CM_1976_F_33.pdf (600.9Kb)

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.I This paper not to be cited without prior reference to the authors

International Council for the Exploration of the Sea

C.M.1976/F:33

Demersal Fish (Northern) Committee

t·io~fw,t

Changes in buoyancy and activity during starvation of cod larvae (Gadus morhua L. ).

by S. Tilsethx

) and T. Strf21mmexx )

l!:..."

^)duction

x)

In 1914 Hjort stated his hypothesis about the critical period in the early

.

life· history of fish larvae. Hjort (1914, 1926) established that the year- class strength of Norwegian herring and cod stocks varied widely and that it was determined early in the larval life-history. According to . Hjort one of the main factors affecting survival through this stage was

the lack of food at the time larvae began to feed.

It is perhaps surprising how little is known about the ecology of .cod larvae. This is apparently due to the considerable difficulties of rearing cod larvae in the laboratory. These problems are still not solved.

Fish larvae will reach a state of starvation shortly after yolk re sorption where they will inevitably die even if food suddenly becomes available.

This "point-of-no return" (PNR) is described by B1axter & Hempe1 (1961) where 50 per cent of the larval population are too weak to feed if food becomes available. The fish larvae may live for a long period beyond the PNR and even exhibit feeding behaviour after they have passed a point in time when they are irreversibly starved (Lasker eta!. .• 1970). Ecologically the point of irreversible starvation is a more important time to establish than the moment of death.

Institute of Marine Research Directorate of Fisherie s 5031 Bergen - Nordnes TToTW8 .. y

xx) Department of Fisheries Biology Univer sity of Bergen.

- 2 -

The determination of the PNR is based on measuring mortality rates of fish larvae deprived of food for varying lengths of time, (Lasket, et aL, 1970). This obviously requires successful rearing of the larvae involved. However, Blaxter & Ehrlich (1974) have described a method where the rearing

problems can be avoided and thePNR can still be determined. They found it possible to do this by studying the changes in activity and b 1l9yancy of herring and plaice larvae during starvation.

In this report we describe the changes in activity and buoyancy of

the larval cod, Gadus morhua L., from hatching to subsequent de~th from

. ^~. .

starvation in an attempt to determine the PNR. The observations will be combined with results of field studies on cod larvae. see (F:34)

this meeting.

MATERIALS AND METHODS

Materials

Artificially fertilized eggs of Arcto-Norwegian cod were obtained in March from the Lofoten area. Eggs from different females were kept separate and were sent by air to the Institute of Marine Research, Bergen, the day after fertilization.

Incubation

The fertilized eggs were incubated in perspex cylinders 15 cm in diameter and 22 cm deep. The incubators were placed in each of five therrnostats.

Before entering the cylinders the sea water was filtered through 50 - and 10

r

^filters. The gas surplus was removed under atmospheric pressure in a horizontal tube. The water then entered and left the incubator

cylinder s through siphons (see Fig s. 1 and 2). The temperature \vas kept constant at 50 C, and throughout the experiment the salinity was 34 - 34. 7°,;'00.

!3~'?Y.§l!l_cy

Thirteen 250 m.1 glass cylinders containing sea water graded ill 0,5⁰/ 0 0

salinity steps from 28 to 34.5^{0/ 0 0} were placed in a thermostat with a constant temperature of SoC. The salinities \vere made up by adding

"

\

i

- 3 -

FILTERS

~~

WATER . INLET

/!CA- ~ oJi\-~i ^: ~ ^{: c;--.}

< I

!

^{I n I • • _ •}

Fig. 1.

Fig. 2.

THE INCUBATOR SYSTEM

• OVERFLOW WATER OUTLET

A E

i~

.0, _

.0' - _ ,_0, _

I

Q)

^{o -}

Cross section of part of a therrnostat containing one incubation cylinder. The system for regulating the water flow is also shown.

A: Inlet level control. B: Air hole regulating the outflow level.

C: Air tube. D: Bottom sieve. E: Airlift.

I

- 4 -

distilled water to sea water of 34.7^{0/ 0 0} salinity. Thirty larvae were anaesthetized in 1: 20 000 MS 222, and 10 larvae were transferred to each of three cylinders. In all instances the larvae were rinsed in the same salinity as that of the exp.erimental cylinder. The neutral buoyancy of the larvae was as sessed according to the method of Solemdal (1971).

