Invertebrates & arine Plants IVlslonResource BranchHalifax, .8., B3J 287

,oc

48•

ess

February, 1979

J.F. Caddy

Fisa

Fi berie and arine Service

anuscript Reports

The e report contain cientific and te hni al information that repre ent animportant contribution to e i ting kno ledge but hich for orne reason rna notbe appropriate for primar cientific i.e. Journal) publication. The differ fromTechnical Report in term of ubject cope and potential audience: anu criptReport deal primaril ith national or regional problem and di tribution igenerall re tricted to in titution or indi idual located in particular region ofCanada. 0 re triction i placed on ubject matter and the eries reflects the broadintere t and policie of the Fisheries and arine Ser ice namel fi heriemanagement technolog and de elopment ocean cience and aquatic en iron­ment rele ant to Canada.

anuscript Report rna be cited a full publication. The correct citationappear abo e the ab tract of each report. Each report will be ab tracted b AquaticSciences and Fisheries Abstracts and ill be inde ed annuall in the er ice indeto cientific and technical publication.

umber 1-900 in thi eri ere i ued a anus ript Report Biologicaleries of the Biological Board of Canada and ub equent to 1937 hen the name of

the Board as changed b t of Parliament a anuscript Report Biologicalerie) of the Fi herie R earch Board of Canada. umber 901-1 25 ere i ued

a anu cript Report of the Fi heri Re earch Board of Canada. The eri namea hanged ith report number 1 26.

Detail on the a ailabilit of anuscript Report in hard cop rna beobtained from the i uing e tabli hment indicated on the front 0 er.

Service des peches et de la mer

Rapport manuscrits

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Les anuscrit peuvent etre consideres comme de publication completes. Letitre e act paraJ't au haut du resume de chaque rapport qui era publie dans la revueAquatic Sciences and Fisheries Abstracts et qui figuera dan I index annuel despublications cientifique et techniques du Ser ice.

Les numero de 1 a900 de cette erie ont ete publie atitre de manuscrit (Seriebiologique) de I Office de biologie du Canada et apres Ie changement de ladesignation de cet organi me par decret du Parlement en 1937 ont ete clas e entant que manuscrit (serie biologique) de I Office de recherche ur les pecheriesdu Canada. Les numero allant de 901 a1425 ont ete publies atitre de manuscrit deI Office des recherche ur les pecheries du Canada. Le nom de la erie a ete changeapartir du rapport numero 1426.

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• Cover design by Chns me Rusk

Department of Fisheries and OceansManus Report No. 1489

February, 1979

SOME CONSIDERATIONS UNDERLYING DEFINITIONS OF CATCHABILITYAND FISHING EFFORT IN SHELLFISH FISHERIES,

AND THEIR RELEVANCE FOR STOCK ASSESSMENT PURPOSES

BY

.} .F. Caddy

Resource BranchDept. of Fisheries & Oceans

P.O. Box 550Halifax, N.S.

B3J 2S7

- ii -

C9 Minister of Supply and Services Canada 1979Cat. No. Fs 97-4/1489 ISSN 0701-7618

- iii -

TABLE OF CONTENTS

ABSTRACT/RESUME.

INTRODUC'f'TON . .

Page

iv

Definitions of Catchabil Effort, and Fishing Power

HAND GATHERING

Catchability and Gear Selectivity.Effort Definition .

DREDGES AND MECHANICAL HARVESTERS.

Selectivity and Catchability Coefficient .Factors Affecting Dredge Selectivity and Efficiency..Definition of Effort in Dredge Fisheries . . . . . .

TRAWL FISHERIES.

Fishing Power.Factors Affecting Catchability and Gear Selection.Corrections to the Effort Unit ......•.

'rRAP FISHERIES .

Fishing Power.Nominal Units of Effort.Corrected Effort Units . .Corrections for Environmental Factors and BehaviorCorrections for Catchability and Gear Selection. .Physiological and Behavioral Considerations....

SUMMARY..

