophores visible in Needham's sac.

Female Immature -- Ovary milky white in colour.

Maturing -- Ovary with yellowish tinge but not transparent.

Mature -- Ovary yellow and semitransparent.

The maturity index was obtained by the use of the following

equation:—

•

in which TL = Total length (mm)

GW = Weight of the gonads (g)

In both sexes, the weight of the gonads was taken to be the

combined weight of the gonia forming organs and the associated organs.

The data for the fishing grounds off Villa Cisneros, Cape Blanco and

Nouakcott were in general treated separately.

The octopus spawning locations were also deduced from the data

on the collection of octopus egg masses. This investigation was biased

because the data were mostly obtained during investigation onboard

commercial fishing vessels. It was therefore difficult to study the

100

— c

50

—•— :ViiI., Cisner05

—0-- : Cap Blanc

: Nouakchott

Reyion conlined

o.

la.

o

lb 40 60 100 •

32

distribution of the egg masses quantitatively but a rough estimate of

the spawning grounds was made by a geographical comparison of the limits

within which egg masses were collected with the distribution of

egg masses.

Section 2.

Seasonal changes Of the gonads

2.1 Seasonal changes of maturity

No seasonal changes in maturity as evidenced by the presence or

absence of spermatophores were found. In fact, making use of all the

male specimens and considering the proportions of mature specimens in

size categories according to total length, all specimens more than 50 cm virtually

long are found to be-emature (Figure 5).

Total Length in

Fig. 5. Occurrence of the matured male by total-length class in all seasons combined.

CM •

20- rr rrl_r Jan.. Feb.

Cap Blanc

• Nouakchott Villa Cisneros

May

10

,dai=1—.■=_Ja

Sine

36

22,

20

20

July

23

20, Aug. 40

a

Region C•mbin•d

Jan., Feb.

219 20 1

20- Mar., 1 495r.

40 -1

-11 016 July

150

100

50

20

60

40

20

60 ri Aug.

34 3

20

20

_-c

Mar.. Apr.

26

20 Male

146

Mar., Apr.

May

1 t 0

June 27

ilL4111.

July

302

60 .1

40.1

20H

mo-1

June

442

60

40

20

20

Sept.

302

Total

22 1

-

._ ,--1 r Jilin -

iiii:pt. - — 57

_

.U 1

• 1 20-

Nov., Dec.

63 —1

50 100

U

Oct. 52 ,

.'2

10 1 Iffiv , Dec. 110 13

Oct.

120

Dec.

100

in C

20 1

Oct.

„4

40

20 1

50

Nov., Dec.

131

100

• •

401 4,91

10

10

10

60

40

20

40 Aug.

20.1 273

--

10 Alm. 60

10 401 Sept.

'-1:1111.1 —

399 .?0 _i___

Sept.

40 40

CZ tare

100 10

Length

:Maturing 111. :Mature

1 0-1

L

Fig. 6. Monthly changes in the maturing condition for female shown in length frequency distribution of specimens. Numerals

in the figures indicate the number of specimens.

co

34

However, remarkable seasonal changes were found in female maturity

(Figure 6). On the Villa Cisneros ground many mature specimens appear in

September. On the Cape Blanco ground there are also many mature specimens

in September, and, although the number is small, all specimens more than

70 cm in total length are found in May and June to be mature. On the

Nouakchott ground, there are many mature specimens in May and June, and

mature specimens are also found in September and October. Considerab1e

numbers of specimens were obtained in July and August at Cape Blanco and

in July only at Nouakchott, but in both places the number of mature

specimens was low. Thus if all regions are combined the mature specimens

are concentrated in May and September, and this indicates that there are

two spawning seasons off the North West coast of Africa, in spring anclautumn.

An investigation of individual sexual maturity and body length

gave the following results. The smallest female specimen assessed as

mature had a body length of 37.9 cm. In the data for the many mature

specimens found at Cape Blanco in September and at Nouakchott in May, the

proportion of mature specimens increased as the body length increased, p34

and the mean mature body length was about 60 cm. The mode of the body length

for these two sets of data was in the 70 cm to 75 cm group. The

number of data at Villa Cisneros was not sufficient, but it seemed that

the mature body length there was slightly greater than at the other

two regions.

Cap Blanc •- * N

Villa Ci-Sner0'.

litnidlchutt

w 20-1

Length class of 50-90 on

o 0 80

Under 50 cm

60 0 2

40 -Î •

.-x ss.

Over 90 cm

Region combined

1- -1- -1- -1

20

60

40 -I

35

2.2 Seasonal variations of the gonad index

Hatanaka (1979) investigated the relations between the gonad

index and a grouping of body lengths. This method is followed in the

present report. The data for males are treated by dividing them into

three groups with total lengths less than 50 cm, total lengths 50 cm to

90 cm, and total lengths greater than 90 cm. The data for females are

divided into two groups with total lengths 50 cm to 70 cm and total lengths

greater than 70 cm.

The month to month variation of the male gonad index was obtained

for each fishing ground and for each body length group (Figure 7). There

Jan. Mar. May July Sept. Hoy.

Fig. 7. Monthly changes in mean gonad index for male hy region rupper;

and by total-length group (lower).

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Author: Series:

Posted: Mon Dec 1, 1997 2:19 PM EST Msg: XTOV-1275-0400 MHS Msg: 0752051101121997/A37441/PBSFOG/11BCOAC53200 From: library@am.pbs.dfo.ca TO: illoofi Subject: ILL request S 556 Gillespie recefeve-

DEc 1 2 )99L

DEC 1 8 19 111451

Studies on the fisheries biology of common octopus of the northwest coast of Africa

Afurika hokusei gansuilki ni okeru medako no gyogyo seibutsugakuteki kenkyu

Hatanaka, H. Canadian translation of fisheries and aquatic sciences;

4741

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• 36

•

was no definite seasonal variation. Males in the important body length

group 50 to 90 cm show high values of the index from April to May on all

fishing grounds and then decline for a while, but there is a tendency

towards a second peak in August/September. When all regions are combined

• and the body length groups are kept separate, the group less than 50 cm

and the 50 cm to 90 cm group show large values in two seasons, April - May

and September - October. The main constituent of the over 90 cm group,

which the fishermen .call "mizudako" is flabby, slow-moving large males,

and their index remains low at all seasons.

Next, the month to month variations of the frequency distribution

of the gonad index in the two female groups was investigated(Figure 8).

Firstly, comparison of the value of the gonad index with visual

assessment of maturity showed that all females with index 100 or more were

judged to be mature, and this was used as a tentative criterion in

investigating the seasonal variations.

The frequency distribution of the gonad index showed definite

variations which were particularly noticeable on the Cape Blanco and

Nouakchott grounds. In the group with total length'over 70 cm at Cape

Blanco, most of the specimens showed high values of the index in May, June

and September, and some specimens in the 50 cm to 70 cm group were found

to have high values of the index in September. At Nouakchott both body

length groups showed similar tendencies, specimens with high indices being

Literally "water octopus", but this name is usually applied to the giant Pacific octopus Polypus (Octopus) dofleini) (WUEHLER) the female of which is often known in Hokkaido by the same name "madako"

as is here used for the common octopus. Translator.

ifee. R•gion Conbined

50 • 70 or Ove . 70 cn

)(4, • • reb. ;

7e

4P1

•

7n, F

1 1 1 1 29 _

70

• • Villa Cisneros Cap Blanc

SO - 70 Cr Ove 70 cn SO - 70 cm (non 70 tr 7 ,r ■ - 1r) ■.a. Il.... 70 ry.

I 70 Jan., 41,

°1 r 1

1 Jan., 7 .!.. 1

201

.

1 I-1 6 En 7 1 I 72 li tl

r 21

-1

. F1 I . 1

I. ri 11 .26 • [1 18 Liii.„ Z1 J Li___.

Mar., Apr.

J . L._ 70„ zoi 7o, n

:n

H.,,,, Mai 20, p,

j May ! i i ! L : , i i, i i Ray

9 IiimaPO .0

] i 1-1 -alaigLia. r 6, • - . 1 1 71 i n—....... i

J –, j — .1 E z . 201 20, 4011 r , fr), noi

..: nj I [1 235 illeme12.4

20 i 1 1 252 June 1 ii

i ii

: .1.11 June Jonc I I_ 153

June

n . n -,-- - ›;

1 El 8 " . LIAMI--ea-

201 •E

- July " 1191

2c-1 1 .:_l y !

no 1

cz, , i.:-. 40

20- . • i July 1 1 1 1 July

F, ',

. 1 !

'...1; 1 re,

1 5 5 I ri 275 201 L

1 T

1 W.... _164 jaini_........ 102 . -

,, ..._ ', 11 , 302.

u ›, 20- ri 4u, F ;`) .

• A.. F. 211. A ug.

Cf)• P r i i ;

0 .! r • 20,1 I I z cr

12 23 1 ...,. 123 [ L i

1,, ' ri 11 rk- 11 153 161

t --. - .... . _ . .. 1.5. 70. 4o,

10I 6

1„.,:-7 Sept.

4

1 I 1 Scut. .

5 nIm. I 1 Sept. 1

r-, Iti [J. 47 ..

,

201,

I _ 1..1 I'S c

Ji 1 .1J1111., nn= : 136

2^ -1. , 17

• 1...17.1_...____--- . ---... -

Nov. .

'0 4u- c,:' 4n, i. . 1 [ : i r I! 1 I 1 u..:

1 J . I ' 2

Oct. 7

8 70-11 !

53

..

1.., -. 0. I ' 114

Oct. ri i 1

72

H.._-..__ .. _ j ii.____ 75 j

-- - . -i .. . ... .1•111... .— „,.. .,,,., 1. -.1 L? - (-1 (le 7 . i.

401 ri :.:, , 1-1 nov.. Dec. ;

1 NOV., ....eC . ri ,, ;

35 I 10 201 I!

I I 50 I ., ' 201[1

1 91 1 ›

52

Li_ _ ...... , _ ...• , :1-.. . , ____- . . 200 400 200 400 7110 400 700 Jot , • 20,, JO, ZOO at'. TOO 4 ( 9 200 4...,

Gonad Index

Fig. R. Moinhly change in gonad illtiCN for female hy length group. Numerals in the figure indicate the number of specimens and

black columns show the matured specimens.

Nouakchott

207

38

found in May and June. There were few specimens in September and October

but the same behaviour was found. At Villa Cisnero the variation was

not so clear, but a high proportion of specimens was found to have high

indices in September.

The seasonal variations of this gonad index agree well with the

results of maturity assessment already described. These two facts show

that there are two spawning seasons. Because there were few specimens

from Villa Cisneros in May and June it is difficult to decide whether

there is then a spawning season, but the male gonad index is at a high

level so it is believed that there are two spawning seasons at this ground

also.

Section 3.

The collection of egg masses

27 egg masses were collected during investigation made on board

commercial fishing vessels (Table 5). These had been laid in the discarded

human artifacts such as rubber boots and oil cans or the large shells which

were mixed up in the catch, and in many cases the parent octopus was taken

at the same time. Various stages were found among them, from that in

which the parent octopus still had good ripe eggs remaining in the ovary

and was considered to be spawning, to that in which the eggs were hatching

at the end of the period of nurture.

The locations of collection of these egg masses reached from the

waters off Cape Garnet to those off Nouakchott. Eleven were taken off

•

Locality Depth Weight of

(N, (m) cluster (g)

Nesting female Material TI, BW GW Remark of nest mm ( ) ig Date

Table 5. Octopus egg clusters observed by scientists on board commercial trawlers.

39

21 Sept. 1972 2015 1735' 74 -- Bucket

8 Oct. 1972 2245' 16'55' 59 --

13 Oct. 1972 18 . 25' 1625' 50 81 Rubber boot 709 • 21

21 Oct. 1972 • 20 36' 17 - 12' 35 380 Rubber boot 652 36

25 Oct. 1972 20 - 05' 17 - 11' 20 416 Shell of snail 651 - - 4

26 Oct. 1972 20 - 11' 17 - 17' 29 S 121 Can of oil 913 2-1 Hatching

28 Oct. 1972 2024'. )7'02' 15 282 Shell of snail 639 3 Hatching

9 Nov. 1972 20 05' 17 09' 21 _

4 Nov. 1972 20108' 1708' 20 181 Shell of snail 589 2 Hatching

6 Nov. 1972 20'19' 1703' 15 162 Shell of snail 615 2 Hatching

6 Nov. 1972 20 19• 17 . 03' 15 101 Shell of nail 522 Hatching

6 NOV. 1972 20 . 05' . 17'10' 22 -- Can of oil

8 NOV. 1972 17 59' 1609' 19 684 Can of oil 855 1. 2

26 Oct. 1974 20 40' 1726' 69 -- Can of oil

10 Mar. 1975 20 14' 17 31' 52 280 Can of oil

26 June 1975 18 05' 16'25. 60 186 Can of oil 9.15 352 Not complete the spawning

11 July 1975 25 04' 15 3.1' 73 370 Tin can No nesting fetnah.

13 Sept. 1975 23 45 . 16'21' 41 562 Can of oil - No nesting fem;de

25 Sept. 1975 23 45' 1615' 39

2 Oct. 1975 20 42' 17 - 23' 57 404 Tin can 770 1546 11'

5 Oct. 1975 22 16' 1703' 61 190 Tin can - No nesting female

7 Oct. 1975 2217' 17'02' 62

12 Oct. 1975 2456' 1524' 39 157 Rubber mat 833 2393 282

12 Oct. 1975 24'56' 1524' 39 1012 Kettle 1145 7965 545 125 g of octopus eggs in stomach

26 Oct. 1975 25'37' 1512' 94

26 Oct. 1975 25'31' 1515' 92 — Can of oil

7 Nov. 1975 Off Cap Blanc — Can of oil

Tl., Total length; 13W, Body weight; GW, Gonad weight

•

0 Nouakchott

Villa . Cisneros

4

s. ••

F " 5 .s

Fig. 9. Distribution of octopus egg clusters observed by scienties on board commercial trawlers.

40

•

Villa Cisneros, thirteen off Cape

Blanco and three off Nouakchott (Figure

9). The depths of collection were

between 15 and 94 m and no special

trends in depth or fishing ground

were found. Of the 27 taken, 23 were

obtained in September to November.

The locations and depths of

collection of these egg masses are in

good qualitative agreement with the

distribution of octopus catches

investigated in Chapter 1 (Figure 2).

The spawning grounds of octopus off

the coast of the former Spanish Sahara

and of Mauritania have no special

areas or depth zones, and it is very

likely that each individual octopus

spawns in the area in which it lives.

•

Section 4

Discussion and consideration

The seasonal variations of the female gonads clearly establish

the existence of two spawning seasons. However the numbers of specimens

in each location and month are insufficient and it is not possible to

investigate the variation from year to year or to specify by any means

•

41

an exact period as the spawning season. In addition, females which have

entered nest holes to spawn are difficult to bring out with fishing gear, p38

and this is thought to cause some displacement in time between the prime

season of maturity as estimated from the catch and the actual spawning season.

When treating the relation between the spawning season and the

fishing season, Mangold-Wirz (1963) and Hatanaka (1970) pointed out that

nesting and death following copulation would result in reducing the catch

and the CPUE. As already found (Chapter 1, section 2), the catch and the

CPUE at Cape Blanco in the average year are noticeably lower in May - June

and in September - October, and these periods are taken to coincide with

the prime spawning seasons.

According to observations in the Mediterr -anean (Mangold-Wirz 1963)

it is reported that the spawning places for octopus are close to the coast

or in shallow places along the coast. The extent of the limits set by

these coastal shallow places is by no means clear, but consideration of

the circumstances in which octopus egg masses were collected suggests

that no such tendencies occur in the coastal region of North West Africa.

We will return to this problem in Chapter 4 on octopus migration.

The egg mass collection agrees well geographically with the catch

distribution, but the number collected in the spring spawning season is

noticeably smaller than that in the autumn, and further investigation

is required.

•

•

•

42

Chapter 3

Age and Growth

Clark (1965) provides an example of a study of the age and

growth of cephalopods in which the assessment of age is based on the ring

patterns observed on the beaks. Many such studies are based on the

seasonal variation of the composition of the frequency distributions of

body length and body weight (Araya, 1967; Squires, 1967; Summer, 1971;

Holme, 1974; Ishii, 1977). This is because in many cephalopods hard

tissue is scarce as an age characteristic and because the life span is

relatively short.

In past reports of the growth of the common octopus in which the

assessment of age and growth is based on the seasOnal variations of the

body length and body weight compositions, Mangold-Wirz (1963) finds a life

span of two years and Tanaka (1979) a life span of about 1.5 years.

In this chapter the age and growth of the octopus on the North .

West coast of Africa are assessed on the basis of the seasonal variation

of body length, and the octopus life cycle is discussed in relation to

the spawning seasons described in the previous chapter.

Section 1

Materials and methods

The body length composition of the catch is divided into a number

of body length groups (called, in the following, the elemental length

groups) by means of fitting normal distributions, and the age and growth

of the octopus is assessed by following the seasonal changes of the mean

length in these length groups.

•

43

The length composition in the catch is estimated indirectly

from the composition n large or small size categories and the length

composition appropriate to each size category. The following materials

were used in this estimation.

(1) Catch tray composition

Operational logs were used from six trawlers, all belonging to

the same fishing company, for about three years from late 1968 to early

1972. The number of fish trays of each octopus size category were totaled

according to fishing ground and quarter year (Table 6). Since differences

depending on depth were found off Cape Blanco, the size category composition

was taken for depths of 40 m or less and for 50 m or more. The composition

between 40 m and 50 m had many of both medium sizes, and they were not

used here. Off Villa Cisneros most of the catch was from 30 m to 60 m and

since no discrimination depending on depth zone was found separate depths

were not used. Materials for the Nouakchott ground throughout the year p40

were not obtained and are not treated here.

Size Size Size Size category (Number per tray) category (Number per tray

LLL 2 - 5 SS 16 - 20

LL 4 - 5 SSS 21 - 30

L 6 - 7 DS more than 31

M 8 - 10 MIX not used

S 11 - 15

Year fvlont h 1..1.1. 1.1. I. AI S SS SSS US Total Dept h (ni)

Table 6. Quarterly compositions of octopus catch by size category in number of fish-trays.