~5!~v5!l

The response of 50 larvae to different light intensitie s was observed in a vertical per spex tube 130 cm high and 15 cm in diameter. The vertical tube was contained in a light-proof observation chamber over which was placed an artificial light source with two lamps. The intensity of one of these lamps, a 1000 W halogen lamp, could be varied by an adjustable auto transformer (0 - 220 V). The other lamp was a 60 W tungsten lamp with an infrared filter. The light intensity was measured at the bottom of the tube.

The tube was protected against the light source and the surrounding air by a perspex cooling jacket. In darkness the larvae were observed by an infrared sensitive television camera (Fig. 3).

Fig. 3.

[-~

/

Tr

³

4

tif51

2

Outline of the observation chamber

Infrared sensitive television camera. 2 Camera stand.

1 3 6

Cooling jackets.

Light source.

4 Activity chamber. 5 Photometer.

",' (} ^'...,

- s -

(1 f

, ' ,

/\

\

The

larJ~l

reactions to changes in light

int4/~i~~\

was observed after

a~~p-

\~ / i \~ I

tion to l~" hours of darkness, when th~I'Pg~,y was \~\~owly increased fro~

o

to 80

O~.,o

lux followed by a slow

rie~ucbi9h

to 109°" lux. The amount

~t

I I f I \ \

larvae sh~\wing swimming behaviour;/;Associated with, ~earching for food ¥Vas observed \dr,. 1 S.

~l'U_i~utes

^after,

;t~!/i'our

^S

a~aPti.on t(\l~"~t

(1000 lUX)

1st

(see Table

'.,~).

^The

\~arval

/fCt!Vlt! and sWlmmlllg

beh~,~~~r

was obserled daily from ha"khing to· su,bisequen. t/ death from starvation:"" '~ r

\ ~ I ... ~ ^--c:

" • ' / ''<\

r

^{r f ::!}

RESULTS '

, i / ' \ I ~

J ~

\.. I-Of.

\" I

I!!'!:!~Y~::~Y \ '>',

I

\\ i

Changes in neutral buoyancy of cod larvae from two different

fe~~es ff~'m

hatching through the yolk- sac

~tage

to subsequent death from

starv~Von l_~:e

shown in Fig. 4. After hatching there was a steady increase in the spJcific gravity of the larvae. The larvae were neutrally buoyant at about 28⁰/bo s alini ty"at-,ha'tehing',' ^C' arl:ci·' they,wer e -heavie

s~c'a

tyo-lk-resnrp tie> n''V'There-'th'e11 we r e

Si (I( ,';,! \! 0 Ilr " () ,\ , [)

neutrally buoyant at about 34. S /00 salinityl'. After the period of yolk re-

,:JHIH 'lAi~ 'j;':JF'-i/\ !~Yl\U (~Y3

sorption the specific gravity decreased and reached a minimum about 13-14 days after hatching where the larvae were neutrally buoyant at about 31-30^0/00

saliRlt~fLi (\Vli'~J nttiJ11if:~J~d.bl~{2;) :di~[J J{br\fb{fE~fit1ife, IfX;I!~(ifidti ~;i'J.~i't6.d~ cre~sear;r

Y..,() jJ,.., .. 1"L"""f"\~ ).-..J'\"-I~J • . . ""is: , 1 . , ,i " , , " " , " ' cr')"B ,., . , . - "''''''-'''''''''''' P'Hl,,\lJ~"lcl .. ,'t.~.,~._ "_,J",, ^J.yffj'tl ~". , . l . , "l~,}l~ ".:.1, .• -

again and as the larvae died .osrrlo:ee,g:ulatilblID.")p'rJdBably!:t£G(;bli~clt a,1J::itll':1:ttpley were negatively buoyant in 34.7⁰/ 0 0 salinity.

~5!~,,-i!y 1

^{001 '"}

(r

^<,)

The

percentag~'\

of

larva~/a~

the surface after 14 hours adaption to!

ark~ess

from hatching t1trough i~he P'e~,io.d"'~,f yolk resorption and death fror .sta{ya- tion, is shown i4 Fig',/S. At hatchi'1\g, 100 per cent of the larvar j,ere ::'at the surface.