REFERENCES

4

44

6

667

8

889

9

101010111212

12

13

- iv -

ABSTRACT

,T.F. 1979. Some considerationsand fi effort in shellfish fisheriesassessment purposes. Fish. Mar. Servo MS

definitionsand their

1489.

of catchabi1ityfor stock

The lems of and effort definition in invertebratefisheries have been reviewed from available literature on four broad

of fisheries hand andfisheries. units of nominal effort have been proposed

power and coefficient identiThese include a of , environmental, and behavioralconsiderations for the species concerned, in addition to the more obviousmechanical considerations which determine the area of influence andefficiency of the gear. Spatial distribution of fishing effort (fishingcrr~,r,,,,rr,,) in relation to the distribution pattern of "thedetermine the effectiveness of a given unit of effort, ane:

of gear saturation (especial in andmay introduce a density-dependent bias into the definition of fishingeffort. Definitions of effort (eg., "days on ground"), which inadequately

the process into its of "search time" and"handl time", may estimate the true pressure onthe stock, over range of population densities. These sorts of bias

misleading when effort data is used in models predictingsustained

Whenever it is possible, it is

to quantify factors affecting fishing powerthat they be used to obtain an estimateis additive units, andand rate.

words catchabipower.

coefficient, s"tock assessment,

RESUME

, J.F. 1979. Some considerationsand effort in shellfish fisheriesassessment purposes. Fish. Mar. Servo MS

definitions ofand their relevance for stock

1489.

On a les problemes de definition du de etde l'effort de des invertebres dans la documentation surquatre grand de peche. Les unites d'effort nominal ont ete proposes,et les facteurs influen~ant Ie pouvoir de peche et Ie coefficient de captureont provisoirement identi Ceux-ci incluent un de

jJuYb..L\.j'..LI...,''::Jiques, environnementales et ethologiques s 'appli-concernees, en des plus

la de deLa distribution (

- v -

de la peche) en regard du type de distribution de l'espece peut determinerla validite de I 'unite d'effort choisi, et avec les problemes de saturationd'engin de peche (particulierement pour les et les dragues), peutintroduire une erreur influencee par la dens dans l'estimation de l'effortde peche. Les definitions de l'effort (par "jours sur Ie terrain"),qui negligent de subdiviser Ie processus de de recherche"et de manoeuvre", peuvent fausser l'estimation de Ia vraie pressionde peche sur Ie stock, en fonction des diverses deCes genres d'erreur sont particulierement trompeurs quand les donneesd'effort sont employees en modeles predisant Ie rendement soutenu.

Chaque fois qu'il est possible de les facteurs influe9antIe pouvoir de peche et Ie potentiel de capture, il est suggere qu'ilssoient utilises pour corriger I'estimation de l'effort de peche, lequelpeut alors s'additionner d'une un de peche a l'autre, et devientproportionnel a l'in-tensite de peche ainsi qu'au taux de mortalite depeche.

- vi -

- 1 -

INTRODUCTION

shellfish stocks held inh the need

the ofAt that

effort andthe gear have

marine fin­as a method of

A meet assessments ofin 1976, the" 64th annual reun

for further research initiatives toshellfish stocks and the definition of relevantit became evident that studies on standardizationgear in relation to fishing exertednot kept pace with similar studies on gear used forfish, the growing of effort limitationmanagement in shellfish fisheries , 19

1be Shellfish and Benthos Committee theconsidered the findings of the special meeting onof shellfish s"tocks, and adopted the following resolutions:

C.Res.19 :5 Attention should beeffort for gears cular to shellfishmeasures should be adopted.

to the definition of fishingfish0ries and that standard

C.Res.1976/5:6 The effects of fishellfish should be given attention.

on the habitat of

This paper attempts a preliminary description and definition offactors relevant to gear performance and fishing effort, while noting thatthe wide of gears used for shellfish harves a seriesof definitions each appropriate to a of gear or harvesting

in invertebratesince, unlike the

was on thesejoint meetings

1957, 1963 ICESexamination

fisheries. Theon development

and local conditions.part in the difficulty

of new gearTheof

of these

Published data effort andfisheries is not available in asituation with to finfish where

ect areas in the 1950 sand 60 s (eg.,effort and the of

on the measurement ofareas has been

in these fisheries to dute has been 1des suited to particular fisheries

lack of standardization plays aon gear performance.