The compositions are shown by depth zone for the Cap Blanc Region.

- -

Villa Cisneros Region

Size Category Year Month

44

LLL LL L M S SS SSS DS Total

199 377 404 867 877 469 427 289 : 3909

663 908 975 1258 1924 1575 1014 409 : 8726 i

240 581 467 538 463 105 11 3 : 2408

483 952 1729 1430 770 666 642 574 ' 7246

16 205 575 1758 2786 1295 649 792 8076

185 466 898 1645 1591 680 248 107 i 5820

30 150 241 300 187 63 39 39 1012

2 18 27 56 58 40 99 31 261

7 241 993 1591 1657 688 448 283 5908

-- 18 43 21 36 18 3 1 143

148 637 412 161 63 14 17 96 1478

34 116 279 586 501 154 92 111 1873

48 393 1700 2578 2177 1969 2091 1332 12288

18 141 270 398 577 322 120 39 1885 ..

1968 I Oct.-Dec.

1969 ; Jan.-Mar.

Apr.-June

• July-Sept,

Oct.-Dec.

1970 I Jan.-Mar.

Apr.-June

July-Sept.

Oct.-Dec.

1971 Jan.-Mar.

Apr.-June

July-Sept.

Oct .-Dec.

1972 Jan.-Mar.

Cap Blanc Region

1968 Oct.-1)ec. 50-100 -• 2 5 16 13 20 11 67

1969 Jan.-Mar. 10- 40 . 1632 4766 2181 870 148 10 2 2 9611

50-100 1142 5504 3965 22-19 1203 415 184 36 14698

A pr. - J u ne 5( ) -- 100 ' 79 168 141 118 67 30 7 1 (ill

July-Sept. 10- 40 138 1532 2613 6332 11283 7479 2643 467 32187

50-10063 157 151 104 31 9 Ill 29 554

Oct.-Dec. 50-100 ; — 1 15 40 • 148 173 215 160 752

1970 Jan.-Mar. 10- 40 461 6954 10675 5456 831 29 3 2 24411 50-100 319 766 508 315 195 94 • 59 35 2291

Apr.-June 10- 40 200 844 528 80 2- 1654 -

50-100 243 752 405 147 46 93 18 11 1645

. July-Sept. 10- 40 • 37 549 1637 4958 9282 6566 3196 2133 28358 50-100 : 57 239 145 54 9 9 3 1 510

Oct.-1)ec. 10- 40 9 420 1078 2032 2668 987 278 29 7501 50-100 • — 12 25 40 53 - 21 16 16 183

1971 Jan.-Mar. 10- 40 867 4485 3313 1829 589 Ill 42 77 11313 50-100 376 843 561 313 179 57 90 17 2366

Apr.-June 10- 40 479 349 154 324 1332 2085 2202 1192 8117 50-100 42 63 48 22 4 1 180

July-Sept. 10- 40 954 5917 12023 21878 19465 8062 4826 3595 76720 50-1(1(1 50 235 218 203 148 85 70 75 1081

Oct.- 1)ec. 10- 40 11 272 337 570 1749 3189 4215 3266 136119 507 100 - 12 46 45 33 76 147 93 452

1972 Jan.-Mar. 10-40 9 317 378 328 142 25 13 1212

45

The octopus classification used in this report is divided into

9 size categories. One of these "MIX" is that in which the number per

tray is not limited, and it is excluded from the data. The "tray" is a

metal tray used when the catch is frozen, and it is common in commercial

vessels to express the catch as the number of trays.

(2) The composition of body weights by size categories

The gutted weight composition of each tray was measured during

two periods of investigation on board commercial fishing vessels, in

January to February and in June 1975.

Individuals collected into the classes "extremely large" (LLL),

"very large" (LL) andrextremely small" (DS) gave difficulty and were

excluded, but in each size category at least 26 trays and at least 250

individuals were measured, the whole investigation covering 275 trays and

3766 individuals (Appendix A).

The differences between the two periods of investigation, summer

and winter, were studied for each size category (Table 7). There were

large differences in the body weights between the two periods, except

for L, LL and LLL categories. These differences show that the standards

described are selected to minimize the differences between individual trays.

The differences between seasons and between regions in the two

investigations were studied by using the differences in the mean body

weight data for each tray (Table 8). This shows significant differences

between the mean values for "DS" but no significant differences between

the other seven size categories.

Period Size category Source of variation

Degrees of Sum of Nlean

freedom squares squares F

-- Table 7. Analysis of variance on size composition among trays by size category

in each sampling season.

46

January and February, 1975

•

June, 1975

Between trays 16 21891744 136823.1 1. 14

1.1.1. . Within trays 31 37113804 1197219 •

Total 47 59005548

•etween trays 19 10430521 548974 1. 9-1'

1.1. : Within trays 70 198(1'2909 282898

. Total 89 311233433

Between trays 93 4074628 177157 1.52

I. , Within trays 134 15640206 116717

Total 157 19714834

I Retween trays 99 2912120 132369 2. 80"

M I Within trays 188 887631-17 47214

1 Total 210 11788517

. Between trays 15 2222-151 - 148163 4. 79'*

S , Within trays 192 5938576 30930

'Total 207 8161027

' Between trays 14 917092 6550)6 3. 8844

SS ! Within trays 252 4254951 16884

Total 266 5172046 •

■ 13etween trays 12 380189 31682 3.42

SSS Within trays 332 307447)) • 9260 :

Total 344 3454659 ,

Between trays 9 2901396 32266. 3. 07* 4

1/S Within trays 4(13 4860919 10.199

Total 472 5151315

Between trays 8 592398 74019 0. 07

1.1.1- Within trays 18 18244019 1013558

Total 26 18836417

Between trays 17 1 2(05250 759132 I. 60

1..1. Within trays 64 3u458205 475909

Total 81 43363455

Between trays 12 18-15421 153785 1.112

1. Within trays 76 1 1443018 15(1566

Total 88 13288439

Between trays 25 6152757 • 246110) 1. 72 .i .•

M Within trays 211 1 1008756 52174

Total 236 17161513

Between trays 32 9803862 306370 15. 76"

s : Within trays 415 8067416 19439

Total 447 17871278

Between trays 18 1733413 96300 8.

SS Within trays 342 40183346 11939

Total 36(1 5816759

Between trays 1 9 748397 62366 6. 75 4.,

SSS Within trays 336 3106420 9245

Total 348 385-1817

' Bet ween t ra•ys 5 7554 115 151081 1.1. 704*

DS Within trays 367 3772830 10280

. Total 372 4528235

* Significant at 5% level ** Significant at 1% level

•

47

Table 8. Results of F-tests on variances and t -tests on means between

two samples obtained in different seasons.

Size Sampling Degrees of . : t Category Period Freedom

Mean Variance 1

' Jan.-Feb. 16 5 1 189 632217

L1.1. June 8 4693 24632 25. 6'4 ' 1. 46

Total 24 4952 429689

. Jan.-Feb. 19 3185 113003

Ll. June 17 323.1 173078 1.53 0. 40

Total 36 3208 141371

Jan.-Feb. 23 2124 27556

L June 1 9 2133 24681 1. 11 O. 16

Total 35 2127 26570 _ .

1 Jan.-Feb. 22 1542 151157

M June 25 . 1597 28105 • • 1. 86 1. 29

Total 47 1571 21997

Jan.-Feb. 15 1097 .- 12119

S June 32 1080 2.1131 . 1.99 ((.39

Total 47 1085 90297

Jan.-Feb. 14 780 3776

,

SS June 18 751 5.105 1.43 1. 22

Total 32 76 I 46(12 __.. .

Jan.-Feb. 12 523 11711 .

SSS June 12 529 2368 2.02 0. 36

Total 24 526 1769

Jan.-Feb. 9 303 712

DS June 5 244 2025 2.84 3. 39 --

Total 14 280 1181 •

** Significant at 1% level

•

•

48

Taking all these facts into consideration, it was decided that

even though there were in some cases differences between trays there

were no changes with region or season of the body weight composition of

the various size categories. It was therefore decided to use the data

from the two investigations as a single unit. Only "DS" differs from

the other trays in showing differences in size, and since there is a

strong possibility that there will be seasonal variations it was treated

separately in the derivation of the growth equations.

(3) The relation between length and weight

In order to transform the gutted weight composition of the trays

into the length composition, equations were derived from the biologically

measured data for the relations between total lenath and body weight and

between body weight and gutted weight.

Logarithmic plots of the body weight and the total length of

each individual show linear relations in each fishing region (Figure 10).

Exponential formulae were derived from these linear relations (Table 9).

A linear relation between body weight and gutted weight was

also found (Figure 11) and a linear formula was derived (Table 9).

•

Table 9. Relationship between total length ,.cin and body weight • g

for the sexes combined.

Body Weight (W . Gutted Weight ' W1,

Number of 1...,•a.I.V. Number of W (YW1, I. all W i ,' Region

Specimens a b Specimens fr aa•

Villa Cisneros 402 6.128 0.3418 404 1.115 6. ;kit)

Cap. Blanc 545 6,487 0.3293 9 34 1.118 6.7m

Nouakchot t 147 7.154 0.3189 109 1. I 1 1 7.399

5

.0

o

3

• ,; • •

' '

•sOr. 5;ci

Body Weight jn gram

• •

•

49

Fig. 10. Relationship between total length and body weight in each fishing region.

Body Weight in kg.

Fi g. 11. Relationship between body weight and gutted weight. Regioits are combined,

•

50

Section 2

Estimation of body length composition in the catch

and search for elemental length groups

2.1 The body length composition by size catagory and estimation

of the body length composition in the catch

The distribution of the body weight composition of each size

category was studied by plotting on normal probability paper. DS was found

to have a normal probability distribution, but the other seven size categories

had logarithmic probability distributions (Figure 12).

Next, the composition of each size category- calculated from the

mean value and the standard deviation was transformed into the total length

composition by means of the equation connecting the gutted weight and the

length. The upper and lower limits corresponding to steps of 2 cm in the

length were found, and the fraction of each in the body weight composition

was transformed by computation into the composition of each tray and then

into the length composition of each size category (Table 10).

The total length composition of the catch was obtained by multiplying

the length composition of each size category by the number of individuals

in each tray and adding the results together (Appendix 2).

2.2 Search for elemental body length groups

The following procedure was used to analyse the total length

composition of the catch into elemental body length groups.

(1) Several trial body length compositions werechosen within a single

body length group and their mean values and standard deviations were

l e

• 011

40,

■ -g

Il 1,91

1: 21,

w ■ • ■

I e eel

L

cr

ell .1

eI

T: 34

7.

U - 7,

• 51

.1!

}I' 4o0

v

nw:

Can 1,011 1400 ('OO

T:

t Se, e 'ee

1 .I

-a» 111 30e 400 0000 611)111 70t1t) /-0(1 , 90.1'1

* Gutted Weight in gram

Fig. 12. Size compositions in gutted weight by size category. • N and T indicate number of specimens and number of fish-trays measured, respectively.

52

•

Length Class

Table 10. Length compositions fitted to normal distribution curve by

Villa Cisneros Reg ion

Size Category Size Category Lengt h Class

Ll..1, LI, I. S SS SSS DS

64- - - 2U- .19

6- . 05 9- . 23

8- . 14 4- . 37

70- .40 6- .51

2- .02 .82

4- . 06 1.32 30 - 1.12

6- .15 1.66 2- 1.72

8-- .32 1.61 •• - 2.45

80- . 01 . 56 1. :33 6-- 3. 37

9_ . 02 . 84 . 91 8 - -I. 62

4- . 05 1. 00 . 50 40 „

•5.91 -

6- .10 1.03 .23 2- 6.72

8- . 17 . 94 . 09 4- . 23 7. 2-1 ,

¶9 1- .25 .72 . 03 6- . 8(i 6, s;-

- . 35 .47 . 01 8- 9 . 42 5. 00

4- . 01 .45 . 29 50 - ■ • . 05 4. 54 3. 09

6- .02 .51 .16 9 - . 211 •, 5.83 1.51

8- . 03 . 52 . 07 4- . 90 5. 49 .53

1011- . 05 . 49 . 03 6- . 05 2. 1 4 3. 87 . 16

9- . 09 . 44 . 02 8- .16 3.50 2.10

4- .13 . 37 60- .47 4. 14 .92

6- .17 .27 2 1.115 3.46 .32

8- .22 .19 4- 1. 77, 2.26 .12

110- .26 .13 6- 2.35 1.12

2- .29 .08 ' 8- 2.43 .45

4- .29 .05 , 70- ' 2.08 . 14

6- .28 .03 ' . 2- 1.44 .05

8- .25 .02 ' 4- .87

120- .22 .01 . ' 6- : .44 ,

2- .17 .01 .18

4- .13 - 811- .07 ,

6- . . 09 2- . 03

8- .06 4-

Total 2.77 4.53 6.68 9.14 1 13.39 18.47 211.69 52.1-1

•

• 53

•

size category. Frequencies are shown in number per fish-tray.

Cap Blanc Region _

1 Size Category Size Category I

Length Class 1 Lengt h Class

1 LIA. 1.1_ L M S SS SSS DS

64- .1)5 20 . 18

(3- .6 2 99 .......

8- . 46 4- . 36

70- . 03 . 99 6- .54

- .81

2- .08 1.53 8

4- . 22 1. 82 30 - 1. 20

6- .01 .44 L66 9- 1.74

8- .02 .74 1.24 4- 2. 61 _ 6 3.75

80- . 04 1. 01 . 71 - 8 5. 12 9- .1)14 1. 13 .31 ,

4- . 16 1. 05 . 13 40 -• 6..15

6- .25 .81 .05 9 - . 0I 7. 55

8- . 38 . 56 . 02 4 . • 38 7. 30

6 - 1.48 6.43

90- .01 .48 .33 8 3. 58 1. 36

9_ . 02 .55 .16

• - .(I .SR .07 50 : .11 5.b2 2.30

6- .1)6 .53 .03 9 . 60 6,27 . 93

8- . 11 .45 . 01 4 - . 03 1. 77 4. 80 . 311

6- . 12 3.32 2.75

100- . 16 .35 8 . - .40 4. 36 I . 20

2- .21 .26

4- .27 . 17 60- . 99 3: 87 .41

6- . 31 .11 9 .. 1J2 2.53 .11

8- .32 .06 4- 2.48 1.25 .03

6-

2.58 .46

110- .32 .03 8- 2.16 .1:1

9- . 27 .02

4- .23 . 01 70- 1.46 . 04

6- .18 .01 .78

8- .14 4- .36 6- . 14

120- .09 8- . 04

2- . 06

4- .04 80- . 02

6- .02 2-

8- .01 . 4-

Total 2.87 4.53 6.68 9.14 I 13.39 18.47 26.69 52.16

• 54

calculated to obtain an idea of the approximate size of the standard

deviation corresponding to a mean value. This was used as a preliminary

study on which to base the following repetitive calculation. p46

(2) In the first approximation, the most evident body length group

(usually the group containing the most individuals) was chosen from the

shape of the total length composition of the catch considered, and the mean

value and the standard deviation of this group were arbitrarily established.

Ath this stage the mean value was set to coincide with the mode and the

standard deviation was estimated to be that established in the first stage.

The objective was to obtain a correspondence between the number of

individuals in the group and the size and mode of a'body length group.

(3) The three values were substituted in the equa -non of the normal curve

to calculate a theoretical frequency of each segment of the total length anel

tle diffennefrom the actual frequency. The equation is

'ti - 2

9,12 f

in which x = the overall length

m = the mean value

a = the standard deviation

N = the number of individuals p47

(4) The elemental length group so established was excluded from the

total length composition, procedures 2 and 3 were repeated and the other

elemental length groups supposed to be present were separated out.

(5) The residuals left when all these elemental length gro -ups were

subtracted from the total length composition of the catch are then calculated.

The above procedures from (2) onwards were repeated and the approximations

•

55

to the mean values, standard deviations and number of individuals were

improved in order to minimize the residuals. Final values for the mean

values, standard deviations and number of individuals in each elemental

group were obtained.

(6) The relations between the mean values and the standard deviations

of the elemental length groups so obtained were used to supplement the data

obtained in the first procedure and used in establishing the standard

deviations in later analyses of the total length compositions of the catch.

The quarterly catch length compositions were analyzed into

elemental length groups by the above procedure. (Figures 13 and 14,

Appendix 2).

There is an almost linear relation between the mean value and the

standard deviation of these elemental groups (Figure 15). When the mean

body lengths and the standard deviaations of the size categories are

plotted on the same graph, the mean value and the standard deviation of the

DS size category lie above the line extrapolated from the other size

categories, so it is taken to be a simple elemental group. In the other

size categories the standard deviations are about half those of the

elemental groups.

•

I

.1

I :e:

Cl.

_

—f

56

1 1

• e

1 :. ■ rr

---. =',--z- -"-> -, t..

1 1

1 , 15

71-11

7 .—IV

Total Length in cm

Fig. 13. Quarterly size com-

positions in the Villa

Cisneros Region and

the elemental length-

groups est imated

empirically by means

of the Gaussian

Curve. Numerals in

the figures indicate

season of the year

(upper and number

of specimens lower).

I, Jan.-Mar.; II, Apr.-

June; Ill, July-Sept.;

IV, Oct.-Dec.

r•,

„F.:7:i \ . •

] ..

,...,

L . , •1 -... .

..1 .. j,

I .:i ____ „--,...,..__,, . _,..

.. i --\----,..-,

.,, . ,•

. . .

Total Length in on,

Fig. 14. Quarterly size compositions in the Cap Blanc Region

and the elemental length-groups estimated empirically

by means of the Gaussian Curve. The legends are

the same with Fig. 13,. -

,

• \

•

ILL

tà

DS

&ei

• • • • SsS Ss

•

( i. gr:

0 • " •

L L

• I I

t cb °o • I I

• • • SS S \ g a;.

