This paper not to be cited without prior reference to the author

International Council forthe Exploration of the Sea

C.11. 1972/J:6Pelagic Fish (Southern) Committee

A'method for determining the fecundity of thehorse mackerel (Trachurus trachurus:(L.))

by

C. T. MacerFisheries Laboratory, Lowestoft

Bundesforschun,,' nl!1O tfür FisG:her i Homburg

Bibliothek

•

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INTRODUCTION

The estimation of fecundity, defined here as the number of eggs

which will be released by a female fish in the next spawning season,

is an important aspect of the biology of fishes. It can be used, ,for,~

example, to determine stock size (in conjunction with data on the '

annual,egg production of the stock) and also as a character far stock

separation (Burd ahd Howlett 1971). , _._ .

The ease with which fecundity can be determined depends upon whether

or not there is a clear size-separation between developing and resting

oocytes at any stage prior to spawning. In some spe6ies, such as herring

and haddock, 'there is a clear separation but with others, such as hake

and pilchard, there is a continuous gradation of oocyte size (Hickling

and Rutenberg 1936), and this latter case also applies to the horse

mackereI. The .problem, therefore, is to determine which oocytes will

develop in the current season •

PREVIOUS WORK

The simplest aPllroach to the problem has been to decide ona criti­

cal size below which all oocytes are considered to be resting. The

critical size is chosen by irispection of oocyte.diameter distributions,

sometimes coupled with histological examination; 'This method was used

oy Andreu (1954)'for Sardina pilchardus, by Petrova (1960) for Sprattus

sprattus, by Komarov (1964) for Trachurus trachurus off South-west

Africa, and by Rao (1967) for Rastrelliger kanae;urta.

-Another method has been to estimate the number of oocytes in a

single batch and to determine the number of batches; a 'critical size'

is sometimes used in this method also.' Th'e.' batch size is usually deter­

mined from the most advanced batch of oocytes, since this is often the '

1

~----- ------------ -----------~

\-only' dis6~~t~~J)~:tb:n.in:oocyto'di6m~ter.distributions.' ,This me~ho(l w~s:~',

used by Clark (1934) and 11aeGregor (1957) for Sardinops eaerulea, but

nei·hi~r 'author was able to givo a definite value for ;the 'numberof :'. ~',·;:.~>,~,,>r:_.l"~~/ ~ ,':'::-':"" ,~ .. '."- .....:~.. ._;.;, '~..::... ~.' '.~:..:batehes.

Most of the studies outlined above have relied primarily upon.\') ~!.... '.' . .;. .., . . . . .'. .:: -,: '

oocyte diameter d~strib~tionsa.s:~"i:ndex 'of the st~ge of. development of... ~. l' ... -,\.' .... ! _..... ...... .... • ••

oocytes but, although such data do provide some useful information, it is

also essential to examine histological material. In this wa:y,. the stage .";': .'. . .~

of development of individual.. oocy'~os:can be relateq to their size.

Chigirinsky (1970) found that, in Traehurus japonicus, the modes in

measurements of whole oocyte diameters did not closely correspond,:.~o:;.. ,;.l

stages.of·dovelopment aoindicatedby:examination of.histological mate-

ri~i·. -:. He. as~ri.ped .tW.s ·to. asynchronous 'grO~t~ of· oocytes, which"e~uses.; •.." . -.". . . - ... ~.. .... . . .. _.. ' ..... ."

ooctyes at a similar. stage ofdevelopment ,to have polymodal .diameter... .. . . '" ~',' ~ . .. .' ... ,;. .... ' ".' .~ .. ~"". ... ' ...distributions. .... . ,'. ;',' ... ' :.":, "~; <~ <.. '. ".~ :.. _.'::': ~

. Polonskii and Tor.mosova (1969) h~ve given values for.:the feeundity.• • ,'.', ....-. ,.'. t- ..... '

of Trachurus trachurus in the English C1?-armol,-_b~t.no,. de~aif.s <>.:f .. th~. !' .

me,thod are; givon. . j •• , .'