Definitions of Catchab Effort, and Power

, twogear characteristics

developed to the ofand Dickie, 1964; Gulland, 1964b):

1) What may be termed thedescription of the mechanics of gearfish behavior in relation to gear.coefficient is defined as q = calA, whichindividuals in stock area A removed by the gearwith efficiency~. This lends itselfphysical characteristics of the gear (Treschev,gear also on fish behaviorrelation to stock distribution patterns (which

for firststudies of

, a catchabilityof

unit area ato measurement ofbut since effective

- 2 -

this also means that effort units must be an exactthe equality F = q'f. This poses majorof fishing effort, or more exactly in this case,

fraction of Fto satisfyin the definition

2) The approach usually adopted into choose a convenient, easily measurable unit of nominal(g) (from log records, port interviews), and ust this for power ofindividual fleet units to arrive at a corrected effort unit (f).The performance characteristics of the gear or units can then bedescribed in terms of the slope of the regression (q) between correctedeffort and resultant fishing mortality rate (F) (determined independentlyfrom catch curves, cohort is, or tagging ) .

A full understanding of the factors underlying performance of fishingunits comparison between these two approaches. Evidently, however,the definition of fishing effort unit chosen should ideal be closelycorrelated with the effective fishing intens exerted in order to minimize

and temporal variations in q. At the same ti~2, variations in q mayresult from changes in fishing power, effort distribution in relation topopulation density (Rothschild and Robson, 1972), as well aschanges in availability by sex, size, and age. Corrections for these factorswhenever possible should therefore be an integral part of fishing effortdefinition. The approach adopted here is to summarize for each type ofgear what effort units seem most appropriate and the considerations thatmay lead to variations in q.

Evidently, in order to sum the individual units of nominal effortexerted by members of the fleet, some account must be taken of their relativefishing power. Thus, the definition of Gulland (1964b) states that "Thefishing effort of a fleet, from commercial statistics, is the sum of effortsof individual units, each computed as a product of fishing power of thatunit and the time spent fishing, or number of Ii We may notefter Saunders and Morgan (1976) that if gear efficiency (p) = cln, where

c '" catch per unit operation from n individuals within the area of gear

infl uence f the absolute fishing power of the gear (r)a c

na p, so that

the f intensity exerted by g units of nominal effort by a given vesselis f == gr. In , because of problems in absolute fishingpower, it is usual to compare effective catches by different fishing unitsto a standard vessel or vessel type within the same time-area stratumto obtain re1a!ive fi power r' (Robson, 1966) before up correctednominal effort units to obtain total fishing effort of the fleet as

nf L: r where the summation is over the i individual vessels in a fleet of

i in units. In general, for all gear types, the conversion from nominal toeffective effort should eliminate, where possible those factors which leadto variations in q, whether due to variations in power, gear configur-ation, or any factors that affect the additivity of the effort unit.

3

Application of Definitions of Fishing Effort andAssessments of Shellfish Stocks

in Population

on the subject_ cons..L~''''"_..L_'''j

types of gear orhas been

definition in terms of fourA necessary SilllViiLi~a'~i'JH

the problem of effortmethods fish

harvesters)trawls

and set gear

1. hand2 •3 •4 .

of effort definitionheadings in

The main characteristics relevant to theeach of the above type of gear are reviewed under

sections.forthe

The uses to which improved estimates of effort andmortality can be applied are briefly reviewed here. T~2 first which ismost relevant to the question of effort definition per se, may beconsidered under the heading of models [eg., Schaefer (1957); Pellaand 'romlinson (1969); Fox (197:)); and other which

status of the stock in terms of the relation-between amount of fish effort and we of ca-tch. The different

approaches are all encompassed within the Generalized Production model,express the in population size over time by: dP/dt = ±HPtm±KPt-qfPtwhere i the size, and H, K, and m are parameters that allow

of a wide range of curves to the of overall catch on fishingeffort. The general s is that catch rises with effort to somepoint (MSY) before with further increases. This treatsthe as a "black box" to which effort is the main input,and an estimate of Id at that level of effort the main output.

it relative limited data isto errors or biasses in the units effort s both

in fi power and gear well as inb of the stock.

to estimatecomposition

relies onsizes from size

virtual population or cohort1974). If an estimate of natural mortality

allow estimation of mortalityof mortality rates with trends in effort,

coefficient q can be detected with age and time.