• •

8

6

4

De

via

tio

n

-o 8

V)

6

4

2

•

g

40 60 G 10C+

Mean Total Length in cm

Fig. 15. Relation between mean length and standard deviation for each elemantal length-group estimated by applying the .Gaussian Curve. The mean length and standard deviation for each cor-responding size category are also shown in the figures.

5'7

•

bo

• • 40

80

o

Villa Cisouroç

- 0-

--0

- -@- •

0 (.1 ID

Cap Olatic

@-J () Q () - --- -0 C 0/

-0 -0 0

0 0

• 58

'SéctiOn 3

Estimations Of Agé and Growth

.3.1 Seasonal shifts of the body length modes

• ,In each quarter, three to five groups can be derived by fitting

normal distributions (Figure 16).

.c

w .

60

40

Feb. tidy

Fig. 16. Mean length of each elemental length-group and possible connections

among the means. Real and broken lines indicate the spring and autumn spawners, respectively. •

59

In order to estimate the growth curve from the seasonal shifts

in length composition the information obtained in the previous section

was used and the connections between the groups were investigated. There sp1'1'n3 atlel au tuynv,

are almost- separate spawning seasons in these regions and the (female)

body length group maturing during the spawning season has a mode between

70 to 85 cm (Figure 6 ). These elemental length groups are the starting

points for tracing the groups back into the past and for looking for possiLe...

connections between the groups. At both Villa Cisneros and Cape Blanco

this encelod which does notieave, any of the groups out, is integrated into a

general method (Figure 16).

3.2 Investigation of sex differences in growth -

Among the elemental length groups found in' the preceding section

there is a large size group with total length more than 90 cm which is

often found during the summer and winter fishing seasons. The relation of

this length group to the other groups is unclear. The relation between

the large group and the other groups, and the differences in growth between

males and females are investigated in this section. fi- Dyy, Cape- ISlavico p49

The four sets of datavfor February and March 1970 and for July and

September 1971 were selected. These sets of data contain abundant data on

body length sex ratios and also contain the large body length group. The

sexes were separated by combining the body length sex ratios with the

total length composition (for details see Chapter 5) and the total length

composition by sex was obtained (Figure 17).

In all four sets of data the largest body length group extracted

from the total length composition of the females was the elemental length

group with a mean value of 75 cm to 80 cm. However, in the male length

1101 III-

Ro Il I .

••••

(1,

o.

; lII

`..t• • i I 1 I ‘.2 I

Total Lenqtli in c iot

1. 1 RI-

Ro t July

LC,

60

Fig. 17. Length frequency divided by sex ratio and the elemental length -group by sex obtained from the application of the Gaussian Curve for four cases in the

Cap Blanc Region. Numerals in the figures indicates the mean lengths.

•

61

composition, an additional elemental length group with a mean of 93 cm to

96 cm was found, and this is considered to be the saine as the large group

greater than 90 cm which was found in the previous section and was entirely

male. No sex differences were found in the body length groups with a mean

length of 80 cm found in the two winter sets or in the body length groups

at 44 cm, 61 cm and 71 cm found in the two summer sets. p50

The presence of a group of males of body length greater than 90 cm is

therefore not due to a sex difference in growth, but is clearly due to a

difference in the elemental length groups. On the basis of the observation

(Wells 1962) that most females die after spawning and nurture, it is

thought that the large group greater than 90 cm is -one year older than the

group with mean length close to 80 cm.

3.3 Estimation of the date of hatching

It is known that octopus eggs need a relatively long incubation

period. By observation of thirteen cases in the Mediterranean and on the

coasts of Europe Mangold-Wirz (1963) established a connection with the

water temperature, and reported that the number of days needed at a locality

ranged from 25 at 24oC to 25

oC to 65 days at 12.6

oC to 19.2

oC. Itami (1975)

deduced a total of 700 to 800 degree days (water temperature times number

of days) for octopus on the coast of Japan.

According to Tixerant (1968), the surface temperature of the water

at the fishing ground 7 miles south of Cape Blanco in June is 16oC to 17

oC,

and 18oC to 19

o i C n October. During shipborne investigations the author

found that the bottom temperature at a depth of about 20 m in October was

• 17oC to 19

oC.

•

•

62

On the basis of these observations the water temperature at Cape

Blanco can be taken to be 16.5oC during the spring hatching season and 18

oC

during the autumn hatching season. The number of days in the incubation

period, as estimated from the results of Mangold-Wirz (1963) and of Itami

(1975) will be 45 to 48 in spring and 42 to 44 in autumn. Then, using the

spawning season patterns found in Chapter 2 in which the spawning periods atld assuvning bhat. 4-5 days aie çc,

at Cape Blanco are centred around 1 June and 1 October,'"the dates for

prolific hatching come to 15 July and 15 November.

There is no difference in water temperature between Cape Blanco

and Villa Cisneros (Wooster 1976) so that the incubation periods will be

45 days, and the dates for prolific hatching will be the saine as at Cape Blanco.

3.4 Estimation of age

According to the results of rearing by Itami et al (1963) octopus

fry reach a body length of 10 to 19 cm in Japanese waters three months after

hatching. If this initial growth is also appropriate to octopus in the

North West African coastal region, the smallest elemental length group around

40 cm found from the body length composition of the catch will be the group

hatched at the previously estimated hatching dates four to six months

previously. That is, the fry hatched in July from the spring spawning

season grow to about 40 cm in length in about four months and are first

caught in October to December. They spawn in spring two years after hatching

when they have about reached 80 cm. However the fry hatched in November

from the autumn spawning grow to about 40 cm length in six months and are

first caught in April to June. They are supposed to spawn in autumn two

years later after reaching about 75 cm. Thus the large (male) group of

mean length 90 cm is about 3 years old.

•

63

It is determined from these calculations that there are two age

groups and age classes among the (normally two) elemental body length groups

found in the previous section, and these can be arranged as age groups

(Table 11).

3.5 Estimation of the growth equations

The Bertalanffy growth equations were applied to these results

(Table 11).

It was pointed out in the previous section that one of the body

weight size categories used in estimation of the length composition of the

catch, the DS size category, was by no means stable either seasonally or

geographically. The small size group in the catch is selected by the net

mesh size and often the extreme small end of the octopus catch is not

used commercially. Only the data for ages over 9 months were therefore

used in deriving the growth equations.

There are differences in the amount of data for individual monthly

ages and mahy values are missing. Consequently, Walford's general "difference

chart" method was not used to find the growth coefficient and the maximum

body length, but the individual values of the mean total lengths of the

elemental length groups and their ages in months were used and the

coefficients were determined directly by means of least squares

(Tomlinson and Abramson, 1961).

The following growth equations were obtained (Figure 18)

Villa Cisneros region

Spring spawners Lt = 123.6 (1 - e

-0.50(t + 0.33) )

L = 121.6 (1 - e-0.48(t + 0.27) ) Autumn spawners

Age in Month Dept h ( m) 4 7 10 13 16 19 22 25 28 31 34 37 • Year class

Year class

1966 1967

1968

1969

1970

Mean

• 64

Table 11. Mean total length in cm at age in month by fishing ground and by spawner.

Spring Spawner in Villa Cisneros Region

Age in Month Year class I- - - -

4 7 10 13 16 19 22 25 28 31 34

1966 - - - - - - - - 98.)) 1967 - - - - 66.6 76.0 80.4 - - - 1968 41.4 52.4 - 64.5 65.0 78.7 80.0 - - - 98.2 1969 46.0 53. 0 55. 0 60.7 67. 0 83. 0 83.2 1970 42.8 - 65.5 66.0 78.0 - 1971 42.0 52.0 - - - - -

Mean 43.1 52.5 55.0 63.6 66.2 78.9 81.2 - - 98. 1 - - - - -

Autumn Spavener in Villa Cisneros Region

Age in Month Year class

3 6 9 12 15 18 21 24 27 . 30 33

1966 1967 1968 1969 1970

- - - - -- 79.5

53.0 62.0 67.3 ffl . 0 79. 5 40.0 52.5 65.8 67.0 77.0 79.6

44.3 44.8 54.8 64.8 67.0 78.1 79.7 - 46.0 46.0 54.0 64.0 -

Mean 40. 0 45.2 47.8 53.9 64.2 67. 1 78.7 79.6

Spring Spawner in Cap Blanc Region

10-40 - - 95.3

50-100 - - - - 94.4

10-40 • -- -- 80.4 95.3 96.5

50-100 - 66.2 79.3 95.0 96.2 96.8

10-40 - 61.0 -- 79.7 83.2 95.2

50-100 40.0 - 61.0 64.6 79.4 82.4 82.2 - 94.7 96.6

10-40 - 60.4 64.8 79.8 94.4

50-100 42.0 49.0 54.0 - 65.0 79.5 80.2 - •

10-40 54.7 62.6 65.0 78.2

50-100 42.2 60.0 10-40

50-100 41.2 - - -

41.4 49.0 54.4 61.0 65.1 79.5 81.9 82.2 94.9 96.-) 96.8

Autumn Spavener in Cap Blanc Region

1966

1967

1968

1969

1970

1971

Mean

Depth Age in Month (m ) • 3 6 9 12 15 18 21 24 27 30 33 36

50-100 ---- --- - - -- . - -- - -- -- 92.0 10-40 . .. • - -- -- - 74.6 50-100 - - 52.2 63.0 77.8 -- 10-4(1 -- - - 73.8 77.6 50-1 00 j - - - 43.8 53.0 63.2 70.5 77.0 91.0 10-40 45.0 53.3 65.2 68.4 75.7 • - 50-100- • --- 51.8 63.8 76.)) 76.0 10-40 42.0 47.0 52.6 65.0 - . . 5(1-100 . - 45.0 52.6 --

, - - 42.0 45.2 52.6 64.0 69.5 75.6 76.9 - --- 91.5

65

7 7

Vi l Id Clynerps

• ..-- -

ing Autumn Spawner Q .- :, Spawner

O'

• 8 0

.0

-

Spaener

Ca n Blanc MB

100 ..1

h()

60.1

o

SprIng Spanner

60-I

41,1 r

Lu 3.0

• Cape Blanco region

Spring spawners -0.70(t + 0.06)

Lt

= 111.7 (1 e )

Autumn spawners Lt = 105.0 (1 e

-0.69(t + 0.05))

where "t" is the age in years and "Lt" is the total length in cm at age t.

A g e in Yea I..

Fig. DI Growth curve of octopus. Dots and circles represent mean length of elemental length-group for spring and autumn spawner, respectively.

•

•

66

Section 4

Discussion and review

The elemental body length groups of octopus in the coastal region

of North-West Africa were investigated by fitting normal distributions to

their total length composition, and the age and growth were estimated from

the seasonal shifts of the mean total lengths.

The principles and procedures used in this report for decomposition

into elemental length groups by fitting normal distributions to a multi-

peak frequency distribution are the same as those described in detail by

Tanaka (1956). With the progress in computers, there is the difference oç erriZr CurVe

that the normal distributions are fitted by direct - computationvinstead of

by parabolic transformation and graphical methods.:

'Although the means and standard deviation and the number of

individuals do not correspond exactly to the actual conditions, it is

theoretically possible to obtain an infinitely close integral. The author

assumed rough limits for the values and the standard deviations to sort out

the elemental groups and compared the standard deviations obtained from the

results with the standard deviations of the individual body length groups

in other independent data. On the supposition that females with ripe

ovaries in the spawning season belong to the same age group, standard

deviations were obtained from the biologically measured data at Cape

Blanco in September and at Nouakchott in May and June. The standard

deviation for mature females with mean lengths of about 73 cm was about

1.4 times that of the elemental body length group of the same length.

One reason for this difference was thought to be that the individuals with

• 67

•

•

ripe ovaries used in the comparison were not neCessary all of the same

age and that there was a mixture containing small numbers of mature individuals' ,

one year and three years old.

Next the octopus life cycle was considered. The results of past

studies of the life cycle of cephalopods have been classified into assumed

cycles of one or two whole years (Squires, 1967; Summer, 1971; Holme, 1974;

Mangold-Wirz, 1963) or fractional year 1% year cycles (Tanaka, 1967;

Mesnil, 1976). In the latter case, two spawning seasons are needed in each

year in order to provide spawning seasons for each generation. For example,

if there are spring and winter spawning seasons, the group born in the

spring will spawn 1% years later in the autumn of the next year, and this

autumn-born group will spawn in the spring of the year after next.

According to the seasonal variations of the elemental length

groups found by calculation among the octopus of the North West coastal

regions of Africa, the group with ripe ovaries at the spawning season has

a total length of 70 cm to 85 cm, and 14 years before had grown to a total

length of 40 cm. No other calculated value for the rate of growth could

be found, so that it would be very difficult to fit a life cycle of 1% years.

Since most of the females die after spawning and individuals with

length more than 95 cm are rare, the lifetime is taken to be two years.

However this is not true of the males, and there is an elemental group

with mean length around 95 cm which corresponds to about 3 years. In

January to March and in April to June 1969 a few individuals in the total

length composition in the Cape Blanco regions were found to have lengths

about 110 cm, and these can be supposed to be four years old.

Ii

2 ‘.i1

'Year

•

o

68

Males about one year old may have spermatophores but further

research is needed to find out whether they participate in later reproduction. .,

The present results on the growth of octopus were compared with

past results (Figure 19). The seasonal changes in body length mode found

by the author are similar to those found by Mangold-Wirz (1963) in the

Mediterranean, but differ greatly from those found by Nixon (1969) most of

which come from tank rearing. In the rearing of hatched fry, Itami (1963)

found a period of about 40 days of floating life during which there is

very little growth. This does not conflict with the results found by the

author and by Mangold-Wirz (1963).

Luropean um Pr s

I.' • , 1•• , ■:• ■ •••

1. , ■ •11 1, oeil

7.

Itani et al(10631 Setu Inland Sed

0

/7 Manan1e-Wirz(19u"3)

/ • /2/r1.'

Meditt•- •- anean Sea

1.0

Age in

Fig. 19. Comparison of past knowleclgès for growl h of octopus.

•

69

Chapter 4

Stock identification and migration

This chapter discusses the stocks which are assumed to exist from what

is already known about the fishing grounds, the spawning seasons and growth. The Seasonal

migrations of the most extensive stock, that on the Cape Blanco ground, are

examined in relation to the ecological characteristics of the octopus and p55

its environmental requirements.

Section 1

Materials and methods

The catch size distributions in the Cape Blanco region (Table 12)

obtained from the field logs of commercial fishing.yessels and from the

statistics of the Far Seas Bottom Dragnet fishery w,ere used to analyze

season and growth related migrations.

The octopus CPUE for each month and for each fishing ground was

determined from the fishery statistical tables. This was assumed to be the

reciprocal of the density of the stock, and seasonal shifts from south to

north and from inshore to offshore were investigated.

Further, the number caught per hour for each age group and for

each month and depth zone was determined from the catch size categories,

and used to investigate the growth and and season related migrations between level

The method used to estimate the number in each age group was the same as that described

in the preceding chapter to obtain the elemental length groups, but the

"MIX" size category was included in calculating the number caught per hour.

It often happens that when the catch is sorted by size there is not enough

to fill a tray of each size and the overall length composition of "MIX" is

expected to be similar to the overall length composition of the whole catch.

•

70

Table 12. Monthly effort and size-category composition of octopus catch in the Cap Blanc Reg ion.

Minutes hauled Year l\lonth De P 111

(m.

Size Category (Ilnit: Ntnnber of lish-trays

1..1.1. 1.1_ I. M S SS SSS DS MIX Total

1968 Dec. 111-30 320 30-50 90 9

- 9 -

50-7o 375 9 5 16 13 20 11 25 92

ip,i9 Jan. 1(1-30 630 12 56 61 10 14 1 26 212

Feb. H) 30 15755 1251 3381 1503 657 127 In 9

_ 1 517 7,179 30 50 813 27 103 37 11 1 22 2111 30 70 11110 59 372 280 272 239 97 16 15 223 160",

Mar. 10 30 5665 261 706 262 77 1 200 1310 30 50 5565 118 635 350 90 :i 109 1363 50-70 15765 374 2046 1530 852 440 148 75 10 . 511 6016 70-1(10 16530 709 3086 2155 1125 52.1 170 61 11 574 8115

Apr. 50-70 21(1 5 F)

70-10)) 505 7 23 20 8 2 . 13 73 •

.) 9 2

-• - 1 May 30-50 350 5 6 9

50-70 5720 70 133 115 1(15 61 27 6 1 67 585 70-100 280 2 6 4 4 4 3 1 4 26

June 50-70 550 6 9

_ 1 • - 13 99 __,

JU1Y 30-50 410 4 1 4 9

Aug. 10-30 19125 20 73 602 2210 4655 3886 1770 241 630 14117 30-50 405 1 4 19 53 90 59 11 4 17 261

Sept. 10-30 23595 103 1352 1881 3754 5993 3212 745 182 875 181(10 30 50 9 395 14 103 108 285 545 322 111 40 94 1625

Nov. 50-70 1725 9 9 9 5 97

70- 100 6(1 .)

_ .)

Dec. 10-30 35U - - - 30-50 270 .,

_ 9 _ 50 70 15380 1 11 37 111 170 206 17,6 213 913 70-100 430 1 3 4 3 5 ., _. 21

1970 Feb. 10-30 22970 101 3326 6519 3467 555 10 278 14256 30-5 11 127 90 302 1167 1263 601 188 27 4 5 166 3724 50-7 11 4070 23 113 83 82 64 33 33 13 63 507 70-100 180 1 1 2 4 2 1 3 14

O

• Table 12. Continued.