, :...~ ...'. ';"1. ...:..~" ~:'!

MATERIAL Alm: METHOnS , " '" : ,~ •.. t'

.< ,~e.. material .i~ derivedfrom,horse~~cke.relsampIes obtained from

the western English Channel over.the. perio~.,.1967,":,,69; there ,were.also ,a .

feWr~~~les from the ,:southcrn half oi'.~~he ,North Sea. . ':

":'_:Art~~ the stage/of maturity had 'been assessed,by eye; the ovaries. .- .... - . ... .' . . ..

were removed from tho fioh and weighed. A small.portion of,.the ovaxy.. ·

was eut off, weighed and plaeed in for.mol-biehromato in readiness for

histological preparation. Sections were cut at 10 I.l. m and stained m''':';<j

ironhaematoxylin: and eosin., " The: soctioned oocytos were measured at a.. '.' '. . . .

magnification:of.x 190 ... The remainde~ of.theov~ was placed .in .!;

Gilson ~ s ·~fixativeand:· periodically shaken, in omer..;to .release the "

oocy~es .from the.· eonneetive. tissue. The .. histologie.al. ma~erial wa~ ·used.

to._as~e.ss the stage .of.. develop;montl,9f .~he .. indiyiC!ual oocytes, and .their ...... ~---_.............. _ __ . . _ ~-. --: ." .

absolute numberwas·ostimated uoing,the·Gilson-fixed material.' In.order~..-:_-..::" :...:_ .....~ .• " ....R....._.:.._:. ,. ,"':.' ! . .. . .• . i ;;':"~~'" _.:,: "~R:"

to correct for possible, .differentiaL shrinkage resulting ·from. the wo,." ..... - :::..:..: ~.:._ _._ '"_ ._'. ",-;'.:: ~_.•_-.:.. .' , .. • *. . R"

methods"of pre.servatio~,.:a·comparison .. of m~an,C?o.cyte d~amet.er.. was..made

for .9.ocytes.· J,el:!S than::Q!.~ .. mm,.usi.ng :f..j.ve.: stago.:.4 (ripe) oyarios •. An.:.'.

analy~i~J.of;;y'~rianco .;.13h()-,~'!~li ..·:that ther~. was ,.:a si~fi~ant diffe~ence .. and .'.

the.mep,n ·of ~esection.~dmate~ial was, .on,'a;v.;n;age,,,:9o?J61e~.s~han th~t

'2

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of'the'Gilson-fix~d'material. This has been taken into account in the

resUlts'- "

, ' After trials using several methods, thenumber of oocytes in each

ovary was determined by taking a subsampIe 'from a known volume ina ­

be~{er, using aspring-loadcd Stempel pipette of 0.5 ml capacity;the

oocytes were firstly stirred vigorously with a largo spatula, using a

Itoand frol motion~ Because oocytes smaller than about 0.2 mm greatly

outnumbercd those of larger sizes, theywere'estimated separately. The

efficiency of sampling was tested by taking 10 replicates for each of

the two size categorics. For the smaller'oocytes (less than 0.2, mm),

thecoefficient of variation was 5.98%;' for'larger oocytes (0.2 mm and

,above), it was 6~58%l, In each subsampIe the oocytes were meas~ed using

• a micromäter(eyepieco at a magnification ofx,50 (1 eyepiece unit =0.02 mm). The' total number of oocytos 'in any 0.02 mm size-grotip could,

thus' be calculated as folIows: .. ':: ',":.:''l: .' V"W:

N = -=rnx-1'V if

, \" ,,'~' .... ;. ~ .:••' •• ", • -. <

", ..

where n = number of oocytes in subsampIe "

V ',': == "volume cf sa.mple, .

V1 - volume of subsampIe , ,

W' = weight of ",hole ovary,

W1 = 'weight cf portion 'of ovaryused in sampIe '

'(Le. the Gilson-fixed portion).