The othermortal rates andof the catch (catch curves)(Gulland, 1965; , 1972rate is available, theserates, and comparison

in the

may be defined as thethe chosen at random by oneq may be due to one or more of the

Ii of one individual inunit of effort, variations infactors:

1. changes in fishing power2. in vulnerability, r~TC>NlT, or stock aggregation.

o

)

stock (Rothschild

power are caused bystay on concentrations

case of whosecatch

, 1975).

the gear is-Lltl]JI",OJOJiblei~he units

shel fish

at definition of effort inbetween search time time fishable

stock) and time the time whichin (Beinssen 1976b). Although it may be

fisheries to distinguish these two withineffort available from commercial statistics, in many

tr . d search time b to increase-le In ex handl tim~ may e

as stock

HAND GA'rHERING

of largely1961), cockle and

(MacPhail andfisheries with the aid ofmanual collection of subtidal shell-

considered wide(Medcof,clam

be

variousrmi

197Gb) .

ty

do not at firstLU_-L£JU L ion gear seof conscious judgement (hand cuI ), or

clam hacks) or be on sieve mesh sizecockle fisheries). These types of fisheries present

certain which may be conveniently presentedto a or lesser extent to more elaborate fisheries.

s pcwhich mayo tine(as in somein an elemenhere since

Effort De

of effort units may best be in terms of man-qrcnuJds, hours underwater by divers, or directly in terms of the

searched, a definition of the between effort andtake into account several additional factors

Whileon the

area of terrainfish

Pielou 1965 may confounda unit of effort exerted at any

Manyotherwise) share with benthic sms

liott 1971), andfish

to1955) is stratification of the catch and

may then beover the whole

(Gullandsubunit areas.

each unit areaand

binomial_, 1974) seems wide

of this ofeffort statisticsnecessary to assessfishing

timeit would seem 1 that the

also be non~random. In addition, thereterms of the minimum CPUE 'that may precede

part of the stock.

2) Thespent , forresultant effort distribution willmay be a distinct cut-off

for another more

3) The definition of a unit stock posed one of the or problemsfor which are at least subtidal, a fraction of thestock may be available to ion. The efficient natureof hand for such as Abalone spp.) and largedecapods may result in the extension of the range of ainto progress distant or inaccessible waters (reefs, etc.)with the result that on stock depletion, effort units measuredin or hours at sea may come to consist of search time asopposed to time the catch. indirect indices oftime spent volume of gases used per trip,may overestimate actual time spent since and inaccessibilityof harvestable densities may

disLi

of estimates a, two types

to themay be

clam dout" of cockles (Franklin, 1972),

fisheries (eg. Dare, 1974 , 1973)mortalities are to make

based on numbers of individuals landed (byis) to underestimates of

Catch and is may the mosttractable to the estimation of the re betweenf effort and sustained for these fisheries;

MacPhail, 1964),and some types ofwhere indirect

good

CU1CU.V'"is •

efficient) s of handan alternative to

2. other (oftencatch statistics may

such methods of virtual

DREDGES AND MECHANICAL HARVESTERS

) ,clams) ,

1959) orwith

(P'''>llrno, 1964).the continuous

terms

and sealwithout,upper and lateralAs a special

Se and Catchab Coefficient

or sampling devicesv,cyall~SmS (Holme, 1964; 1 1956); and

used to measure distance travelied by dredges(B'JurD,e, 1965) have indicated that mesh selection

are towed to fullness, or if other debrisand Gibson, 1956). Under these

correlation between selection factor and volumeoccur as noted by McCracken (1963) for otter trawls,

extent. As a result, the range of sizes partiallymay extend over a wide of the available

72) , cull undersized individualsexerted of teeth

reduce the amount of debris. obs),

fraction ofThis must be

if the terminalthe bot'tom.