Depth ' Minutes Year Month (m) , hauled Size Category (Unit: Number of fish-trays)

LLL LL L M S SS SSS DS MIX Total

71

O

•

1970 Mar. 10-30 17055 81 2597 3055 1476 114 5 -- - 110 7438 30-50 3945 184 365 267 158 88 6 8 2 35 1113 50-70 5640 291 600 382 195 98 43 20 13 26 1668 70.L100 1320 4 52 41 34 31 17 6 9 5 199

Apr. 10-30 660 20 33 15 5 2 • - - 3 78 30-50 240 6 10 6 2 -- — — - 2 26 50-70 3960 193 545 271 74 28 20 16 II 31 1189 70-100 120 5 13 7 5 2 1 1 34

May 10-30 13055 174 798 502 73 -- — - - 266 1813 30-50 ' 300 — 5 4 1 -- -- - • - 2 12 50-70 1975 6 66 4-1 12 5 1 1 - - 16 151 70-100 530 4 14 9 5 5 1 1 7 46

- June 10-30 ' 310 -- -- - - -

-- - --

- 3 3 30-50 , 1505 6 5 5 1 — — - - -- 5 99

50-70 • 14615 35 114 74 51 6 -- -- -- 124 404 70-100 135 -- — --- — --- - -- -- -- --

July 10-30 13250 23 26 97 497 1150 1163 840 555 147 4598 30-50 ' 740 1 2 — — 1 1 1 4 2 12

Aug. 10-30 51895 . 6 199 1024 3465 6548 4352 1996 1280 827 19697 30-50 ' 2465 — 11 22 79 86 39 26 30 34 327

Sept. 10-30 ! 15820 •, 7 238 397 779 1342 965 321 154 232 4435 30-50 : 1990 , 1 75 97 138 155 47 21 19 26 579

Oct. 10-30 ' 5790 : 9 26 16 49 379. 344 101 7 139 1073 30-50 , 10335 . 4 132 202 242 315 168 85 92 226 1466 50-70 1080 — 12 25 40 53 21 15, 16 13 196

Nov. 10-30 . 11215 — 8 45 469 1297 477 111 "13 141 2561 30-50 ' 1000 • — 1 1 4 5 7 6 — 25 49

Dec. 10-30 ; 13715 ' — 363 986 1473 912 121 28 I 110 3994 1971 Jan. 10-30 ' 22285 135 1493 1342 831 271 57 20 22 172 43-13

30-50 50 — - — - -- - • - -- - - - • 50.70 2285 -- 4 4 70-100 1080 , 1 2 3

Feb. 10-30 18055 562 2100 1139 524 174 24 6 15 138 4682 30-50 9785 119 332 238 172 94 22 5 2 47 1031 50-70 2675 . 32 96 61 44 52 15 6 6 20 332

Mar. 10-30 8705 137 513 393 186 75 23 13 39 77 1456 30-50 3930 103 169 117 52 19 1 1 1 :35 49s 50-70 10300 314 688 473 257 121 39 13 11 108. 2024 70-100 510 . 30 59 26 12 6 3 1 9 146

, Apr. 10-30 • 950 65 48 19 8 3 1 - 13 157

30-50 4215 368 221 97 46 25 3 - - 4-1 80-1 57-70 1580 42 63 48 22 4 1 • 18 198

May 10-30 1085 6 11 7 • 12 28 51 78 79 5 277

June 10-30 15265 120 133 58 267 1279 2030 2124 1113 212 7336 July 10-30 60330 ' 145 706 2675 6949 8480 4394 2647 1588 666 2825 0

30-50 170 , — 3 6 14 9 2 — - 34 • Aug. 10-30 82760 ! 95 1576 5478 10463 8270 2633 1249 831 912 31507

30-50 27440 . 471 1568 1472 1550 1386 983 991 657 616 9694

Sept. 10-30 15610 1() 490 1145 1372 645 201 253 551 138 4805 30-50 55680 , 822 4408 3414 3899 2376 688 510 572 1172 17861

Oct. 10-30 17820 3 122 121 127 306 740 1559 1826 75 4879 30-50 360 — — - 2 1 2 3 4 4 16

' Nov. 10-30 19025 : 8 131 110 238 1341 2375 2538 1320 2 3 1 8302 30-50 4830 — 34 183 278 131 115 204 182 11 1138 50-70 2770 — 12 46 45 33 76 147 93 6 458 ...

1972 Jan. 10-30 3460 9 264 322 238 127 24 13 --- 36 1083 i

•

•

72

See the number in the "MIX" tray of the catch was used to

multiply the mean numbers obtained from the eight size categories with EIX omitted,

and the number obtained was distributed proportionally between the number of

individuals in each of the elemental groups.

The mutual relations between migration and ocean environment were

discussed with regard to such matters as seasonal changes in the environment

as causes of migration.

Section 2 p57

Identification of the stocks

As was found in Chapter 1 from alelysisof the present state of the

octopus fishery, and as can be seen by considering the quantity caught in

each region off the coasts of the former Spanish Sahara and off Mauritania,

there are three quite separate octopus fishing grolinds. These are the Villa

Cisneros -region stretching from 22oN to 26

oN, the Cape Blanco region centred

about 20o

N to 21°N, and the small Nouakchott region at 17oN to 18°N. The

variations in the CPUE in each region show that the three regions are

Practically separate (Figure 20), the quantity of octopus caught in the

border regions between them remaining small throughout the year with no

season in which the CPUE is high.

The morphology of the fishing seasons (Figure 3) and the annual

variations of the CPUE (Figure 4) also show evident differences between

the fishing grounds.

Thus consideration only of the actual catching conditions leads to

the belief that there is little interchange between the fishing grounds,

and that the stocks found on each ground are to a great degree local

and independent.

p58

• IF

114

claim • UB

ODOM BOOM. DOOM

DOM Da • • CID

B B •

BIM DODD

BEM In • • •

le • • NI • DB

LE5 25 0 .1

• 1.1 11

• aciry

I . '

el

•CI • • • • CICI• • • • on

RCM MD. ODfl •

. •

CI Atti-1 • • , • *toe • 'be •

20'N .11 • • I I OK*

• 6, 1 • I•

3: •

Bri•n • CO

ulna OD B.

(Jin. ( Jun e ) ly

• • a

• • •

• 1 o

• •

0 O, •11 • •d

HD

• 4 •.

ti ra wpm

BE • ff BIM

• BB BIB o

_.2

BC • I •

1117J Mo,. )

BOB • B BB A; r.)

•

20 . 0

20 -N

NIP si t • 1 • •

• • • • •

• 10

• •

Re Bari ma (Sept )

•

73

• • 25 • ODD

DODD ODD'

• e

117

BBB BB BIZ

3IB CID ddri

CIDCIS CIC111'

CI

••B B•B

MOM BOMB

Clan MOON DOD CICID

DD

rr, riCIDB

L1.L_K•J • 10

• an an

ma DB (July)

• - • 1 kg; DI

•

OS Da

ODD MID DOB

DOD'

8 *'" • ODD.

OD IBC •

BBB u. • P Oct.)

BIBB BB • • I

• , • 1 •

o

111 ,1 BC

BM

(Aug.)

ma

-;

Loi_VJ • O

Do •s DC•1.•

. • 01120 0 P2

D •

Bare

BC • 0 MCI

BOB BB BB

•

0 N

(Dei.

0._L• l • ••• • el, • • I•

0:0-10 kg/hour, 6:10-50, • •100-200, • 200-300, .300-4 00 ,

Fig. 20. Monthly change in catch per unit of effort by block for the vessels of 550-1500 GRT. The averages of tIlree years from 19711 to 1972 are used.

•

74

Two spawning seasons during the year were found by the analysis

in Chapter 2. Discussion in Chapter 3 of the seasonal changes of the

modes of the length composition showed that there are two mutually independent

life cycles, with those spawned in the spring spawning two years later in

the spring, and those spawning in the autumn spawning two years later in

the autumn. These are taken to be independent spawning stocks.

However in the area off Nouakchott there is only a summer fishing

season, and as was found in Chapter 2, there is a very close relation

between the fishing season and the spawning season. It is very probable

that in this area there is, quantitatively, only one spawning stock.

These separate stocks have been deduced from the data which have ssure_t cl

been gathered on body length in the catch. Thev extent of the separation

mbeperhaps be lowered if we knew • the life habits of the

fry and larval periods.

Section 3

Stock migrations

3.1 North-south migration

The north-south migrations of the octopus were investigated by

using the catch statistics of the Far Seas Bottom Dragnet Fishery, which

give the CPUE for the three years from 1970 to 1972 during which operations

extended over a large area (Figure 20), and using also the catch by region

and year shown in Chapter 1 (Figure 2).

The CPUE is low throughout the year in the boundary region between

the Villa Cisneros and the Cape Blanco regions, and in addition it is a

•

75

seasoal region of extremely small catch. There are non_ags in the changes of CPUE

which would demonstrate.migration of the stocks between areas, and there

is no tendency towards a negative correlation. It is therefore considered

that there is no north-south migration between the fishing areas.

The CPUE in the region from 19°00'N to 19°30'N between the Cape

Blanco region and the Nouakchott region is very low and the quantity

caught is extremely small. It is therefore considered that there is no

FUrtherciore, the continental shelf aroun interchange between the two regions. 21 30 1 N and 19 30I3 constituting the boundary between the fishinr grounds is narrow and for topographical reasons, a norbk- Se u Ek ynlvati.n LS lurdty posstel e__

Both the Cape Blanco and the Nouakchott regions are of limited

north-south extent, and it would be difficult to investigate north-south

migration inside the region. However in the Villa Cisneros fishing ground

- which is relatively long from north to south, there are differences in the

geographical distribution between June-January and February to May, but

there are no oscillating changes in the separate CPUEs, and there is no

suggestion of north-south migration inside the fishing ground.

3.2 Deep-shallow migration

The size category composition on the Cape Blanco ground (Table 12)

was arranged according to month, depth zone and elemental age group, and Qç ea4.1 a3c. srcup

converted into the number of octopueCaught per hour (Table 13). This

was plotted by month and depth zone (Figures 21, 22).

The Figure shows that the spring spawner group (Figure 21) is

first caught half a year after hatching and is regularly present directly

after becoming one year old in July - September. At this season the CPUE

for the one-year old group is very high in the depth zones 30 m and less

than 50 m, and the group occurs mostly in shallow places. The increase

76

— -

Table 13. Catch per unit of effort in number ; hour of each age-group by

mont h and by dept h zone in t he Cap Blanc Region. Broken

line indicates the t urn Mg point of t he age.

Spring Spaw ner A ut UM n Spaw per Year Month

Dept h ( Age in Years ( A ge in Years . Ages combined

ui Op 1

I I 2 1 3 0E 1. 2

1968 Dec. i 10-30 -

50-70 34 65 241 344

1969 Jan. 10-30 5 85 36 14 142

Feb. 10-30 71 67 10 15 9 -

30-50 • 30 39 4 75

50-70 31 63 29 81 200

Mar. 10-30 - • 31 39 8 79

30-50 . 41 35 3 79

50-70 8 77 41 3 35 165

70-100 7 97 61 5 31 200

Apr. 70-100 32 14 2 3 5 9

May 30-50 16 4 9 - 99 ,.....

50-70 l0 14 7 15 48

70-100 211 10 6 1 1 6 5e

June 50-70 5 8 13

I July 30-50

A ug. 10-30 422 106 162 703

'

30-50 359 28 149 553

Sept. 10-30 408 30 133 591

30-50 -- 351 70 103 5.10

Nov. 50-70 3 31 •17 ,,,)

Dol. . 50-70 17 18 58 9::

70-1011 5 14 39 fill

1970 Feb. 1C-30 -- 212 38 5 262

30-50 - 69 39 14 116

50-70 25 20 9 23 81

70-100 12 44

Mar. 10-30 - 126 42 170

30-50 4 49 3(1 21 110

50-70 11 47 32 .).i

-- 12o

70-100 24 26 12 37 1 on

Apr. 10-30 --- 14 14 4 3 36

30-50 --- 17 12 3 33

50-70 14 45 34 7 4 8 112

70-100 14 38 30 6 21 109

May 10-30 -- - 27 15 2 44

30-50 10 I 1.1

5 11-70 2 16 8 3 28 • -

70-100 6 12 8 1 ' 13 - 39

Depth (m Year Mont h

Table 13. Continued.

Autumn Spawner f Age in Years Ages combined

0+ 1+ 2-i 3+ 01 1-1- 2 1

1970 June 30-50 -- 3 1 1 4

50-70 — 4 3 - -- 3 - 10

70-100 _ __ .. _

July 10-30 — 223 - - -- 179 40 447

30-50 — — -- --- 20 3 - 25 , Aug. 10-30 -- 213 -- - 97 70 387

30-50 — 51 53 97 133

Sept. 10-30 - - 155 35 49 250

30-50 - 86 - - • 38 6$ 17 210

Oct. 10-30 -- 163 181

30-50 -- 51 40 24 124

50-70 • 7 9 64 28 • 170

Nov. 10-30 -- 169 -- - - — 16 - 195

30-50 -- 7 • --- 42 - - 49

Dec. 10-30 -- 58 • - - - - - -- 96 160

1971 Jan. 10-30 •-• 46 23 -- 9 - - 84 . -

30-50 — -- - -- -.,- — •

7 (1-100 1 - • - -- - 1 -

Feb. 10-30 3 45 33 -- - 1 (1 . ' ... 9:1

30-50 --- 41 9 9 -- 23 1

50-70 9 18 10 • 98 66

Mar. 10-30 16 33 16 11 78

30 50 1 22 12 3 5 44

50-70 4 34 20 4 15 78

70-100 4 39 33 8 - • 17 - 100

Apr. 10-30 -- 13 16 12 4 46

30-50 - - 15 17 13 6 53

50-70 - -• 23 11 4 3 42

May 10-30 Unident i lied Unident i lied -11-1

June 10-30 Unident i lied Unidentified 678

July 10-30 -- 191 130 87 433 --

30-50 — 17 - - - - - 101 120

Aug. 10-30 — 116 - -- 44 103 -- 277

30-50 , — 74 - - - - 121 59 290

Sept. 10-30 -- 35 11 - - 126 85 967

30-50 — 41 --- - - 43 69 31 191

Oct. 10-30 -- 40 -- --- 445. -- 506

30-50 -- 11 — • 58 4 • - 75

Nov. ' 10-30 44 146 — - - - 425 634

30-50 33 23 — --- — 161 47 274

50-70 36 — -- — — 177 19 240

.1972 Jan. 10-30 — 82 30 — , -: — 34 - - 152

77

Spring Spawner (Age in Years

•

78

•

100 r a-

Al » . =

-=

•

100 r May

1--- 0-- —0----sts----=-- 6—

... 0.

1110 .-

200

100

z 400 I- =

300 I-

Moe C

100

'o

400 r

200 -

100 -

X

o,,,

z 511.1.

30u r 0

\ ---0

Ion .

---X

A 100

L Sec.

200

100

IfIt) [ 4 •lan. 0

e \ • -- 0

\

lob [ \ I eb.

0— 'A------,8:---•--<".4—_o___ ..._x

IIIIT

10 5 r Arr.

L ,__.__.-_-_-6- -x

i .-- 1110 r

X — X —• —X —

•,1•1.1

L

e a r

L.

I ,

0.

1011 t. .

L

111 0

..1 ■ •

o 0 0 0

( :1966 PM; — x :loI 0 — — 0 :14t.,

Jul e

Aug

o

1 '0 1• ,.

rn

0 .

100 r Jon.

Ps.

100 r Ma,.

L

x 100 r

100 L._

I 10 1. 50 Il.. itl ■ •

Depth in m Depth in

Fig. 21. Monthly changes in catch (number) per unit of effort for spring spawner by year class and by depth zone in the Cap Blanc Region.

•

(It t.

à °

g. .

....... .

—o o—

x - --x

L._ _ • _ '

30 511 7;; 100

0. 41LI

,1111

e

*Sot.

..1111

Lu

n_ •ggt,

79

0

.21111 0

•it, e

• 0

t r I All i.

X

Depth in m

■-1,t1 .

III•

L.._ ,,,-___,-6---,.-_---t, _ , „ r

I ' L,__,.,_.,_

x ,,. 0 ....._ 0

o_. x.

,.,..1. ------„,

L

e-- g

—

i ■ It [

:111 r t-- e r

g g

li• 311 51 7.1

Depth in ,m

- -x 0-•-O

Fig. 22. Monthly changes in catch (number; per unit of effort for autumn spawner by year class and by depth zone in the Cap Mani- Region.

•

•

•

80

in CPUE then shifts to deeper regions and at about the spawning season

in April (age 1.8 years) the CPUE reaches the same level in all the depth

zones. At the beginning of the spawning season there is no catch, but this

group reappears in each depth zone at the beginning of the next winter (age

2.7 years) and the catch disappears on the arrival of the spawning season

for the second time.

The autumn spawner group (Figure 22) appears in shallow water in

July to November, from 9 months to a year after hatching. The rise in the

CPUE gradually shifts into deep water, and as the winter fishing season,

January to March, begins (age 1.3 years) the CPUE becomes uniform or

slightly higher in the deep places. As the summer fishing season, July and

August, approaches it increases but drops with the spawning season in

September and October, and finally the catch disappears. p61

These changes of CPUE may be supposed to be influenced to some

extent by the octopus distribution. The spring and autumn spawners in the

Cape Blanco local stock differ somewhat in age at appearance and in their

dispersion at their full age, but they appear on the fishing ground at

about one year, shift from the shallows and migrate offshore and disperse

so that in less than two complete years they become evenly distributed in

all depth zones. After this they spawn and apparently do not migrate

again before they die.

Next, the Far Seas Bottom Dragnet Fishery statistics were investigated.

They are inferior to those used in the previous investigation because it is

not possible to sort out the age groups in the stock, but they have the

advantage of representing the activities of many fishing vessels whose

main target was octopus.

•

•

81

The region of greatest catch on the Cape Blanco ground was

divided into three blocks each 30 minutes from east to west between 20°N

and 21% (Figure 23), and the catch perhour in each block was àetermined

(Table 14). The data used were from four classes of fishing vessels,

including the many ice-hold ships based on Nouadhibou, the 300- 500 ton

class ships, the 550 - 1000 ton class ships and the 1000 - 1500 ton class

ships. The variations in their level of catching efficiency are

discussed in Chapter 6, Section 2.