As a ruXther test,thc method was compared'with the one used at the

Lowest~ft laboratory for tho estimation of fecundityinherring (Burd

and Howlett 1969), in which a weighed subsampIe is taken. Ten replicate•estimates of the fecundity of one horse mackerel ovary weremade, using

both methods. The oocytes ifore first sieved'to exclude those below

about'O.36 nim, since it i9 tho largor oocytes which are most likely to

be tinderestimated by',the' prosent 'method. The meari estimate by the

weigh~'disubsanlple'method",as:87 690 (SE ± 674) and thatby thc volume

subsaInple mothodwas 84 14Ö"(SE ± 2 642). Thera 'is no statistically

signi:ticailt difference betioreen themeansof these estimates.

RESULTS

(a)' 'The gro"rth of the oocytes , . .~ ,

'. ~: :'.,'

The distributions of oocyte diametors per matlirity stage are shoWn

in Figures 1 ahd 2." Th'ö Gilson;"fixed ma,terialwas used for these,'',t.

3

. ,

measurements and each histogram is based on three....<?yaries. The. maturity

stages are as folIowo: 1 - immature (virg:1.n), 2 ~ mature but res~ing",

3 ":'" deyeloping (earl:,..), 4 - developing (late ), 5 ~. ,ripe, ;(hyn,lin9 ' eggs ) ,

6 - running, :7 -partially.spent, 8- ..spent, 9 - reco,;ering., . D~~a~?r.

stage 6 are not included in .tho figuros, since no running fomales.were, .' '. •••• t • ",. ... .: '.

obsorvcd in the saIDplcs. In Figure. 1, the ordinate iS,on,a.log scale ... '.: . .' .. ' ~.. .. ' .

to overcooe thcproblem of the great.d1sparity in nuobers between thc ". " -. ".

largo and small. oocytes. In Figure 2, the. same datafor.::tp.e smller. .: '

oocytesonly;.ar.eshown on:an arithmetic scale.. >. , ,., ,',

,. In immature or resting fish there are very few oocytes large~ than.' . ':. .... .

0.,1 mIn, and thisalso applies.to post-spawningmaturity stages.(8 and9)., •. -.< '. . .- ,-' .. , .. : ',.

~This suggcsts that the division between resting and developingoocytes .

.~~ ~tage 4 (~he stage ~t Which f~cundi~~S been est~~t~d).~~?~s ;a:~'. •about .0 •.1 .. rom, p'iameter. In Figure .1. the growth of the oocytes up. to .the

'. ... ., '.~: . - .' ". '. '. ....';' '. ..,

ripe stage (5) can be clearly seen•. 'There appear to 1?e at least three.~ '. • t . ' \.

batches of oocytes but the only clear one is the last bateh, which

comprises the hyaline oocytes.

(b) 'The criterion of oocyte development.

Some authors (o.g. Andreu 1954, Clark 1934, Rao 1967) have taken

the prosence or absence of yolk as their cri~erion of whether an oocyte

is resting or will develop in the current~eason. An earlier sign of

development, however, is the appearance of vacuoles in the 'cytoplasm,. .,I. .' ..~ 1 . •

caused by the presence of oil globules w1U:~ are removed in the histologi-

calprocessing (Fulton 1898) •. According to this author, SUCll vacuoles>. -' < • " .~ '.' • ..l. , , ..' ..,' ..• -. •. •

ar~Il~eseI1:t,.'.Qnl..y.~n those specie~.. ~n which the planktonic. eggsalso have •

. c.lil :~~o~ul.?s; however, this is not truefor ..the cod, for ~xample

(tvoodhcad .' and "loodhead 1965). , ...'~ ,~. ~.. .' . .' '.,

.+n th~ presen~ work, therefore,following Chigirinsky (~970), the.'. '.. .'... . .; , ..

criterion,of developmcnt was .taken to bo the presence of cy~oplasmic" . r' '. f' :. '. ,......" ,'. • ~ .... ' " '. -'

vacuoles •. Idcally, each ovary used for a fecundity determination should.•..': .•,',' ".'. .:.. ., ". '.' ' ':. ' . . ...... ..•••• ·..._'.1 '.' '.