are relativefor benthic or 'thic

odometers have beenon bottom, the same studiesis poor if theblocks the incircumstancesof contents maybut to a moreretained the

which means thatthe (Baranov,distribution

type may be

definition of1918) may have to be

the species (

gearthe of and

pern,af>s, is the effectof the stock referred to earlier,

effort in terms of area swept bymodified in the light of the, 1975; Allen, 1976).

as weather may affectof the , even the

contact with the bottom (Baird 1959).over the same ground

the effects of the in

Seasonal factors suchsignif increased "j LUH!".Lll'1

addition of pressure to maintainGear may also vary onparticularly due to recess ofmod and the bottom

(Chlamysshow active

will

Behavioral. More activeoffshore

behavior which can affectto some extent on

makesthanselective

has aH.L':)l"e::> t proba­

size atAnotherunder­

dumping of1974) .

dredgesmesh screen

'rhe generaldestructive) than manual

of the fishingfurther

Another major problemthe incidental damagemoss rakes to other

consideration.

due toclose contact with the bottom, may exertindicated of individuals

at size may be caused contactthe gear. , for some

size selective ; andof seems to occur

which it is just fromcomponent of incidental mortal is that causedsized individuals from deck either duethe catch and cull , orBoth factors may contribute to deathto once returned to theused for many infaunal bivalves mayin the delivery chute (Franklin, 1972) or manual

is that this type of gear isundersized individuals

the of modificationharvest

consideration (eg., deGroot andthat occurs in fisheries this

Possible incidental(eg., lobsters Scarratt 1975) may

influence on effectivepower calculations

of effectivecarried out at sea.

have anunits in

influence ratioo the catch

r.be used to strati

summation. Crew size maytime where

F

effort andand efforttime over

Def ion Fisheries

can

and the(Table 1).the

as at sea or on thebe may introduce a dens bias toestimate of effort This because the ratio ofsearch time and time to timeand process the catch will both vary with abundance. Moreeffort units may be either defined in terms of the time on bottom

the gear which may then be converted into area (where area= gear time on bottom effective width). If

the gear is unselective and saturated the number of towsthe gear or gear of a known capac may be more

measure of pressure. In this caseof influence of the will be a function of

of shellfish in the material retained

8 -

THAW], FISHEHIES

, andtrawl fisheries

957; 1960; 1974discussed here,same gear.

capture of reptant:Considerations

Fishing Power

Two groups ofdefinition:

may pose sl_~.. ,_-.l different ems in effort

1)width and area swept may be thepower, and

crustacea) where effective t.rawlfactor in fishing

the water column (eg.,movement off bottom occurs (eg., many

headline cross-sectionalin volume.

which may bewhere least some

) f

mouth are

2)

squids) orcommercialarea of the trawl

powermeasures ofto some

istics may be usedcorrelated with

is not an

errors inbecause of

~~HL,.~_L) ), but also

indications that effective trawl width not include theconsideration for burrowing

The use of the "areaeffective effort (Hoydal, 1976;

loss of over the headlinebecause factor" by

power

There aretrawl wings ifspecies such as

may leadCarlsson, 1976),(which can vary

which fishermen

Factors and Gear Selection

on bottom temperature1976) f and may also

affected.for groundfishby-catch of

gear that

variescontent and Munch-Petersen,

Behavior of males and females may beof trawls may be less clear cut than

"'''';::''1.Lll,::!, and in many cases there is a swhich has several

catch.

varySelection

s due tosmall groundfishminimizes fish

Effort definition. The here are similar to those well definedfor groundfish trawl fisheries. units of effort may be either in days spenton the ground, number of tows, or distance gear of a known type,width, or cross-sectional area or volume.

- 9 -

Corrections to the Effort unit

The of effort d(~fj nition in mult "Hl'l'cies fi,,!lerie;; has beenaddressed ('lsewhere (eq., Anon., 19()(); Ke j(). This mi:l.y tw par-ticular important for those showinq contaqioufl e1istr ion <melmarked substrate preferences. This may make it necessary to apportion effort

subareas of known substrate or habitat Penn and Hall 1976) in orderto arrive at an effort measure that is related to theexerted by the gear.