The annual course of the CPUE in each of the blocks, A, B and C,

show differences between the seasons of plenty in 1967 - 1970 and in

1971 - 1975. In 1971 the course is unusual, with a peak of CPUE in June

when, in other years, the fishing is slowiny down.

was excluded and the mean values are plotted in Figure 24 for the years

1967 to 1970 and for 1972 to 1975.

The plots show the following two general distinctive features.

1. At the beginning and middle of the winter fishing season, from

November to March or January to March, the CPUE is always high in the

coastal block.

2. In other seasons there are no significant differences between the During the 4 first years, the values for block A in April and

different blocks. May are different but this variation seems to be due to the fact that the operations were limited in this block at that time.

The first feature is probably influenced by the high density in

shallow waters of the spring spawners, which as shown by the analysis of

year classes (Figure 21) appear in summer in shallow places and remain

till about February of the next year.

The second feature, the absence of difference between the blocks

during the summer, may, it is suggested, be due to a balance between the

p62

Consequently 1971

I •,'

82

•

•

23. Outlitle of topography and breakdown of blocks in

tIII Cap Blanc Region.

'Fable 14. Catch per unit of effort in kg , tour of •Common octopus ip three

blocks of the Cap Blanc Region. lireakdo‘vn of the blocks and its

symbols are shown in Fig. 23.

- • ---

Year Block Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

1967 A 722 734 535 -- 684 285 46 15

B 463 617 411 316 287 225 31 1130 623 39 23 263

C 39(1 543 371 336 301 '233 18 1083 777 10 :12 99

1968 z-1 239 218 -- - 82 - 837

li 380 307 208 224 416 222 526 773 549 637 555 1-19

C 277 234 18-1 20-1 128 68 152 992 597 334 -199 490

1969 A --• 461 148 0 35

B 554 566 373 303 174 84 45 553 648 118 129 144

C 439 5112 426 334 122 69 . 91 521 565 ' 146 112 119

197(1 A — -- 386 77 134 • 39 257 625 94 162 35 2

B 187 456 224 159 147 25 202 337 466 267 224 306

C 113 341 238 141 134 25 106 319 319 216 152 138

1971 A 304 417 271 174 520 451 327 127 180 501 403

B 258 259 208 170 306 420 368 272 209 210 300 337

C 151 241 216 168 150 169 279 256 131 137 163 172

1972 A 384 297 265 155 269 315 82 173 220

li 343 298 267 213 151 205 233 324 206 69 139 2 06

• C 277 296 294 232 196 86 120 470 297 53 140 96

1973 A 292 379 365 278 101 61 157 320 107 158 177 268

li 241 266 273 237 137 66 153 227 124 97 138 215

C 281 224 194 284 168 63 128 242 194 128 106 172

' 1974 A 312 269 238 176 213 75 160 379 159 246 313

li 231 241 195 177 114 72 204 251 183 Ill 181 210

C 178 166 182 221 246 79 78 290 201 160 179 196

1975 A 425 531 406 262 192 141 119 238 136 87 136 177

. B 217 197 214 183 117 113 232 209 131 54 109 139

C 198 156 191 171 156 78 303 244 149 56 30 107

800

1967 - 1970

0 0

600-1

o .c

400-1

200 -

• / 0

• 0

I. f

I .. .... •

• -u

\

Block A

0 — Block A

-'s• O

Block B

x•- 1

r • .

......

Block C

1972 - 1975

/ x ..

■

..•

. X

•._ . •

400

CI. o

200

• 83

• Mar. tidy July

•

Fig. 24. Monthly changes of mean standardized CPUE's in threc blocks in the Cap Blanc Region. Breakdown of blocks and its symbol are show ii in Fig. 23.

•

•

•

84

abundatiLly

small size group which is present in the shallow waters and the large

size group much of which has dispersed into the offshore waters. No data

for water deeper than 50 m were obtained in summer in the yeargroup

analysis, but, apart from the weight, there is a tendency, as shown in

Figure 22, for the autumn spawners which when nearly two years old form

an important component group, to disperse from February onwards into deeper

waters, and this trend is thought to be a distinctive feature of the summer.

Section 4

The relation betWeen - migration and 'ocean environment

4.1 Outline of the ocean environment

The following is an outline of the ocean'environment in this

area as derived from reports by Allain (1976a, 1976b), Fedoseev (1970),

Huyer (1977), Ingham (1970), Jones et al (1970), Mascareno (1970), the

Japanese Fishery Agency (1972) Szekielda (1976) Szekielda et al (1977),

Tixerant (1968), Wooster (1976) and Wozniak (1970).

The North West coast of Africa lies in the North East trade wind

zone and is washed by the southward flowing Canaries current. The ocean

and weather conditions are calm and stable throughout the year.

Largely because of the variation of the NE trade winds the

Canaries current is strong in winter and weak in summer. The equatorial

counter current also develops during the summer and flows north along the

coast as far as Cape Verde, and the influence of this warm current is

felt as far as the Banc D'Arguin. In vertical section, the water is

Central North Atlantic water down to about 500 m, and this water mass

forms most of the Canaries current.

• 85

•

The influence of the Canaries current makes the surface temperature

relatively low, and except in local coastal areas the surface temperature

lies approximately between the limits 17° to 26°C. To the north of Cape

Timiris there is upwelling throughout the year and the vertical gradient

of the water temperature is consequently small, the bottom temperature in

areas less than 100 m deep on the Cape Blanco fishing ground remaining all

year in the range 15° - 19°C.

There is little inflow of fresh water and the salinity maintains

relatively high value, 36 °,/ or more all year. It is lowest in June oo

because of the changes in the activity of the upwelling, and increases

with the onset of winter, but the range of change is small, within the

limits 38.8 o/ to 36.4 / . As with the water temperature north of oo oo

Cape Timiris, there is no vertical salinity gradient.

pec-ause. or the interaction between the Caturin current and the coastal topography,

there are several eddies along the NW coast of Africa. There are notable

eddies near Cape Garnet, near Cape Blanco and north of Cape Verde, the

eddy near Cape Blanco being present throughout the year.

The upwelling current is produced by the action of the ocean

current and the NE trade wind. In this region, the wind blows almost

all the year from the NNE with force 2 to 7 (annual average 4), and the

strength of the upwelling responds to the weather changes. There is up-

welling all year between 20 °N and 25°N, and in the Cape Blanco region it

is most active in June, reducing the water temperature and salinity. Because

of this there is neither thermocline nor halocline.

Because of the upwelling, the nutrient salt concentration in the

region becomes very high, and the basic productivity reaches a chlorophyl

p65

86

concentration of 10.0 mg/litre or more. This is the maximum concentration

found, except in special circumstances such as the red tide.

4.2 The relation between migration and environmental changes

Causes reported to lead to octopus migration include changes of

water temperature and salinity (Rees and Lumby, 1954; Tanaka, 1958;

Une, 1959; Itami, 1975; Mangold-Wirz, 1963; moue, 1969), turbidity

(Cousteau and Diole, 1972) and water currents during the planktonic stage

of the larvae (Rees and Lumby 1954). The seasonal variations of water

temperature and salinity on the Cape Blanco ground will be compared here

with the seasonal variations in the well-known fishing grounds in the Seto

Inland Sea, , in the Northern Mediterranean (Banyuls-Sur-Mer), and in the

Caribbean (Caiman Sea) (Figure 25).

The surface temperature in the Mediterranean varies from 11°C

to 21o i C n the Seto Inland Sea from 8

oC to 26

oC. According to Tanaka (1967)

and Itami (1975) there are seasons outside the optimum temperature

range. In the Caribbean the water temperature enters the optimum range

only during the season of lowest temperature, and according to Voss (1973)

the fishing season for octopus on the coasts of Puerto Rico is from

October to March when the water temperature drops. However on the Cape

Blanco ground the seasonal variations are small and the temperature

remains in the middle of the optimum range throughout the year.

The salinity varies seasonally on the Cape Blanco ground considerably

less than in other regions. Throughout the year it is less than in the

Mediterranean and higher than in the Seto Inland Sea. In the Mediterranean

and at Cape Blanco it is notably higher than the level obtained from

Itami's (1963) ideal ocean water density.

87

(I r

0

- '-,--°-

Te

mp

e ra

ture

!rrejular feedinq /

( Taenia, 190) a .

, f

/ /..,,,,N C,

9..--x-... '''''

,, ---x —„ ,,,

",.,___-•\ ''.:., o 5 x/

x , ...------..

......, .--.--. • --..........-

/0/ 61/ • Cap [Shun

(1 IX era 11 1 , 1 967;------ . ---.... / ' • I . ,I., ' , . . , . • s •

› 0 i.,, c>., ' -... ." ,,,

/ / ,,, • ,..,

J,,,,-. •"/ ,s'd

X

x Irrequiar (recline (nat.],

1 1

Jan. >tar.

IV•tIlt el ■ att.......ollo; t

X..... x .......X —*— X, X "

X-' ,....-x —,

/ X X (I ---x-•e'.. ..ilt ■ Iti•

• X --,‹ _

N

X.,..„

• ----- • .---- • ----- • • ------• • ---- • — • _ ____-,• --

Su Inland ',ra (et imu ■ ramp-

-0 - --° (1t,a..1. I qt, • 0 - 0 0

o

-

(I ■ 1•1111.awa 196.1)

- —

\

1 1 1

. Mar. Ma y Je Iv apt. .

Fig. 25. Seasonal changes of surface water temperature (upper", and salinity ( lower) in

well-known fishing areas of octopus.

io

Lop 1.1am. --- •

(Tueront , 196F■ ;

Ilt

•

88

Thus the seasonal variations of temperature and salinity at

Cape Blanco are less than those at the other regions and since a stable

environment is formed, it is to be supposed that these variations are

less likely than in any other regions to be factors in octopus migration.

Moreover, the temperature in the octopus' habitat lies within the known

optimum range, and regarding salinity, the salinity selectivity of this

species is thought to have a fairly wide range.

Section 5

Discussion and reView

Analysis of the CPUE according to depth on the Cape Blanco

ground shows that the small size group appears finst in the shallow places

and that after gradually dispersing into all depth.,zones it apparently

grows, spawns and dies without migrating.

This immobility after first appearance may be explained as due

to the stability of temperature and salinity throughout the year in this

region so that the causes of migration are extremely feeble when compared

with other regions. However the appearance of the young age group in

shallow water and the migration during the planktohic period from shallow

water to offshore, though now suggested, remain as problems for the

future.- The data are sparse but the following can be proposed as the

principal reason.

On the coastal side of the Cape Blanco region there are the wide

Arguin Bank and Levrier Bay, and on the outside there is the fishing

ground at depths of 15 m to 100 m (Figure 23). The area of the Arguin

Bank is twice that of the fishing ground, and there are reefs and sand

bars less than 10 m deep along its outer edge. It is difficult to operate

there, and no data have been obtained concerning the distribution of

• 89

•

octopus on the bank. However there is a counterclockwise eddy all year

in the Cape Blanco region (Fedoseev, 1970) and on the west side of Levrier

Bay the current flows to the north (Tixeerant, 1968), and the environmental

conditions in these shallow regions are thought to be unsuitable for the

growth of octopus.

Thus a considerable part of the octopus larvae hatched on the

offshore fishing ground will be transported by the eddy or by tidal

currents during their 40 days of floating planktonic existence (Itami 1963)

onto the Arguin Bank or into Levrier Bay. They will very probably there

make the change to bottom dwelling in shallow water. p66

Cousteau and Diolé (1972) have found during diving observations

that octopus will strongly avoid turbidity of the Water. According to

Tixerant (1968) the North wind in the neighbourhobd of Nouadhibou Bay is

strongest in May and June with 18 to 20 days per month when the wind

velocity is 10 m per second. The octopus which have adopted bottom dwelling

in the shallow part of the bay will have increased in quantity by growth,

and are thought to migrate offshore to avoid the turbidity in the seawater

originating from wind-blown sand which filters down to the bottom.

•

• p67

90

•

•

Chapter 5

Other biological information

This chapter discusses the results of investigations of sex

ratio and of the stomach contents of the catch.

Section 1

Materials and methods

The sex ratio was obtained for each month and body length class,

using all the specimens whose body length or body weight was measured.

The number of specimens from Cape Blanco used for this purpose was about

8000 (Table 15). The data from Villa Cisneros and Nouakchott were

unsatisfactory and were omitted. The body lengths-yere determined from the

body weights, using the relationship given in Table, 9.

The data used for analysis of the stomach contents were obtained

from frozen specimens thawed and measured in the laboratory and from

measurements made on commercial fishing vessels directly after capture.

In general, the octopus chews its food small, grinding lAi th itsodontophore

in order to swallow it. Species identification in the stomach contents

was difficult, but in investigations on board ship 'direct comparison with

simultaneously caught benthic animals was possible so that identification

was relatively easy and exact. However, detailed species identification

of the crushed stomach contents was not possible with either of the two

sets of material, and in this report they are handled in five groups,

fish, crustaceans, cephalopodsshellfish and the unknown.

The shipboard materials were used to determine the weight

composition of the observed types of stomach contents. In investigating

• 91

•

the seasonal variations of the dietary condition and the weight of the

stomach contents as seen in the number of empty stomachs and in the food

solids, the total material measured on shipboard and in the laboratory

was used. This was carried throughout the year, using the Cape Blanco

local stock.

Section 2

The sex ratio in the catch

There are seasonal variations in the sex ratio of the catch from

Cape Blanco region (Table 15 and Figure 26). As the body length increases

the sex ratio declines, but it is found throughout:the year to be about

50% up to a body length of 50 cm. At greater lengths it declines gradually

and at lengths greater than 85 cm it falls off very rapidly. Only males

are found in the class longer than 115 cm. This increase in the proportion

of males is attributed to their longer lives as compared with the females.

As to the seasonal variation of the sex ratio, the quantitatively

important classes 50 to 90 cm in total length normally show a slight

preponderance of males but in two seasons, March . and August to

September, the proportion of females becomes large. Directly afterwards

in April to June and in October, the proportion of females rapidly decreases.

Section 3

The stomach contents

3.1 The species composition of the stomach contents

The quantitative proportions in the stomach contents are shown by

fishing region and date of collection in Figure 27. There are some

r-

Oct .

5'14

14,

51

50

50

5(i

Jan.

•

51,1 X

tr)

•E 50 1

504

Feb.

.ct

• I; Ce

50

a.

Apt . -Joule

50

Jul é

Jan.

Frequency

Male Fem.

Feb.

Frequency

Male Fem.

Mar.

Frequency

Male Fein.

Length Class (criu Sex

Rat in Sex Rat io

St.• Rat io

,*

92

lIt 115t e5

Total Length in C Total Length' in cm

Fig. 26. Monthly changes in sex rat in in the Cap Milne Region. Curves a,re tin (.(1 by oyus'.

l'able 15. Sex ratio (percentage of female by mont h and t pia! -lengt h class in the

Cap Blanc Reg oit.

30 8 9 53 94 93 49

3(1 11 11 50 31 2-1 41 1 o

35-- 9t; 25 49 75 71 49

40- 55 45 45 156 96 38 • 1 lou

45- 9 85 78 48 145 142 49 .

- 5 71

50- 83 96 54 169 162 49 5 5 50

55- 59 71 55 195 140 42 11 13 54

60- 62 42 40 162 147 48 14 15 -9 D-

ti5- 75 41 35 149 112 43 99 38 63

70- 70 29 99 128 129 50 35 33 -19

75- 59 90 25 144 93 39 3U 44 59

80- 43 90 32 114 86 43 36 41 53

85- 30 13 30 91 45 33 34 47 58

90- 27 7 21 . 86 26 23 44 13 93

95- 16 5 94 58 10 15 24 7 — 3

100-- 13 4 24 49 3 6 24 3 11

105 10 I 9 32 1 3 33 Il

111)- 8. 0 17 1 6 29 0

115- 5 • - - 0 16 .._ 0 15 -- 0

.,120 . 1 --- 0 • 8 — 0 - - -- --

Total 746 517 41 . 1849 1311 41. 359 265

•

Table 15. Continued.

Male Fem. Rat io Male Fem. Rat io Male Fem. RI l in Male Fem. Rat io

' - 30 1 100 9 3 60 I 100 . 1 7 88

30- 1 100 . 2 3 60 2 0 : 1 2 • . 33 -35- 1 3 75 5 -- 0 1 8 89 : 3 11 79

40 - 1 2 67 4 9 69 9 5 36 16 15 -18 45-- 9 1 33 11 19 (13 7 8 53 16 11 47

50- . 3 4 57 24 16 40 14 21 f1 0 19 95 57 55 • 3 3 50 52 34 -10 19 :1 9 63 28 17 38 60 3 3 50 64 57 47 33 39 51 Ili : 1 ii 69 65- : 4 0 76 58 43 58 44 43 25 31 55 70- 1 1 50 66 49 43 47 60 5 (1 311 62 67 75- 5 1 17 38 28 4 9 43 43 51) 26 411 61 80- 3 4 57 22 24 59 26 19 42 36 31 46 85 12 5 29 19 8 30 15 -1 21 211 8 94

90- 14 3 18 14 1 7 14 0 27 9 7 95- . 13 14 52 6 • 0 1 100 8 1 11

MO 15 0 5 0 6 0 105- - 13 0 3 0 1 0 9 1 33 110-- 13 • 0 1 0 -) - ' 0 115-- 7 0 -- - 1

:120 17 -- 0 - - 1 0

Total 130 46 26 414 309 43 290 285 51.1 292 303 51

Oct. Nov., Dec. Tot al Length Class Frequency Sex Frequency Sex Frequency Sex (cm)

30 1 1 I 50 36 -15 56

30 - 3 4 57 1 4 80 55 19 17

35- 7 11 . 61 9 100 118 131 53

40- 14 17 : 55 11 7 39 266 197 43

45- 23 14 38 11 16 59 302 297 50

50- 95 23 • 48 13 10 43 .17:- .,.).) 162 50

55- 29 15 34 9 11 : 55 405 1311 45

60-- 95 99 47 15 16 52 39-1 177 49

65- 30 20 40 16 14 . 47 455 158 44

70- 33 99 40 15 15 50 425 400 48

75- 25 18 42 ' 14 (i : 30 381 293 43

80- 9- .,/ 8 23 12 6 - 33 319 239 43

85- 22 9 ' 8 ' 18 3- 14 267 135 3-I

90- 14 I 7 6 0 246 53 18

95- 3 9 40 3 0 131 40 9', -a

100- 5 0 6 0 123 10 8

105- 1 -- 0 1 0 96 3 ,

110- _. --- _ - - 70 1 1

115- --- -- --- -- - 44 0 . .