be .oxamined. histologicallyandthe.praportion of developing oocytes .in ...lA J • , .'~..' \.... , • .' , '. ~", .... ;., •. ' .;, •• ". • ) - •.,. ~,~.

each ·sizo~hould be determined. ' However,. this would involve an ,enormoust, , •• I. ,. '" . . , '. • • : • ~.' .,. " ........ . . • • '. • • ' ••.•},' ~ :.' •~ .:.. .•~ .;, J • ;

number of meo.surements;. a simpler method, used in the present. work,. is; .....' I : .;, . ' ..:. - , ., . ' , •• ... , ~ . : .' : • .' '. ..: ' •. ' .,; •

to take mean values derived from axamination of a selected number of

ovaries. Table 1 gives the results of the examination of 12 stage. 4·>: :.1 ...... ',.,;

horse D:l.ckerol ovaries • They W'ere used as a 'pool' .fram 'which a .minimum, ',' , ... .:... .... ,~.... . ._', .~'.' ': .~~....:. :.. '. \. i

of 150 oocytes.in.ea"ch 0.02 mm size-class,wero classified as being~ ; .'. • f ~ '. .:..., '. .:, • • • • ;.1.' '. i ~,,' , ~..., ...','

vacuolated.or non-vacuolated~,.Fewer,oocytes were examined in size~ ", . '.;, . . '.. '.' ' ..~ .!.classes in which all oocytes ware of the same type. Only those oocytes

: 4

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•

which were seetioned through the nueleus were ineluded in the data.

Vacuoles first appeared in,oocy~es with a diameter of 0.08 mm, andat a

size of 0.16 mm all ooeytes had vaeuoles. Yolk droplets first appeared

in ooeytes measuring 0.24 mm.

':By applying the data in Table 1 to the quantitative estimates gi:ven

by, equation (1), an estimate of the total number of" developing ooeytes,

percvary eould be obtained.

The ooeyte size at whieh vaeuoles first appear is alsothe approxi­

mate size at whieh development ean first be detected in oocyte diameter

distributions (Figares 1 and 2). Further supporting evidenee for .the

use of vaeuolation as a eriterion of development is presented in

Figure 3(a), whieh shows "the proportion of vaeuolated,ooeytes bymonths,

per selected maturity stages (usually the most eommonstage). A maxi­

mum is reaehed in stage 4/5, after which the proportion falls rapidly.

New oocyt~s do not appear until stage 9, as is, the',ease ,in other fish',

species (Andreu and Pinto 1957, :Bowers and Holliday 1961"Woodhead and

Woodhead 1965), and' so the decrease in the proportion of ,vaeuolated, " ,

ooeytes must be due to an' absolute deerease.

(e) Resorption of ooeytes. ,

As in other fish species' (e.g. Woodhead and W06dhead 1965), both

pre-and post-ovulatory degeneration'of oocytes is obser..:red in thehorse

mackerel. Prior to spawning 'this oeeurs only in oocy'tes 'with advanced

yolk formation, but in latermaturfty stages less advaneed 'o'ocytes may'

also:be affeeted.

Figure 3(b) shows' the p~oportion,~'6f yolky ooeytes undergoing' ,~",

degellerat:ion~ by month arid selected maturity stage. In:'stage 4, the':;

'mean:poi'eentage was 6.31 (SE ±1.15, n= 22) and this is 'aiJ. estimat~ of:'

the e'xten't"of resorption in 'the most"advanced (first) bäteh of' oocyte's. '

No eorrectioll to a fecundity estimate would be:lleeded in 'ihis 'oase, ':,~'

since these ooeytes would almost eertainly disintegrate in Gilson's

fluid. However, some correction would be neeessary if this figure also :;

applied to subsequEmt batehes~'The estiinatetor stage 5 ovaries was

3.73% (SE ± 1.83, n= 11rbut tliis value earlnot be ascribed 'to:aspeci­

fic batch, since it has not beeIifotmd possible todetermine how'maiJybatches'have been 'spawned in~ pa;ücuiar ovary. Resorptiori"is, h~wever,

clearly more' vtll-iable' in: batches s~bsequent ;;0 the' first,',: sirtce'6f the

11 ~tage 5 ovaries 'examinecl, 7 had no resorbing' 'O'o6ytes, . whil~'t·,. in the

oth~rs, the'percentage ranged from 2.12" to 15. 55 ~ ., '.', .:: ..: ~. !