TRAP FISHERIES

The problems in defining practical measures for f power andfishing effort for " gears such as fisheries have been reviewedby Hancock and Simpson (1962), Simpson (1975), and Bennett and Brown (1976).In addition to mechanical considerations such as size and design, sizeand shape of entrances, and escape holes, and the presence or absence ofone-way valves (all of which may vary on a basis), fishing powerof traps depends to a extent than for "active" gears on physiologicaland hehavioral considerations; some of them understood, and few ofthem fied.

The sequence of events outlined in Bennett and Brown (1976) summarizethe main factors affecting the trap capture process. This is (with somemodifications) :

Process

condition

freshness of bait(food moult

condi t:ion)

(

( -

( ­(

attraction

locating trap:

(

( ­( -

response timedirected walk

effects of temperatureconflict?)

on locomotory

- soak time

to

- inter-,avoidancecannibalism)

dimension oflimit?) + trap size

- number of individualssaturation)

attraction,(predation,

(upper size

in trap (gear

escape from

size of mesh, lathescape

- self-destructof lost

presence of

to prevent ghosttraps?

~ 10

Fishing Power

power could be determined from gear efficiency(the number of individuals as a fraction of those thegear) and the unit area of gear influence (number bait/populationdens In however because of difficulties in absolute

power, fish power of a trap should be calibrated againstsome standard trap des and bait before effort over the whole fleet.In doing so, it should be borne in mind that interactions and contagiousdistribution (Paloheimo, 1963; Sinoda, 1970) may biasresults, on trap location and In situations where gearsaturation is to occur, the average power a fishing

may if rate is a function of availablespace in the ion density (Munro, 1974).

was then obtained from a

Unit area of gear influence. Miller (1975) quantified this parametercal trap catch of crabs op-il-io) against

underwater photography. An experimental estimate of f"fi:ect2Ye area fished24100 m. While

ab d'.. of approximatecre enslty .

noting that this t:ype of estimate may be affected by a number of factors suchas soak time, time, and of to bait(which latter may be to decline with distance approximately accordingto the inverse square law), it is interesting to note that the olfactoryresponse threshold for HomaY'us amer-icanus to freeze-dried cod extract(McLeese, 1973) 0 5 to 1 X 10- 4 g/k leads to a similar predictionfor the order of of a. that 1 Ib (453 g) of bait offresh fish may 60% of its we as "attractant", itwill on dilution 5 g/k provide 1.824 x 10 3 m3 of attractant. Ifwe postulate a laminar tidal flow and confinement of attractantd to wi 0.5 m of bottom, a similar order of magnitude for a isyielded as with Mille's calculations.

Nominal Units a

Number of and fished have both advanced as unitsof effort in fisheries Both measures may contain s cant errorsor biases as mortal indices, and this applies equally to less precisemeasures such as on and num:b(~r of

Corrected Effort ts

Soa~time. It is widely1970; Bennett and Brown 1976; Skud,linearly with time in the water, butmay be expressed the equation:

(Sinoda, 1970; Rothschild et al.,1976) that trap catch does not increaseincreases towards an asymptote which

-RS(l~e ) (Gulland, 1955; Monro, 1974)

where Cs is catch after S soak days an asymptotic catch Coo at a

11 -

rate by coefficient of capture R. addition ofunderestimate to"tal effective effort (

of are left than the standard soak time. Forsimilar reasons, trap-days in the water overestimate effective mortlalityif allowance is not made for dec power with time over longersoak times. This of bias serious, since soaktimes are 1 to occur with effort and low biomass as fishermenuse more gear and also at and low effort when traps are moreliable to be saturated even with short soak times. An ustment for soaktime can be made if of the above are known bynominal effort to a common soak time Tusing:

s

Corrections for Environmental Factors and Behavior

It may be questioned whether corrections for these factors should beto the effort unit or to the catchability coefficient. In

the latter is to retain its usefulness as a parameter of thebetween effective effort and (ideally

variance in q to pure error), good quantitative informationavailable on the influence and magnitude of any factor on the effectivenessof the gear should be used to correct the effort unit. For example, iffishing power is a linear function of temperature T and Wilder,1958) in relation to some minimum at which power is

zero:

where r standard power.