-

120 -- - 27 0 ' -

Total 287 180 39 ! 151 110 42 4518 3326 -12

93

• Apr.-June July Aug. Sept. Length Class Frequency Sex Frequency sex Frequency sex Frequency Sex (cm )

Male Fem. Rat in Male Fem. Rat in Male Fem. Rat io

•

19-21, July Total (88(J speciment) 31, July-2, Aug.

4-30, July Total (1343 toot ) 8-16, June

Contents Weight

per lndi per BW g (g. kg)

N um ber Contents Weight

of Specimens Per Indi Per 8\\ *

g (g-kg

•

•

94

•

• Cap Blom» 20 - 13 - 20 - 2e' N Depth 18 - 23 In

Nouakchott 18 13' - le 4L' N Depth 16

• 1 :Fishy, Cr:Cruslateans Ce:Cephd1ople. Sho 111 11) :uIIfleu,

Fig. 27. Food items in percent of stomach contents. The data were obta ined - in 1975.

Table 16- Stomach contents weight per individual and per unit of body weight by

size class in two samples obtained from the Cap Blanc Reg ion in 1975.

19, • uly - 21, July 31, July - 2, Aug. Total Size

Class Number Contents Weight Nu„ber

of of • g) specimens per lndi per BW . , ( g )

(g/kg) rn peciens

•

0- 2 0. 10 L 00 9 O. 10 1. 00

200- 9 O. 51 1. 70 1 U. 00 0. 00 10 . O. 46 1. 5%

400. 21 2,84 5.68 17 0.76 1.52 3S 1.l1 3. i'' )

600- 39 1.91 2.73 28 2. 18 3.11 67 2,1)2 2. SS

800- 56 3.7(1 4, 11 53 4. 31 4. 79 1119 4. 00 4. 4-1

1000- 52 3,26 2.96 70 3.7l 3.44 1 99 3.5h

1200- 65 2.44 1. 88 77 4. (13 3. 10 142 3. 30 2. 51

1400- 35 6. 07 4 , 05 67 5. 19 3. 46 102 5. 49 3.66

1600- 31 3,96 2.33 57 6.1' 3.61 88 5.39 3.17

1800- 16 4.56 2.40 41 9.26 4.87 57 7.94 4.1S

2000- 12 7.34 3.50 27 8.20 3.90 39 7.93 3.7i

2200- 13 6.88 2.99 22 6.43 2.80 35 6.60 2.8:

2400- 8 11.81 4. 7 9 14 4.45 1.78 22 7.13 2.85

2600- 6 10.75 3. 98 9 2.37 L88 15 5.72 2.12

2800- 2 (1. 00 0. 00 7 5. 11 1. 76 9 3. 98 1. 37

300(1- 5 6.72 2.17 8 9.35 3.1)2 13 8.34 2.69

3200- 3 6.5)) 1. 97 1 27.5)) 8.33 4 11.75 3. 56

3400- 9 3.55 I. 01 1 0.1(0 0. 00 $ 2. 23 0.1;1

3600- - - - . --

3800- 1 0.1)0 0. 00 1 1). 00 0. uh

4000- I 0. 00 0. 00 •- - • 1 0. 00 0. 0.!

Total 379 3.90 2.92 500 5.08 3.40 879 4.57 I 3.91

Indi, individual; BW, Body Weight.

9 5 .

•

•

differences between regions, but the largest group is shellfish at 45%

to 61%, with 19% to 34% fish and 20% of cephalopods and crustaceans combined. ,

Ehrou5hout the, ce,Ei3ueu , 13c ods.

In both regions the spacies composition changes in octopus diet ̀ ."

Sc.l15 occ-up/ an inlportenk positfon, but preference for particular species is weak and it is suggested that

the octopus preys on whatever type of food is at the moment available.

3.2 Seasonal variations of stomach contents weight, and state of diet

In order to determine the relation between body weight and weight

of stomach contents, the weight of stomach contents per individual and

per unit of body weight were obtained from two samples collected in depths

of 18 m to 23 m in July and August in the Cape Blanco region (Table 16).

The Table shows that in both samples the weight of the stomach contents

varies with the size class and an'increase of bodY" weight is accompanied

by an increase in weight of stomach contents. However the weight of

stomach contents per kilogram of body weight is in the range 2 g to 4 g

in almost all classes, and there is no trend with weight category. It

is therefore concluded that the weight of the stomach contents per

kilogram of body weight is an index of the seasonal variation in feeding

activity. The similarity of the trends of the seasonal variations in the

weight of stomach contents per kilogram body weight and in the feeding

condition is shown in Figure 28. There is an increase in the weight of

stomach contents in December to April and in July and August when the

rate of feeding is high, and in the periods when the weight of the stomach

contents diminishes, May - June and September - November, more than 50%

of the stomachs are found to be empty. It is therefore concluded that food

seeking is active in December to April and in July to August, and that it

is at its lowest in May to June and September to November.

p71

97

In the range 50 cm to 90 cm the sex ratio is high in March and

in August to September, and decreases rapidly directly afterwards in April

to June and in October. Octopus in the body length range 50 cm to 90 cm

at these times are for the most part found to be those which are a full

two years old and are spawning. Previous analysis has concluded that the

spawning seasons are in May to June and in September to October, and the

reductions in the proportion of females in the catch correspond with the

spawning seasons when the females are caring for the nests. The increase

in the proportion of females directly before these seasons suggests a

shift in activity in which they come out of the nest holes.

The feeding activity is high in July to August and in December

to April, and is low in September to November and in May to June. The

high periods suggests high activity before spawning, the low activity In these grounds t the active fishini; period was

suggests the spawning periods. December-April and July-September. This correspon to the period of active feeding. The period of

reduced feeding corresponds to the period of reduced fishing. To summarize the above, feeding activity becomes low during the

spawning season because the period outside the nest hole is short and

the activity declines. The season of good fishing is that produced by

food seeking just before spawning, and it is suggested that a reason for

the feeding activity is the accumulation of nutrition before turning to

spawning and nurturing.

p72

• 98

•

Chapter 6

Resource fluctuations and optimum catch

The present conditions of fishery operations and the biological

information obtained in the foregoing chapters are used in this chapter

for preliminary calculations of the fluctuations of the octopus stocks

and of the maximum sustained yield (MSY). The optimum utilization plan

is related to the age at which the stock is first caught and to the

catch mortality coefficient.

As already discussed, there are three separate octopus fishing

grounds off the coastal regions of North West Africa, and they are

believed to consist of highly independent stocks. On the Villa Cisneros —

ground the Japanese fraction of the catch is much less than the Spanish

fraction and there are no detailed statistics of the catch other than the

Japanese statistics. For this reason statistics sufficient for analysis

of the stock fluctuations are not available. The fishing history of the

Nouakchott fishing ground is short and the octopus stock is not large so

conditions are not satisfactory for analysis.

The stock fluctuations analysed in this chapter are therefore

principally those obtained from the statistical data provided by

Japanese fishing vessels about the Cape Blanco stock.

The octopus stock on this ground is estimated to consist of two

practically independent spring and autumn spawning groups. In order to

comprehend the present condition of fishery operations in relation to

each of these groups, estimations were made of the number of individuals

in each spawning group for the years 1967 to 1971, and attempts were made

to estimate the natural mortality coefficient in relation to the number

• :99

caught. Next these parameters were used to estimate the optimum age for

catching to start and the catch mortality coefficient for the MSY from

the quantity which is expected to appear. The catch statistics from 1967

to 1975 were also used to calculate the maximum equilibrium catch and the

optimum catching effort. Coordination of the information obtained from

this investigation led to conclusions regarding the management of octopus

stocks in this region.

The various basic data mentioned in the analysis"and the

materials and methods are discussed in detail in each section.

Section 1.

Estimation of mortality coeffients

Using as a basis the number of individuals in the hourly catch

by month-and depth zone obtained for each age group in Chapter 4 (Table 13),

the numbers of individuals (stock size) by age group and spawning group

were estimated from the area of the fishing ground in each depth zone and

from the area swept in one hour's trawling. The data from 1969 to 1971

were used to estimate the stock size of four year classes of.spring

spawners from the 1966 class to the 1969 class and of three classes of

autumn spawners from the 1967 class to the 1969 class (Table 17).

The fishing ground area was limited to that between north

latitudes 200 and 210 and at depths between 10 m and 100 m. Octopus

occur outside these limits but since important fishing grounds are not

formed it is thought that the quantity is small and that no important

bias will be introduced into the estimations. However the shallow water P73

water on the Arguin Bank is thought to be an important area of distribution

during the larval period, and i -43exclusion is expected to cause a large

error in the stock size at the recruiting period.

Spring Spawner

Year Year Month Age

Class

TabIt 17. Stock size in number x 10 8 ) by year class in the Cap Blanc Region (20 00

21 00'N). Figures of CPUE with parentheses are extrapolated from those in

the nearest depth-zone.

100

Dept h Zone rm )

111 30 30 50 5o 70 70 100 • Total

t 576 mile 2 . 649 m 468 in iii 2 214 mile ' Stock

CI'llE. N CPUE N Cl'I:E E. N CPIT. N Size

1966 1969 Feb. 2.6 67 941 39 617 20 331 29; 151 2411

Mar. 2.7 39 548 :15 55-1 41 • 68 61 318 1888

May 2.8 (4 ) 56 4 63 7 So 6 31 2:11

1970 Apr. 3.8 4 56 3 • 17 7 80 6 31 215

1967 1969 Feb. 1.6 71 997 30 475 63 719 63 329 -- 2520

Mar. 1.7 31 436 41 6-19 77 879 117 506 2470

May 1.8 (16 , 225 If 253 11 160 1 )) 52 liqU

19f)) Feb. 2.6 38 534 39 617 9 10-3 1'251

Mar. 2.7 42 590 30 ,175 32 365, 12 1-33 1-193

Apr. 2.8 1.1 197 12 190 31 358 . 30 157 931

May 2.8 15 211 4 63 8 91 8 42 407

1971 Mar. 3.7 - - 3 47 4 46 8 42 135

Apr. 3.8 12 169 13 206 4 46 , 1- 21 4,11

. 1968 1969 Aug. 1.1 . 422 5929 359 5683 ( , 11611

'

Sept. 1.2 408 5732 351 5556 11288 - , .

1970 Feb. 1.6 212 2978 69 1092 20 228 28 146 4445

Mar. 1.7 126 1770 49 776 47 536 26 136 3218

Apr. 1.8 14 197 17 269 -15 514 38 198 1178

May 1.8 27 '379 . 10 158 16 183 12 63 78:1

1971 Feb. 2. 6 33 464 22 $-18 I)) 111 • It) 52 978

Mar. 2.7 16 .995 12 190 21) 228 33 172 815

Apr. 2.8 16 225 . 17 269 11 126 :II 57 677

1969 1970 .

Aug. 1.1 213 2992 51 807 t ! 3800

Sept. 1.2 155 2178 86 1:161 . :1539 :

(Ici. 1,3 163 22911 51 807 72 822 ;1919

1971 Feb. 1.6 45 632 41 649 18 205 , 18 94 1581

Mar. 1.7 33 464 22 318 34 388 39 901 Ho 1

Apt - . 1.8 13 183 15 237 23 263 23 1211 803

O

Table 17. Continued.

Autumn Spawners

Depth Zone tin ; l'ear

1'ear Month ' Age 10-30 30-50 50-70 70 100 Total

Class (576 mile , . (649 mile , •468 mile , . 211 mile , Stock CITE N CPIIE N CPUE N • CPI N Size

1967 1969 Feb. I. 3 81 925 81 423 13-17

1969 Mar. L 3 35 4(10 37 193 593

May 1.5 t - - ) - 15 171 16 81 255

Aug. 1.8 162 2276 149 2359 '1l9 1701 , 149 778 7113

Sept. 1. 8 133 1868 103 1630 : 103 1176 , 1113 • 538 5212

1970 Sept. 2.8 -- -- 17 269 t 17 , 19-1 . 17 89 552

1968 1969 • Aug.'. O. 8 106 1489 28 443 1932

Sept. O. 8 30 421 70 1108 1530

197 0 Feb. 1. 3 5 70 14 222 23 263 , 12 63 617

Mar. I. 3 -- — 21 332 22 251 ' 37 193 777

Apr. L4 3 42 8 91 21 110 2-13

July 1.7 40 562 3 47 , 3 34 3 16 659

Aug. 1.l 70 983 27 -127 : 27 , 306 . 27 III 1860

Sept. 1. 8 49 688 63 997 '63 719 63 329 2731

Oct. 1. 9 2.1 380 28 320 28 1-16 816 ,

1971 Sept. 2. 8 31 491 • 31 • 353 • 31 162 1i ilH

1969 1970 July 0. 7 179 2515 20 317 2831 , ,

Aug. 0. 8 97 1363 53 839 • . 99 02 .

. .

Sept . 11.1-I 49

35 2 38 602 1(19:;

1971 Feb. 1. 3 10 140 23 361 . 28 320 . 28 I 1G 970

Mar. 1.3 11 155 5 79 15 171 17 8(1 191

July 1. 7 87 1 922 101 1599 f 101 1153 : 101 527 4501

Aug. 1. 8 103 1447 59 934 ( 59 • 673 • 59 • 308 3362

Sept. L 8 85 1194 69 1092 c69 t 788 • 69. 360 3434

N. Stock size in each depth zone.

101

•

•

•

102

The area swept is taken as the product of the separation between

the front wings of the net and the distance trawled. The mean area swept

per hour in the total data is 0.041 square miles. It is suggested that

vulnerability varies with season but there are no reliable data concerning

this, so it has been assumed that all octopus in the swept area are caught

at all seasons.

The numerical stock size calculated by this method (Table 17)

show that the largest estimated number is that of spring spawners 1.1 to p74

1.2 years old, and that the number gradually diminishes thereafter. The

autumn spawners are recruited at 0.7 to 1.0 years and the estimated value

is normally highest at 1.8 years. This is thought to be because the

period of recruitment is in fact longer than that 'of the spring spawners,

so that the total is not taken into account. In tile following investigations

the data - on spring spawners is therefore principally used (Figure 29).

The number of individuals in each age group per ton of catch

was obtained from the number caught per hour by month and age class for

each spawning group (Table 13) and the numbers caught for each month and

age group were calculated (Table 18).

Mortality coefficients were investigated bn the basis of these

data. The monthly decrement in stock size (the difference in stock size

between the previous month and the succeeding month) was compared with

the number caught during the month (the sum of half the numbers caught

in the preceding and succeeding months) (Figure 30). According to this,

the estimated stock size 1.5 years old in August and September is less

than the number actually caught, so that the size is evidently under-

estimated and is not suitable for estimation of the mortality rate. The

a,

1

• In

1 .

I

• 103

1 , • ,

--0 r--

o o

14C: le.

A ije a n (1 Mont Il

Pig- . 29. Changes of stock size in number of spring spi.iwiteis in t he Cap Mane I ■ egion.

•

Table 18. Monthly catch in tomber by year class in the Cap Blanc Region , 20 . 01r-21 ofi'N). Figures for n t in

parentheses are assumed values.

Spring Spawner Autumn Spawner

,

Year Month Catch Year Class Year Class

in I 1966 1967 1968 1969 1967 1967 1969 tons ,

' , ro N n 't N n 't N n t N n t N n„ t N n a N

1969 Feb. 3699 154 570 180 666 45 166

Mar. 4229 140 592 219 926 79 334 .

Apr. 1910 114 218 260 497 24 46

May 498 79 39 163 81 -• 166 83 •

June 322 235 76 147 47 -

-

July 606 • - - 682! 413 150% 91 (163 , 99

Aug. 4722 - • - 682 1863 962 1237 163 462

- •• -

Sept. • 5758 - 627 2990 203 1169 53 194 -• - ,

Oct. 781 i• ; - 1 700) 547 100 1 78 (1200 , 937

Nov. 664 800) 531 -- "2325 1544

Dec. 1130 . - 3-15 390 — 1107 1251

. 1970 Jan. ; 674 ( 1 / 1 350% 236 !25- 17 Feb. 4256 89 379 370 1575 25 106 Mar. 2545 - -- 116 295 298 758 29 74 Apr. 1032 28 29 133 137 173 179 32 33 May 1143 15 17 126 144 228 2(11 , 8 June 218 125 .)- ..., 171 38 121 26

July , 1612 753 705 136 219 608 569 Aug. ; 7018 656 3405 217 1523 303 1573 Sept. . 7830 611 3768 8 63 211 1652 147 902 Oct. 2035 665 1353 119 242 207 421 Nov. . 181(1 856 1549 100 181 . ..

Dec. 2295 2.11 537 387 888

1971 Jan. 2272 . 138 314 277 629 51 123

Feb. 3469 141 .189 205 711 71 246

Mar. 2330 16 37 . 117 273 212 194 79 184

Apr. 820 73 60 106 87 113 9:1 34 28

May 1043 1(16 111 113 118 34 1 • 35

June 2896 16307 . 113 :127 34) 98

July -1890 - 219 1071

Aug. 5102 289 1474 ‘I

Sept. 1837 17 89 163 268 192 .

n't. Number of individuals per ton: N. Catch in number.

•

111■ I

o

o

7.7111

105

-01-111à--1

•

Age and Month

Pig. 30. Mont hly decrement of stock size in numlwr 'open column and catch

in number (black column) for spring spawner in t he Cap Blanc

cg ion. Some decrements and en tehes are slum . n bimont hi y.

Table 19. Estimated total mortaltiy coefficient by spawning stock and by vear class

in the Cap Blatte Region. The mortality caused by ineubat ion is not included.