5

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(d) Feeundity

Tbc resultsof the determinn.t:lbn'of the number of develc/ping ooeytes

in stage 4 ovaries'are shoWn in FigUre 4. From the regressiorl1line, the

values range from 168 000 ooeytes in fish of 25 ~m'to 860 000 in fish of

38em length~ As is usual 'in fccundity studies, the vClriance is rather

large. Tbe presemt feeundity estimates 'are eomparable with those found, '

for the Blaek Sea horse mn.ekercl but eonsiderably higher than those found

forthree öther stocks (Table 2).

DISCUSSION:

Tbc estimates of the total number of dcvelopingoocytes shown in

FigUfO' 4 are best' describedas esti:inates of potentiaifecunditY~' sines

not-all"of the ooeytes wiil necesoarilybe shod. ' At the end of the. . .., f ."., '. . . '.. ',' "

spawning season, any developing ooeytes which have not maturod suf-

fiei~riÜy' tb be shed are resorbed. It is, however, extremely diffieult "

to quantlfy"this faetor, sinee degenern.ting'ooeyteswould almost 'eer~ . ,,':; .

tainly Ciisintegrate in Gilson's or ~ other preservingfluid.· Judgi!~g",

by histological seetions, howeverr the numberof ooeyteswhich ~re': ,';

resorbed is eomparatively small. " _,_,,';'-',

'It may :be, as Chigirinsky (1970) suggests, ,.that the number of" .

ooeytes matured in a .partieular season dopends upon the environmental- ,'::

eonditions in that· season,:so,that, for a givcn size of fish, the aetual­

feeundity might varyfrom' year to year.·' Tbis meehanism would' eonstitute

a more flexible eontrol of foeundity than oceurs in species such as the

herring. , Here,.-the fecundity.appears to be fixed immediately after

spawning in the, previous year,(Bowors and"Holliday 1961)" and 0.1though

this is 'probably also'the situation, withinlimits, in the horsemaekerel,

there would :appen.r. to be a further~, Inter, control, ,brought about .by. ,

asynchronousoocyte developmont and the phenomenon of resorption. , ..

SUMMARY , 1 • , '

1 Inthe'ripe ovn.ry of·thc'horse maekerel there.is'a continuous.grada-

tion in oocyte, size,' so;.that thereis no' elear size .separation

.between restingand dovelopingoocytes. ..

2 ,Tho eriterio~,of development istaken to bevaeuolation of theccyto-'

, . plasm of th~ J oocyte., Tbc proportion of, vacuolated' ooeytes -in. each'"

" .size~class·is determined histologically and ·the,' results"are applied

to quanti tntive estimatcs made .by subso.mpling oocytes': preserved in '

Gilson's fluid.

6

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•

f-- ---~----- --- -

•

3 Both pre~ and post-oVulatory resorption o~'ooeytes oeeurs. The

effect on fecundi ty. estimates: is diseussed•.".. ..,.,

4The numb_~r of developing ooeytes in 35 stage 4 ovaries was deter­

mined~ Values ranged,from .168.000 ooeytes ~or'a fish length öf·

25 em to 860 000 oocytes for a fi~h lerigth o~ 38 em.,'5' , It i8 suggested' that· thenumber of ooeytes aetuall;Y released in' a

..particular year may depend upon environmental eonditionsin that

year.

REFERENCES.

ANDREU" B., 1955.', .The sexuality o~ sardines. Proe. teeh. Pap. gen.

Fish. COlll1. I'Iedit~~r. " No. 3 ~'. 45-60.