On~

dat after hatching\the number of larvae at

tl;~

^{s rfack}

I '

decreased,

reachi~g r

minimum of

le~

than five per cent or;,Ahe _

:~

day

a~ter

hatching,\ JFrom

th~S

^period

(i)n~d,~he,

percentage

in/reasedtunt~~

the thuteenth day

~ter

hatchmg,

whe~ more,.~~an

80 per cint of tht

la~~ae

were found at the ,Stlr,face after a penod of 14 't..).ours adaptlOn to dar'kne~s,

On the fourteenth /day _I

~~,ere

_, was a significant

ch:~~g,y,

_'. as ,.the larvae _I

~ied _I ^r.

_~t, ⁱ

and sank

toth,_e.:~pttpm_, ~f:,(the_,j;uJ?,e.j~igr'_.,5'fd?-tt.e,dl~u'~)~"_,,Tlu;.e.e,.,.da.yt~,

^{Idee r}

;'q ,~I I ~; { 0; 8. a ^{~ 0}

all the larvae were dead. ^1\

(J (/1 ~ ~! 'j Y ,r\ \-~ ,r;{ :) T "~~ e v ;\ (] .~~ V .~J

g (1 i,rb j r;,d' ( C l ' 't I);~ ,) d I ,\ "~ ') ,t:,V! ,h) [:.0:) V"I.I),t B 't,o !) gnj nC':I;) "19

er

^,r;

^,gn:

~JJ~'''''fb[ bCCib.u ;', inrll,;<'J ,r'o!,LS,Il"LGJn rno'I) fUB~1b JUnu ,,){lu'\ :HU 110

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..

3

34

33

32

>-

!:: 31

; j ~

<t

III

30

2

28

o

Fig. 4.

w u

<t LL 0:

::l III

w :r

I-

~ w <t

>

0: <t -' LL o

100

SO

6 ..

I I I I I I I I

I I

,

^I

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'I

/ /

,

/ /

'I I /

2 (

I I I I I

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4 /

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,

I I I I

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6

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8 t

EYS

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~ I

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\.... ___ ... ...._ ... _ .... - - - - 0 /

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10 12 14 16 18

DAYS AFTER HATCH I NG

Changes in neutral bouyancy in the cod larvae death from starvation and --- larvae from two different female fish.

from hatching to are based on

/ •

\ /

' \ 0... /

/ 0 /

/0_0 \ /

\ I \ /

\. _{\ 0_0} / \/

0

\ // /1\

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w Cl

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Fig. 5.

2 4 6 8

EYS

+

10 12 14 16

DAYS AFTER HATCHING

Percentage of starving cod larvae at the surface from hatching until death from starvation. --- Percentage of dead larvae On the tube.

-7-·,·d,·,

The percentage of larvae at the surface (0 - 5 cm), of t~~~C!l~~~8-!.1~J?.~~~~

the bottom (125 - 130 cm) of the tube after two hours adaption to light at 1·00.:0.)

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During the first 48 hours of larval life the cod larvae exhibited little loco- motory activity. Most of the time they floated motionless at the surface.

At the end of the second day their activity increased. Three different swimming patterns were observed. The distinctive feature of two of these patterns was that the larvae executed a brief but intense burst of swimming where the tail and body beat from side to side. This burst of swimming lasted for less than one second. The frequency of the two intermittent swimming patterns was different. The highest frequency was associated with active swimming up and down the water column in response to changes in light intensity. The lowest frequency was observed when the larvae

started showing feeding behaviour, and was closely as sociated with larvae searching fo'r food. The frequency typical for these swimming patterns is shown in Table 1. The third characteristic swimming pattern was an avoidance or escape reaction. The larvae swam continuously for several seconds. This reaction was strongly stimulated when tapping the wall of the tube, or when the larvae accidentially collided with the wall or other larvae.

Table 1. Differences in frequencies during intermittent swimming;

Age in days

3 4 9

I: Swimming associated with responses to changes in light intensities, II: Swimming as sociated with feeding behaviour.

I II

Frequency of bursts/min

Frequency of bursts/min

mean

+ -

^{, -}SD N mean

+ _-

SD N 40.2

+

^{0.18 11 27.3}

+

0.27 6

- -

41. 1

+ -

^{O. 19 13 28.2}

+ -

0.26 9 40.5

+

^0.17 9 26.4

+

^0.24

l z J

- ^-

The larval activity in the dark was studied by using infrared light and an infrared sensitive television camera. After 8 hour s adaption to light the intensity was slowly reduced to 0 lux and the I R - light was turned on.