Tota fi effort may then be

T'

lineartemperature,

teml=,eI~ai:UJre-spe,cj_f ic

T' is the at which the catchability ceases to be afunction of temperature. If catchabi is not related to

Paloheimo (1963) effort may be usted forleve

r L:T

data is available on the effect of on feeding

12

Corrections for and Gear Selection

Variationsenvironmental and(5), and(1974) from an

where a-d are linearused resultstemperature

regression parameters.Wilder (1958)) = q )

authors t:o

ends of the size spectrum:LO.'-Ale;:» and on

entrance holes mayas may exit

fishing or

operates at bothmeshes

hole diameter).(Stasko 1975)

,1976) both in

selecton small individualsof large individuals (entrancedetermine compositionholes (Krouse and Thomas, 1974;"lost" traps.

Physiological and Behavioral Considerations

seasonal variations in q have been observed for1962), and in vulnerablil to traps

in summer following mou , declining as(Chittleborough, 1975). Catchabil may also

tidal rhythms (Hancock, 1962), abundance of natural preynt.rclslJeclfic attractants , 1970), and avoidanceWhile it may be difficult to correct for some or all of

to have the most serious impact on Deluryof catch per unit effort within a season (Hancock,

effort may be unaffected as as thela·tive in duration in relation to short-term

Differentialmanyisthe next moultshow diurnal and(Simpson, 1975 ,(Hancock, 1974).these factorsestimates based1965) annual

effects.

SUMMARY

effort definitionof fishing powerof gear used in

the main problem is inwhich are additive for

of the fishingphysiological,

and fishingpower and

has not been

terature relevant to fithat measurable units

are available for mosteg. FAO, 1976). However,indices oflinear dens

the gear. A number ofas well as thoseiden·tified as

in most

A reviewin invertebrateand nominal fishellfishconverting these intoall units

exertedand distributional,strategy) have beencatchabilityelucidated.

The problem areas seem to call for further attention:

1) What is the nature and extentexisting measures of fishing effort, for

factors inand

13

fisheries, and how can these be corrected for beforein models to determine levels of

the units

2) What is the relative ficance search and handl timeas components of fish effort, and what should be the relative contributionof the two as to models?

3) What is the extent of indirectin those shellfish fisheries where discards -'-':J""_k _L"~'. e-

In relation to gear types:

4) An improved understanding andpower in trap fisheries seems called for.

of factors affecting

5) For those gear wherehabitat of shellfish (eg., dredges,of effort on the long-termbe investigated.

has atrawls), the effec~ of

of the

on thesustained level

grounds should

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..Table 1. Factors affecting fishing power, effort definition, and catchability in invertebrate fisheries.

fishing power (r) = p x a

Trap design, sizesize, entrance

diameter

trawl, cull

changestrawl

Dredge width xdredgesMesh size, toothspacing, manual call

) Type terrain, soilconsistency, under­water visibility,amount of cover

) DepthTine spacing, sievesize, visual call pt.

Factors affecting catch­(q) and selectivity

Physiological state(moult condition,activity)

Inter- intraspecificcompetitionGear inter-reaction

7) Migration, seasonalavailability, changes

1) Trap/vesselpower

2} Soak time, gearsaturation

3) Environmental factors

1) Vessel fishing power2) Multi-species effort

correction

Individual fishing

1) Vessel fishing2 Multi-species

correction

Adjustmentscorrected effort

of fishing

1) Number hrs,dayssearching, digging

2) No. dives/volume ofbreathing gases used(scuba)

3) No. divers, diggers

1) No. trap hauls2) No. trap days fished3) fished4) Days fran port5)' No. trip36) Fleet size/No. traps

in fleet

and 2 corrected forsoak time)

Hours dredges on bottom2) No. tows

Days fi.:hed4) Days on ground5) Days from port6) No. trifs

Fleet size

Hours in water. tows or hauls

Days fishedDays on ground

5) Days from port6) No. trips7) Fleet size

Area raked,or searched

Area swept = dredgewidth x towing distance(per unit operationper time)

Area (or volume) swept= effective trawl

towing distance(per unit operation orper time)

(per effort unitor time)

(durL"1g a standardsoak time)

Handgathering

Dredge