. .ANDREU, B. and pnrro, J. S., 1957. Caracteristieas histologieas y

biometrieas deI ovario de sardina {Sardina pilchardtis Walb.)

en la maduraeion, puesta y recuperacion. Origen de los:~Ovocitos•

.:Notas Estud.'.Inst. BioI. mar., Lisb., Tomo VI, No. 17, 21::pp~;· ,­

BOWERS;A. B. and'~QLL+DAY, y.G. T., 1961. Histologicalehanges in the

gonad assoeiated with .the '. reproduetive eycle of the' herring

(Clupeaharengus L.). Mar. Res., No.' 5,16 pp.

BURD, A. C. and HOvffiETT, G. J., 1969. English herring fisheries:

feclll1dity studies. ICES Pelagic Fish (N) Committee, C.M. 1969,

H:28 (mimeo).

BURD, A. C. and HO'tlLETT, G. J., 1971. Feeundity of Bank and Downs

herring. ICES Pelagic Fish (U) Committee, C.N. 1971, H:18.

(mimeo) •

• CHIGIRINSKY, A. I., 1970. Tbc nature of oogenesis and feelll1dity in the

Japanese horse maekerel Traehurus .iaponicus (Temminek and

Schlegel). Vop. Ikhtiol., jQ (6), 1005-1011.

CLARK, F. N., 1934. Maturity of thc Californi~ sardine (Sardina

eaerulea) determined by ova diameter measurements. Fish. Bull.

Calif., No. 42.

FULTON, T. S., 1898. On the growth and maturation of the ovarian eggs

of teleostean ~ishes. Rep. Fishcry Bd SeotI., no. 16, Part 3,

Sei. Invest., 88-124.

HICKLING, C. F. and RUTENBERG , E., 1936. The ovary as an indieator of

the spawning period in fishes. J. mar. biol. Ass. U.K., ~,

-311-317.

7

•

KOMAROV', Yu.,A., 1964.' Some data on. the, reproduetion of" the 'horse mackerel

f'rom the south-west Mrican' ,coast. "Trudy atlant. nauchIio-issled.

Inst. morsk.ryb. Khoz. Okeanogr., VYP" 11, 87-99L!n Russi~.,

MACGREGOR, J. S.~' 1957.. FeClUldity of' thePacif'ic sardine. Fishery Bulle

Fish Wildle Servo U.S., 2I (121), 427~448.

PETROVA, E. G., 1960. On the feclUldity and maturation of'the Baltic sprat.

Trudy 'vses. nauchno-issled.Inst. morsk. ,ryb.Khoz. Okeanogr.,

.ß, 99-108 fJ.n Russi8:!i7.

POLONSKII, A. S. and TORMOSOVA, I. D., 1969. The spawning of' the horse

mackerel of the north-east Atlantic and the distribution:ofj,its'

eggs and larvae., Trudy atlant. nauchno-issled. Inst., ryb.', Khoz.

Okeanogr., Vyp. 23, 27-48 lJn Russia:il.

BAO, V. R.,,;·1967. Spawning behaviour and feclUldity of the Indian mackereI,

Rastrelliger kanagurta (Cuvier), at }~galore•• , Indian J. Fish.,

14, 171-186."

vlOODHEAD, A. D. and \.,rOODEElill, P. M. J., 1965. Seasonal changes in the

physiologyof' the Barents Seacod, Gadus morhuaL., in relation

to its environment. I. End~crine changes particularly affecting

migration and maturation. Spec. Publs int. Commn mlAtlant.

Fish., No. 6, 692-715.

',:. :~ ... "

•

•: ,.:

.. '. ! '.

~ ..'..~..~,~

,)',"

\ ....

, ,

, ~;, 1/,'

'[.

... ~,.. '

,8

. ',:' .'." " ~ '.

.. ....\.' ".'

... _" Table.1 .The ..propor.tion of oooytes, in. histological sections .. of.s.tage. 4 horsemackerel. ovaries.__ "which have vacuoles in the cytoplasm. The diameters have been corrected to the ~quiva­

lent measurements obtained'from Gilson-fixed material (see'text)

,'. :

Oocyte diameter (nun)-.

0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0; 18 Ö~2Ö+~'" .'\ : ".

--

Vacuoles absent 47 165 224 267 148 70 19 6 0.'