Spawner Year Class Period in Age Survival Rate Total Mona I it y

Spring 1967 1.6.2.6 1.00 11. 00

1.7-2.7 1.21 -0,19

1.8-2.8 2.70 - 0.99

Mean 1.64 - 0.39

9 . 7-3.7 0,09 2.-10

2.8-3.8 O. 47 0.75

Mean O. 28 1.56

1968 1.6-2.6 0.44 0.82

1.7-2.7 0.51 ((. 68

1.8-2.8 1.15 -0.14 : Mean* O. 48 0.75

Autumn 1967 1.8-2.8 O. 21 1.55

1968 1.8-2.8 0.74 O. 31

* Value for time period of 1.8-2.8 in age is excluded.

•

106

data for February to April show the same tendency. When the monthly

mortality coefficients are calculated from the number present and the

number caught, the catch mortality coefficients obtained for March to

May are 0.2 to 0.4 and the natural mortality coefficients are 0.2 to 0.8.

The annual mortality coefficients deduced from these monthly mortality

coefficients are extremely high in comparison with the overall mortality

coefficients to be later computed in yearly units,, and it is thought that

they do not represent true mortality coefficients. The reason is

believed to be underestimation and change in vulnerability between March

and May.

For these reasons and in order to take account of the changes

in vulnerability, the overall annual mortality coefficients were estimated

by using the changes of stock size at the sanie season after one year

(Table 19). Since it is believed that in the period from one year to

two years oldi most of the females spawn and die, one hall' of the one-year

old stock size was used. For part of the spring spawners of the 1967

year class from 1 to 2 years old and of the 1968 class from 1 to 2 years

old this gave an unacceptable estimate, the total reduction coefficient

being negative. However the other six examples were all in the range

0.7 to 1.6 and the total monthly coefficient excluding spawning was

estimated to be in the neighbourhood of about 1.2.

This total reduction coefficient depends principally on 1970 to

1971, and as discussed later (Table 23) the fishing effort from July 1970

to June 1971 was almost the same as that from July 1974 to June 1975.

Thus it is estimated that the total mortality coefficient in recent years

will also be in the range of 0.7 to 1.6.

107

The stock size of each year class at the end of the fishing

season and at the beginning of the next season is at just the same level

and sometimes the estimated value is found to increase (Figure 29). The

fact that no diminution is found between the fishing seasons suggests

that the natural mortality is of negligible extent.

Section 2

Maximum equilibrium catch and optimum fishing effort

2.1 Standardization of fishing effort

Medium sized and small ships operate with octopus as their main

objective on the Cape Blanco ground, and the fishing effort was standard-

ized for each of the types based on Nouadhibou Bay._ There were ice-hold

trawlers of 100 to 200 tons and refrigerated ships.of 300 to 550 tons, 550

to 1000 tons and 1000 to 1500 tons. Of these four types, those of 1000 to

1500 tons were operating at the initial season and are still operating in

the same way at the present time. Since there has been relatively little

change in the mode of operation and the fishing capability of these vessels

this vessel type has been chosen as the standard.

Firstly, the quantity of octopus caught per hour by each type of

vessel in each month and in each fishing tlock(30 minutes of longitude wide)

were determined from the catch statistics, and the value of the ratio to

the standard vessel type was used as the index of the fishing efficiency

of each type of vessel. The yearly values of this index have a normal

frequency distribution but each vessel type shows wide variations and there

are considerable variations between years (Figure 31).

The share in the total octopus catch for the year and the octopus

CPUE were determined for the three types of vessels other than the ice-hold

108

et' :,0 1II, I I ,

•

I 1,01 al 11 -‘3 1. ! e t

rat 7

Pat, I

1,. .;

•

; I I 9e,

i

n

7 n ll f. .-^-1--7...aj P r , n

2 1 i 7:.•

' n. a — — a --7 •i" 110i"t —1 : fi 11 1971

11 .

— r--r- --1 --rra al t.) C I

I

I f: -I 1977 : • , .7

ix. I a . . r n-Reefell; -r---,----•-

—rat - 1 , , xi ..--=.-1---..-1—g - ---, n r.

1 u -1 U.: J .,: 1 "4 7 •

I -.• 1 a a Ii6j1=e'r . . . --t--141' .1.1—r‘=.-- ■ 13.

i 977: 10 -I

197 1 . 197• I

r

•, , .

CPUE Index

Fig. 31. Frequency distributions of CPUE index for three vessel types against the standard

trawler, 1000-1500 GRT type, in the Cap lilanck Region. Black columns indicate the

number of blocks where the fishing efforts moa- than 10011 Min", were ex Pended Per munf

. ,

•

•

•

109

trawlers (Figure 32) and were used in the following study. In the three

years 1970, 1971 and 1973 the standard type vessel had a lower share of

the catch than the other types and it is thought that this was because

other species of fish were the main objective of the operations of this

standard type at some seasons. However the CPUE varied in such a way

that the standard type of vessel had a CPUE midway between those of the

other two types or perhaps closer to that of the 500 to 1000 ton type,

and in the above three years it was less than that for the 300 to 550 ton

class. Thus even with vessels which are wholly concerned with octopus,

there are seasonal and annual variations in the conditions of operation

and these are thought to result in changes of the CPUE.

The effectiveness was therefore standardized by using the data

for the two most prolific seasons, February to March and August, when

almost all the fishing vessels operating have octopus as their main objective.

The mean values of the ratio to the standard ship types of the octopus

CPUE for those months were calculated (Table 20) for blocks A and

(Figure 23). In addition, in order to reduce the scatter of the index

caused by insufficiency of the amount of data, operations with less than

100 hours in a month to a block, with less than 60% share of the octopus

or with a difference of more than 20% from the octopus share of the

standard type were excluded.

The CPUE index obtained for each year in this way showed definite accord i>13 t. the. type- cf ve.ssel.

variationsnFigure 33). The catch efficiency of the 300 to 500 ton class

with respect to the standard type increases as time passes. The catch

* Sic, but Figure 23 gives blocks B and C. Translator.

p79

550-1000 GRT

300- r.,!-.() GPI

s ./ 100C) - (501) GR1

110

vs

(i)

In

o

0

• •-■

0 -C

01

80

60

40

600

400

Lû

o

Year

rig. 32. Annual changes it) octopus share of the total catch , upper and citioh

per unit of effort (lo v er) by vessel type in Ole Cap Blanc Rvgion.

•

• •

Table 20. Octopus share of the total catch, CPUE and CPUE index for each vessel type by block and by selected

month in Cap Blanc Region. The values less than 100 hours hauled, less than 60% of octopus share

and more than 20% of difference in octopus share against that of standard type were excluded from

the calculation of mean CPUE index, and these values are shown in parentheses.

--- Month . _ Vessel

Year Block Type

. 1 Item

February March August ____

300- 550- 300- 550- Standard Standard Standard

550 1000 550 1000

300- 550-

550 1000

1967 B Share (%) 83 88 89 ! 76 73 83 96 92 95

CPUE (kg/hour) ' 362 830 585 201 497 429 726 1227 1265

CPUE Index 0.62 1.42 1 , 0_47 1.16 1 0.57 0.97 1

C . Share (%) (100) -- 88 84 (47) 81 90 (60) 92

CPUE (kg/hour) (383) - 542 199 (340) 384 619 (300) 1083

CPUE Index (0.71) - 1 0.52 (0. 89 1 1 0.57 (0.28) 1

Mean CPUE Index 300-550: 0.55 550-1000: 1.18 .. ..

1968 B Share ( % ) (58 i 73 70 . 138) ' 49 57) 93 95 93

CPUE (kg/hour) (138) 415 302 (81) t 2281 234 1 463 930 795

CPUE Index (0.46) 1.37 1 (0.35) I 0. 97 . (1 . 1 0.58 1.17 1

% C Share ( %) ( 40 ) ( 22 ) 69 '.331 '20 51) ? 19 '1 '1001 ' 93

CPUE (kgliour) (118) (1001 247 ;74 1 ( 100 1911 , 57) .1804) 998

CPUE Index (0.48) (0.40) 1 (0. 391 i 0.52 , ' 1) 1 0.1 (6) 11.81 ) 1 ....,

Mean CPUF. Index 300-550: 0.53 550-1000: 1.27

1969 B Share ( % 75 73 87 63 , . 71 62 76 94 86

CPUE ( kg ' hour ) 343 . 610 538 308 - 455 320 313 634 742

CPUE Index 0.53 1.09 1 0.96 1.42 I 0. -12 0.85 1

C Share (% 1 172 ) 88 SO ' 80 83 7:3 .7 , • 33' ' 77

CPUE (kg hour t 274 ) 667 497 339 59" 394 15:: '73 549

CPUE Index ( 0.55 ) 1.34 1 0.99 1.32 1 (0.03) 0.141 1

Nfean CPI*: Index 300 -550: 0.74 55(41000: 1.20

August March February

Ice- 300- 550- hold 550 1000

300- 550- Ice- 300- 550- Standard •. hold 550

Standard

550 1000 1000

Month !

Vessel Type ice .

Item , hold Standard

95

306

0. 92

71

152

0.66

96 91

443 333

1.33 1

71 68

268 232

1.16 1

t

Share (% )

CPUE (kg/hour )

CPUE Index

- 84 89 83

-- 347 54 7 455

--

0.76 1.31 1

88

222

0.69

78

194

0.81

(94) 89

(234) 322

( (1 . 73) 1

51, 72

(198 , 239

(0.831 1

Share (%)

CPUE (kg/hour)

CPUE Index

- 82 (26 78

- - 326 (88' 331

- 0.98 (0.27 , 1

550-100(1: 1.27 300-550: O. 80 an CPUE Index

85

229

(1. 92

61

176

1

72

276

1.57

69

147

((.84

Share (%)

CPUE (kg/hour)

CPUE Index

-- 83 76 69

- 278 264 241

.-

1.15 1.11) 1

Year Block

1970 B

Me

1971 B

79 73

336 249

1.35 1

C Share (% )

CPUE (kg/hour i

CPUE Index

_ i 83 ) 63 62

• ( 250) 255 243 -

- '1.03 ) 1.05 I

67 68

244 225

1.08 1 I 1

71

103

0.46

77

273

1

78

.241

0.88

82

108

0.10

Mean CPUE Index 300-550: O. 75 550-1000: 1.17

U:72 B Share ( % )

CPUE (kgihour ,

CPUE Index

75 79 80

229 3.1:1 297

(1.77 1.15 1

94

249

. 0.84

(94)

(184 ,

((.72

69

268

1

,85)

(415 )

(1.55■

(91 )

(316,

1.1$ -

70

957

1

73

305

1.19

(97'

(211 .

(0.79 .

78

959

(I. 98

91

516

1.12

79

270

0. 90

87 86

439 460

0.95 1

69 74

301 999

1.01 1

(65) 30 -9 ,,

(246 : , 129 300

( (1.82 ? ( 0.43 I

C Share t

CPUE ( kg/hour

CPUE Index

Table 20. Continued.

•

Mean CPUE Index Ice- hold: 0. 84 300-550: O. 91 550- 1000: 1. 1 18

Month Vessel

Type

Item

February

Ice- 300- 550- hold 550 1000

Ice- 300- 550- h old 550 10(10

Ice- 300- 550- 1101(1 550 1000

Ma rch August

Year Block Standard Standard Standard

• • • Table 20. Continued.

1973 B ' Share (% ) (82) (68) (72) 156) (81) (67. (66) 144 (911 (74) (75 , .55)

CPUE (kg/hour) (185) (229) (237) I 217) (198) 219) (319) ! 141) (130. (218) (337,, 2161

' CPUE Index (0.85) (1.06) (1.55) (1) 11.40) 1.55 1 (2.261 )1 ) (0.60) 1.01' (1.56) 1 1 )

C Share (% ) ---- (67) (58) (42i ' )92; (63) (56) )43) )1011; 66 72 67

- CPUE (kg/hour) --- (250) (281) )132) (212( .215) 11921 :97 , .208. 193 993 246

CPUE Index - (1.89) (2.13) W 0.99) ‘ 2.22) :1.98) '1 ■ ' 1.08 ■ 0.78 1.19 1

Mean CPUE Index Ice-hold: - 300-550: 0.78 550-1000: 1.19

1974 13 Share (%) 92 74 76 83 (88; 68 71 69 96 91 91 85

CPUE ( kg/hour) 185 223 236 244 • 142, 159 227 187 178 929 283 251

CPUE Index 0.76 0.91 0.97 1 .0.76 0.85 1.21 1 0.71 0.91 1.13 1

C Share 1%) --- (70) (64) 58' - '75' 71 61 .100 90 '31- 89 ,.

CPUE (kg • hour) - (170) (161) :160. - '1931 193 177 )176. 3(17 '136 306

CPUE Index . -- 1.06) 1. 1.011 11 1.09 1. 09 1 ((.58' 1.11(1 r 0.44 ' 1

Mean CPUE Index Ice-hold: 0.74 30(1-550: 0.92 550-1000: 1.1 (1

1975 B. Share 1%) (92) 75 70 66 99 83 90 8" 99 . ‘ 91 , 84 ' 64

' CPUE ( liglhou r ) (210) 145 190 150 195 176 213 181 10 (1 , . 177 230 , 189

CPUE Index )1.35) 0.94 1.22 1 1.08 (1.97 1.18 1 ((.85 . 0.9:4 , 1.22 1

C Share ( »;, 1 (100) (55) (57 ■ 70 : 86 78 87 100 • • 96 )97 .

CPUE (kg/hour ) (333) (106) (1 )171 190 .À 158 167 214 211 '2111 340. .

CPUE Index (1.75 , )0.56) 10.56) 1 • (1.74 0.78 1

Mean CPUE Index Ice-hold : 1.08 300-550: 0. SS 55() 1000: 1.06

co

4.1 1.0 -o

0.8

0.6 • Ui

o

Ice-hold Trawler

against 300-550 GU x 550-1000 GRT: A

standard type:o o X

Y O.:2 0

A A

300 - 550 GRT

• 114

0

1.0

0.8

0.6

1.4

1.2

550 - Inau

1.0

19'68 1970 ' '

70 197; 1974

Yea r

Fig. 33. CPUE indices and the regressions against year in the Cap Blanc Region.

•

115

efficiency of the 550 to 1000 ton class is about 1.2 up to 1970, but since

1971 it has gradually declined. Data for the ice-hold vessels which can

be compared with the standard vessels are difficult to obtain, but after

the ratios with other types had been calculated they were transformed to

ratios to the standard type.

The values of fishing efficiency of each vessel type were not

used directly, but computed values based on straight lines or curves were

used instead (Figure 33).

It is believed that the fishing efficiency of the 550 to 1000 tons

type exceeds that of the largest standard type because this type of fishing

vessel and its equipment for processing octopus after catching have been

optimally planned. The provision of fishing equipment and the operating

methods of the standard vessels are also influenced by the need to catch

mackerel and other species at other seasons. p83

2.2 Estimation of CPUE and fishing effort

Standardized values of fishing effort for February March and

August were obtained by using the standard vessel fishing efficiency

indexes found in the foregoing section, and from them were calculated

monthly values of CPUE (Table 21).

Next the information given in Chapter I was used to provide the

quantity of octopus caught in each of two seasons, a summer fishing season

from July to October and a winter season from November to June, and

standardized seasonal fishing efforts were calculated by using as representative

values the standard CPUE values for August, February and March (Table 22).

1966

1967

1968

1969

1970

1971

1972

1973

1974

1975

3712 5934 626

1815 6610 275

2243 4129 543

3245 7635 425

3135 12284 255

2865 10081 284

2532 9629 263

2421 11037 219

2721 12608 216

Winter

(Nov.-June)

Summer

July-Oct.)

Catch (tons)

CPUE (kg/h)

Effort (hours)

Catch (tons)

CPUE (kg/h)

Effort (hours)

116

Year

•

Table 21. Monthly catch, standardized fishing effort and CPUE of octopus in

the Cap Blanc Region.

February March August

Catch Effort CPUE Catch Effort CPUE Catch Effort CPUE (tons) (hours) (kg/h) (tons) (hours) (kg/h tons (hours, t kg h,

— — — 16 1084 15

2402 5973 402 5908 5334 1108

955 4943 193 10364 13136 789

2986 7680 389 * i.i876 6258 460

1083 7681 235 4835 13975 346

2259 10683 211 4175 15619 - 267

2518 9409 268 5107 14797 345

2258 8538 264 4021 17207 234

2569 13370 192 3219 12497 258

3031 13691 222 25.16. 11931 213

• Fishing Season

Table 22. Catch, standardized CPUE and estimated fishing effort by

fishing season in the Cap Blanc Region.

Item 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975

— 22724 12402 24512 12444 17299 18611 13862 13778 18061

— • 514 234 466 330 233 276 264 206 219

— 44210 53000 52601 37709 74236 67431 52508 67046 82470

698 13213 36239 12035 186.19 13822 10218 9668 8310 6007

— 1108 789 466 346 267 345 234 258 213

630* 11925 38326 26163 53899 51768 29617 41316 32209 282();

* Estimated by CPUE for 1967.

•

117.

It would be desirable, in evaluating the stock, to distinguish

between the spawning season groups, but long-term data for this do not

exist. The relative proportions of the spawning groups during the summer

and winter seasons were found from the monthly numbers of individuals ( -5,61c.is)

in each spawning group for the three years 1968 to 197e. In the summer

season there are about 52% new recruits from the spring spawners (1.1 years (i.8 )' ears old) (o.S. years old)

old), about 40% parent groupvand 8% new recruitâutumn spawners. In the

winter season the parent group of spring spawners (1.7 years old) and the

large old males (2.7 years old) make up together more than 80% and the

autumn spawners (1.3 years old) less than 20%. It appears from this that

the spring spawners form the important part of the catch and that the

operating fishing year can be considered to be a summer season together

with the following winter season.

Consideration of the relation between the CPUE of a summer

season with that of the following winter season shows a high positive

correlation (Figure 34) except in 1967/68, thus suggesting that the fishing

seasons have been appropriately classified.