0• _. __.OM ....~ ....- -. \' " ....... .- . ". _..... .-.... - .. - •...~

Vacuoles present 0 0 0 2 17 83 133 114 59 115.-

%with vacuoles 0 0 0 0.74 , '10.30 54.25 87.50 100 ' 100 100. 'l.-&4

....... " _.__., -.....

Table 2 Previous data on the fecundity of Trachurus compared with the present data

Species

T. japonicus

!. mediterraneus ponticus

T. traehurus

Area

East China Sea

Elack Sea

Off west coast of Africa. ..

Feeundity(thousands)

48.5-410.4

e. 500

50-170

Size range(em)

22.0-40.0

c. 35

22-38

~\uthor...

Chigirinsky (1970)••• A' _

Saf'yanova and Revina (1960) ­quoted by Chigirinsky (1970)

....-Komaröv'( 1964) "..

_. . 21~29. ... ._.)?.()lop.f3~~! .. ~d T()rmosova (1969)!. traehurus

T. trachurus

.EnglishChannel~North,Sea " ." ._I~::209

Western :J1,nglishChannel ""168:"785 25-37 .• Present data

50·010·01·0

0·1

50·010·0

1·0

0·1

50·010·0

1 .0

0·1

• ~ 50·0a. 10.0E0 1·0lf)

0·1c

~ 50·0Cl 10.00.....

1·0c~u 0·1L~0-

50·010"01 ·0

0·1

50·010·0

1·0

0·1

50010·0

1 ·00·1

0·02 0·1 0·2 0·3 0 4 05 0·6 0·7Oocyte diameter (mm)

0·8 0·9 1·0

Figure 1 Frequency distributions (on a log scale) of oocyte diameter in the horsemackerel. Each histogram is based on measurements from three ovariesfixed in Gilson 's fluid. The number above each histogram indicates thematurity stage. The vertical broken lines are for reference.

40 (1) 25 (5 )

30

20

10

o

20

15

10

5

o

30

20

10

Cl>

0. 0Eoeil

20c

Cl>g15~

cCl>u~ 10

(l...

5

o30

30

20

10

o

50

40

30

20

10

o30

(8)

•

•

o 3

(9)

02

15

10

5

25

20

(4 )

o002 0·1

15

10

5

25

20

o02 0·3 002 01Oocyte dlometer (m m )

Figure 2 Frequency distributions (on an :l rithmctic scale) of ooc~'te diameter in thehorse mackerei, using the same data as in Figure 1. Oocytes larger than0.3 mm are not -shown. The vertical broken lines are for refercnce.

+ indicates less than 0.3%.

8 9920--0--0-0II,

I,III,

:17

(b)

II

II

4 :p: 74 X 4 I

3 3 "N ....~ ..:n:_ 5/---'c r - 6/:± I ±--'ti..L-.....LI---I_.A--~~--I

J F M A M J JAS 0 N 0Months

100

20

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0...

3

(0)

7,~7

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0III,,,,

I,

\\\\\\,\

OL..-.....L.-......A.--.L.----I.-.L..-~__'____L.._____l~l}_...J.--J

JFMAMJJASONDMonths

30

10

&20o+ICQ)ULQ)

0...

Figure 3 (a) Tbc proportion of non-yolky oocytes which contain cytoplasmic vacuoles.(b) Tbc proportion of oocytes with advanced yolk formation which are

undcrgoing resorption+.Each point is based on histologicaI seetions of three ovaries, and shows themean ± one standard error. The number adjacent to each point indicatesthc maturity stage.+for stage 3, in which thcre is no advanced yolk formation, all oocytes withyolk were counted.

7

1·0lf)

co

E 0·8

lf)Q)+J~

g 0·6oClC0-

o 0.4Q)

>Q)

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o

130 cm 135cm

o

138cm

10 20 30 40 50

The cube of body length (cm x 10-3 )

Figure 4 The number of developing oocytes related to the cube of the body lengthin stage 4 horse mackerel ovaries. A line fitted by regression und n5%confidence limits are shown.

60