The catch size and the fishing effort (Table 22) were therefore

summed up for each fishing year and the CPUE for each fishing operational

year was calculated (Table 23).

According to this, the highest CPUE was obtained in 1968/69 when

the catch was 55 000 tons, about twice that of a typical year. The fishing

effort peaked in 1970/71 and has recently tended to decrease. Since 1970/71

the CPUE has been stable in the range 216 to 293 kg/hour and in recent years

the stock level has not varied much. The CPUE, which may be considered as

the reciprocal of the stock density, is at a level of about one half of

that during the first season of exploitation.

p84

o 750

500 0,

0

o

4- 250 o

Lai

0_

'7 ■■

0 7 .2./73./

71/720 0

73: '4re 74/75

• 250 500 756 1000

CPUE (kg/hour) of Sumer Season

Fig. 34. Relation between CPUE's in summer and in the following winter

fishing seasons in the Cap Blanc Region.

Table 23. Catch, fishing effort and estimated CPUE by fishing

Year (July-June) in the Cap Blanc Region.

66/67 67/68 68/69 69/70 70/71 7172 72/73 7374 74 75

Catch (tons) 23422 25615 54751 24479 35946 32433 24080 23446 2. 6371

Effort (hours) 44840 64925 90927 63872 128135 119199 82125 108362 114679

CPUE (kg/h 522 395 602 383 •281 272 293 216 230

Item

•

•

119

p85 2.3 Maximum equilibrium yield and optimum fishing effort

Part of the group forming the summer season were recruits during the

summer season of the preceding year, and the 2.7 year old group is caught

during the winter season. Both these are important target groups during

the preceding fishing year. It was thereforedecided that the means of

the fishing effort in the year considered and the fishing effort in the

previous fishing year would be appropriate for use in the General

Production Model (Gulland 1961). However, since, as has been mentioned,

1968/69 was a year of abnormally high catch, it was omitted from the

following investigation.

A negative correlation between the fishing effort and the CPUE

was found, and transforming the regression equation between them into the

equilibrium catch relation, the following maximum equilibrium yields and

the corresponding optimum fishing efforts were obtained (Figure 35).

Y/f = 523 - 0.0024 f (re-0.78)

fs

= 108 434 (hours)

Ys = 28 344 (tons)

where Y/f is the CPUE (kg per hour), f is the mean fishing effort (hours),

r is the correlation coefficient, Ys

is the maximum equilibrium yield and

f is the optimum fishing effort.

This shows that the CPUE has been stabilized since 1970/71 with

a fishing effort close to the optimum level.

Comparison of the MSY with the present catch shows that the

catches of the three years 1968/69, 1970/71 and 1971/72 exceeded the MSY.

Since 1972/73 the catch has remained near the MSY with a virtually optimum

o

o0 o

120

50ti

7r.

Ileart F t ;mu.; t t ■ ) ■it

• r ou

a

tz

tie

eon

-0

Fig. 35. Relation between fishing effort and CPLIE, and estimated equilibrium yield curve in the Cap Blanc Region,

Fishing Mortality Coefficient C F )

Fig. 36. Yield per recruit for male and female octopus in the Cap Blanc Region.

•

121

p86

•

•

fishing effort, which is LO the short life-span of this

species and to the matching of the trawler fishing gear to the

characteristics of the catch.

Section 3

Maximum yield per recruit

3.1 Determination of parameters

Beverton and Holt (1957) have developed the yield per recruit

formula.

X_ e—mée e-Ite Fh÷ m (/ _ e.-

hO

in which R = the number of recruits

M = the natural mortality coefficient

tr

= the age at recruitment E.:, bhe.. 3rouncls

k = the growth coefficient in the Bertalanffy growth equation

Y = the annual yield

F = the fishing mortality coefficient

te

= the age at first capture

W = the theoretical body weight limit

to

= the intercept of the Bertalanffy growth equation on the X axis

t = the maximum age in the catch

An

= the number depending on n

A = +1 o

A1 = 3

A2*

= +3

A3

= 1

122

It has been shown that the males and females of the octopus on

the North West coast of Africa have different life spans and that there

are different recruiting patterns for the spawning groups. Thus Y/R in

the above formula is to be found separately for the limiting body weights

and maximum ages of the males and the females, and these are afterwards

to be combined to estimate the optimum age of first capture and the

fishing mortality coefficient for the whole stock. The following was

established by using the growth equations of the Cape Blanco groups for

the growth parameter.

Recruiting of the new groups to the fishing grounds begins for

both spawning groups in a normal year in July to August, and at this

time the spring spawners are about 1.1 years old and the autumn spawners (èt.)

about 0.8 years old (Table 19). The age at recruitdentvwas therefore

taken as 0.8 years.

The maximum age at catch (t x ) is different for males and females.

It is 2.0 years for females, since most of them die after nurturing the

eggs, and for males it was taken as 4.0 years because males 3.8 years

old have been found in the catch (Table 17).

The natural mortality coefficient (M) is estimated to be

extremely small, as has been shown in Section 1, and in this section Y/R

was calculated with provisional values of M of 0.1 and 0.2.

There is little difference between the spawning groups in the

parameters of the growth equations shown in Chapter 3, so the mean values

of the growth coefficient (k)vand of to (-0.057) for both groups were

used. The parameters of the growth equations were used for the limiting

•

1 . t •

(-) t ,

trt

3 .

t•- ta

1:11

o

L .

n o.z 1 OM

II1t11t ,

• 124

3. t1

a .1!

I , --r— —I- I

Fishing Mortality Coefficient ( F )

Fig. 37. lsopleth diagram of yield per recruit of octopus in the Cap Blanc Region.

Shady areas indicate the status of fishery in 1975. —

•

125

The large males more than 2 years old have flabby bodies and are known as

"mizudakon and they have less commercial value. Thus the optimum first

capture age is that which corresponds to the lower maximum, and is found

to be 1.6 to 1.7 years.

The investigation in Section 1 of the overall mortality coefficient

shows values for recent years in the wide range of 0.7 to 1.6, and the

mean value was used here, the approximate recent overall mortality

coefficient being 1.2. The age at first capture of the spring spawners

has been estimated at about 1.2 years (Table 18). However the autumn

spawners gradually begin to be recruited from 0.7 years but the quantity

caught and the number present in fact become maximum at 1.7 to 1.8 years.

Thus on the average the recruiting age can be thought of as around 1.2 years.

Thus consideration of the present conditions suggests that the

exploitation of the octopus stock on the Cape Blanco ground has remained

at a slightly unused level. If the age of first capture were increased

to 1.6 years and the fishing mortality were simultaneously increased by

about 30% there might perhaps be an increase in the Y/R of about 10%.

However increase of the fishing mortality alone would not be effective

management, since it would reduce the Y/R.

* "Mieldako" see my note to Chapter 2, Section 2.2 translation page 46. Translator.

•

126

Section 4

Discussion and review

The stock sizes for each month were calculated in Section 1 by

the area statistics method, and attempts were made to estimate the (E)

mortality coefficients. The exploitation rates'eobtained from the resulting

monthly estimates of stock size and from the quantity caught gave very est.ivna_Liolis o si o ck size_ cle_pcnclivI9 orl the- se a $CY1S ahol

high mortality coefficients, and this indicateemonthly variations in

the vulnerability. In order to eliminate this variation in vulnerability,

the overall mortality coefficients were found by comparing the numbers

present in the same month in successive years, and it is thought that

the values obtained in this way, even though not completely stable, were

not far from the truth. p88

Next the equilibrium yield curves were estlmated by the Production

Model, though the estimated values did not distinguish between the spawning

groups. However, since the number caught by year class and by spawning

group showed almost identical values from 1969 to 1971, it was thought

that there was little difference in stock density between the spawning

groups. Also, although there are differences in the relative importance

of the spawning group composition according to the fishing season, no

characteristic variations with season or place could be found. It is

therefore thought that the stock fluctuations of the two spawning groups

are similar, so that the errors involved in taking the spawning groups

together in the analysis are not thought to be large.

Taking together the present condition of the fishery and of the

stock, the equilibrium yield curves estimated by the Production Model,

and the stock exploitation picture given by the Yield per Recruit Model,

•

•

127

the exploitation policy for the octopus stock at Cape Blanco is thought

to be as follows. It is believed that the present utilization of this

stock is comparatively good and it is determined that there is no need

for the immediate introduction of new fishing controls. p89

Some fishing regulations have already been introduced by the

coastal country Mauritania. The territorial sea was extended in 1972 to

30 nautical miles and the number of fishing vessels entering the territorial

sea was limited, and the use of nets with mesh size less than 60 mm

prohibited, and considerable parts of Levrier Bay and the Arguin Bank

were closed to fishing. Normally, the age of first capture is reduced as

development of a fishing operation proceeds, but in this region there is

no such reduction because the shallow water areas in which growth during —

the larval period is believed to occur are protected. This regulation

should be maintained in the future.

On the other hand the basic data for stock assessment on the

Villa Cisneros region, where stock exploitation has been faster, are not

sufficient, and no stock analysis has therefore been made. This should

be given some future consideration.

Fishing regulations are completely absent from this region, and

there are practically no topographical obstacles to fishing in the region

of growth during the larval period. The age at first capture is lower

than at Cape Blanco. The CPUE for Japanese vessels in this area has

declined since about 1971, and the catch by Spanish vessels has had a

downward trend.

Consequently a decline in the octopus stock level on the Villa

Cisneros ground is forecast, and the introduction of regulations for the

methods of stock harvesting and fishing is necessary.

•

128

Summary

The Japanese Southern Trawling Fishery on the North West Coast

of Africa was developed in 1959, and the catch started to decline from

its peak in 1968, causing concern about a deterioration of the state of

the stock.

This report collects together the fisheries biological researches

concerning the common octopus, which has become the primary operational

objective of the Japanese fishing fleet, and the results obtained are

summarized below.

1. The growth and present state of the octopus fishery

1. Three octopus fishing grounds for Japanese vessels are found off

the coasts of the former Spanish Sahara and off Mauritania. These are

offshore from Villa Cisneros, Cape Blanco and Nouakchott.

2. Two clear fishing seasons are found at Cape Blanco. The fishermen

call them "summer season", which in a normal year is from July to

September, and the "winter season", normally from December to April.

At Villa Cisneros the octopus fishing season extends from autumn

to spring with two peak periods.

At Nouakchott the fishing season is from July to September.

3. The Cape Blanco fishing group was developed in 1965 and has been a

centre for Japanesafishing vessels since 1958. The common octopus forms

55% to 74% of the catch, and operations are carried out with the single

octopus species as the main objective. The fishing effort is steadily

increasing but the mean yearly octopus catch is at the level of 23000 to

36000 tons and is remaining comparatively stable.

• 129

•

4. The Villa Cisneros ground was the most rapidly developed, and at

first sea-bream was the main objective. Then there was a shift to squid and

octopus, and octopus constitutes 10% to 30% of the annual catch. Since 1969

the fishing effort has been concentrated on the Cape Blanco region and the

relative importance of Villa Cisneros has declined. The octopus catch has

also changed to a decline since the peak of 26 000 tons in 1967/68 and the

CPUE has also declined. The 1974/75 catch was only 900 tons.

5. The Nouakchott fishing ground, the last developed by Japanese vessels,

was developed as a subsidiary to the Cape Blanco region. Sea bream and squid

were at first caught, but there have recently been operations in which octopus

was the main objective, and the annual octopus catch has reached 3 000 tons.

6. The coastal industries and fifteen distant countries conduct fishing

operations off the North West coast of Africa from 9°N to 26°N and in

recent years the total catch has reached 2 330 000 tons. The total octopus

catch has been 4% to 14% of the total, at a level'of 70 000 to 130 000 tons.

91% to 96% of the octopus catch is taken by Spanish and Japanese vessels,

and in recent years the Spanish catch has increased.

7. Octopus forms about half the catch for both Spain and Japan, and

operations are conducted in which octopus is the main objective. In addition

Korea and the Mediterranean countries Italy and Greece include octopus in p90

mixed fishing. The Spanish and Korean octopus fisheries are entirely

confined to the coast of the former Spanish Sahara.

2. Information on spawning seasons and locations

1. Male individuals more than 50 cm long have spermatophores at all

seasons. Most females mature at about 2 years old, and at that time

their body lengths are within the range of 55 to 100 cm.

2. On the Cape Blanco ground two spawning seasons centred on May to

June and on September to October were found.

130

•

3. So far as could be determined from the data used, two spawning

seasons in spring and autumn are suggested at Villa Cisneros and at Nouakchott.

4. Octopus do not concentrate into definite ocean areas or depth zones

to spawn but are believed to spawn in the areas in which they live.

3. Age and growth

1. By applying the exponential curve to the total length -weight relationship, we

obtained the following relational formula .

0.34 Villas Cisneros grounds : L = 6.13 W

Cape Blanco fishing grounds : I = 6.49 W 0.33

Q. Nouakchott fishing grounds : I = 7.15 W 32

in which L is the total length (cm) and W is the weight (g).

2. In past studies on common octopus, there were life cycles of the integral-

years type (2 years) and of the non-integral-years type (1.5 years). In the present

study, we estimnted that the integral-years type applies to the cycle of the common

octopus

3. Most femPles mature and spawn around two years of age and die after the

nurturing is completed. It seems that SOMB males live as long as four years.

4. I the 45 days estimated (on the basis of what we know about other regions)

to be required for hatching are added to the above estimated spawning period,

on the average the hatching days are in mid-July for spring spawning and in

mid-November for autumn spawning.

5. There is not much difference in the graNth of mnles and of femeles. The growth

is shown through the following curves.

Villa Cisneros -0.50 0.33)1 spring spawners It = 123.6 11 - e (t + -0.48 (t + 0 .27) autumn spawners Lt = 121.6 11 - e

• 130a

Cape Blanco -0. 59 (t + 0.05)i autums spawners Lt = 105.0. 11 - e

in which t denotes the years and L the total length (cm) at the age t

4. Stock identification and movement

1. The octopus on the North West Coast of Africa are believed to form

three geographically completely independent stocks on the Villa Cisneros,

Cape Blanco and Nouakchott offshore fishing grounds.

2. The spring spawning group and the autumn spawning group have life

cycles which are in principle unrelated, and they are thought to be

independent spawning stocks.

3. On the Cape Blanco ground the small size gr9up are principally recruited

during the summer from places of shallow water, and they disperse to all

depth zones on the fishing ground during the autumn and winter. They are

believed to disperse to an almost uniform density in all depth zones and

thereafter to live completely without migration.

4. A considerable portion of the octopus fry hatched offshore are

displaced by Mum currents and tides to the Ardlin Bank and to Levrier

Bay, and it is very probable that they change over there to the bottom

dwelling life.

5. Other biological information

1. The sex ratio in the octopus catch at Cape Blanco (the proportion

of females) is about 50% for those of overall length up to 50 cm but

decreases at greater body length, and drops more rapidly at a body length of

85 cm, those larger than 115 cm being exclusively male. The large

proportion of males in the large sizes is due to their longer life span.

- spring spawners Lt = 111.7 31 - e 0.70 (t + 0.06)i

• 131

2. The sex ratio is found to vary with the season, the proportion of

females in the principal body length classes being greatest in March and

in August to September and declining rapidly thereafter in April to June

and in October. The reason for this change from an increase to a decline

in the female proportion is believed to be the nurturing of the eggs.

3. The stomach contents of octopus at Cape Blanco and Nouakchott during

the summer are, in proportion by weight, 45% to 61% shellfish, 19% to 34%

fish and 20% crustaceans and cephalopods combined.

4. The species identified in the stomach contents vary with season and

place and the octopus is believed to prey on any species which it is able

at any time to utilize.

5. The weight of the octopus stomach contents was high in winter and

summer and low in spring and autumn ab Cape 131anco.

6. The seasons of inactive feeding are those of poor fishing. An

important factor in the formation of a fishing season is the activation of

feeding behaviour with food seeking before the spawning season.

6. Maximum equilibrium catch and optimum fishing effort

1. Using data from the active fishing season for mosi fishing vessels

operating with octopus as the main objective, the ratios of the CPUE olocrakin9 or; Ce edânco

between different ship tonnage types'ywere used as indexes of their

fishing efficiences.

2. From an analysis following the General Production Model, the maximum 2E Cape. Ed2vico

sustained yield and the corresponding optimum fishing effort were''estimated

to be as follows

Maximum sustained yield 28 000 tons

Optimum fishing effort 108 000 hours. 3. The recent level of fishing effort is practically optimum and the size of catch is close to the meximum equilibrium yield.

132

4. The octopus stock size was estimated by the area statistics method

and used to find the mortality coefficients, the recent overall mortality

coefficients being in the range 0.7 to 1.6. The natural mortality

coefficient was estimated to have a low value.

5. The Yield per Recruit Model gave large differences between males and

females, and combination of the two leads to two maxima, resulting in two

values for the optimum age of first capture. However, considering the

state of the fishery, the lower maximum value and the corresponding age of

first capture of 1.6 years were considered appropriate.

6. The exploitation of the Cape Blanco octopus stock is now in a

satisfactory condition. It is forecast that an increase of the age of

first capture and a simultaneous increase of the fishing effort could increase

slightly the yield per recruit.

7. It was concluded from the results of both models that no change to new

fishing regulations was urgently necessary.

8. On the Cape Blanco ground it is thought that various fishing regulations,

particularly the prohibition of fishing on the Arguin Bank and in Levrier

Bay, which are believed to be the growth areas during the larval period,

have effectively stopped a reduction of the octopus stock. However on the

Villa Cisneros ground, where there are no regulations, the stock is tending

to decline, and it is thought urgently necessary to introduce assessment

and regulation.

9. In order to progress to regularized stock exploitation, it is necessary

to understand clearly the relation between ecological characteristics and

fishing equipment and methods, and, by maintaining statistics of the catch

per spawning group and of the population, to understand the reproduction

coefficient and the amount produced by each year class.

133

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O

•

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•

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•

-t

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tI

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À

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145 -c

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JAN p left

Date Due

SHODART, CO. Cat, No. 23-233-003 Printed in U.S.A.