The larval swimming activity was significantly reduced and as the larvae were positively buoyant in 34.7⁰/00 salinity sea water, they slowly' rose to

- 9 - '.1:

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o

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actively towards the surface. This was used to test the response o!.:t~~y,., ~l(lj

larvae to changes in light intensity (Table 2).

\',rih (\;.' .. 1 .I,.!,:: i I:'" I';] :-,1) . . ),;{ 'J~if ~'~,li.'!ji':)' '-id T~W'i??-an&es • ~ • _ " _ in the larval locomotory , ~c.tiyity;, an~dfe,~,:p0rt\SEii9.'it9 Shfir'~i~An",ri':~ , .., , . _ J ~ , • _ _ • ' : \ • "L . . , ,.

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'.. ' " ' , " ) - - - - , - .

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Jrpn;:t

sta.rY,if,tt9R,.,f,

.. _~,f . .f.,-"t.,.), " , I " " \ I . , I , ' " .i,~"_,_,, , . I . " , \ . , , \ . "., . ' , . " ' " ,~ , \ ,1,.,,(,:.,) 1 / , ) \

" c l ! , ! ^{i>1 ~~., ,': ~" ,} r\) "_',:'!}"} :-Id-^l n,()

Table 2. The activity of 50 cod ~frv<3:~ )rolll. 1)A:t~Nn'~)H,:s'7~~yquE).q,t'i f~lec;p(\fJ;!

}rOlll sta,rvation: showing I:,/Th~('p~rc~ptag~ (!9f ~);a~f~~', ;P,1r{?r,p/;'jt.~Hr intermittent swimming associated with responses to changes in light intensity and II: Intermittent swimming associated ;~~tl:l''':';':'I(i feeding behaviour. The percentage of passive unspecified larvae

;~-; .' ) . 1 i also includes larvae in the .. ^llhe,ad ,doy.rA" .. Po,sition.

J t r " ! ^{j \} " " " .1.., " . ~ ^f J ~ ')'~ :-,l,:',f'

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, , I, ,~ , :' Activity Pas:slve(,L, " l , ' I .. : I.~\ ,:

~E;' ;'l.h ^{11.), "} ~:'( \/ tL-' ^1.4 1..,.

Per cent Per cent ^{;" \}

. . . f_

.. ' J.un j

Ag'eJirt, " intermittent intermittent, 'un speeified,~ 11,,1·( 'Head"

P'ead

^{,,' ,} "r'-'1

D?-.ys:),.

v,

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10 ,. ,

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- 10 -

At hatching the cod larvae did not re spond to changes in light intensity.

After 14 hours adaption to darkness the light was slowly increased from

o

to 80 000 lux and all the larvae then floated more or less motionless at the surface.

The activity of the larvae increased from the second day to the fourth day after hatching when more than 90 per cent of the larvae showed intermittent

swimming associated with feeding behaviour. Swimming activity was high throughout the period of yolk re sorption until the eleventh day after hatching.

On the thirteenth day a marked reduction in activity occurred and the first three larvae were observed in the "head down" position (described by

Blaxter & Ehrlich, 1974). Four days later all 50 larvae were dead.

Discussion

The exact determination of the "point of no return" presupposes a success- ful rearing of the larvae through the critical first feeding stages. This has not yet been accomplished for cod larvae. However, changes observed in the cod larval activity and neutral buoyancy during starvation could

indicate the PNR as observed for herring and plaice larvae by Blaxter and Ehrlich, (1974). The changes in neutral buoyancy of cod larvae living on their yolk reserves are very similar to those of plaice larvae observed by Blaxter & Ehrlich (1974). During starvation there was a decrease in the specific gravity of the cod larvae probably due to an increase in water content and protein catabolism. This change in specific gravity increased significantly from the ninth day after hatching, reaching a minimum on the thirteenth to fourteenth day.

The change in activity, i. e. the larval ability to respond to changes in light intensity, decreased from the eleventh day and dropped to less than 50 per

cent from the thirteenth to the fourteenth day. At this stage the larvae were truly moribund because from the thirteenth to fourteenth day onwards the specific gravity increased again and the fish larvae in the "head down"

position were observed. Marine fish larvae are known to be hypotonic to the surrounding sea water (Blaxter, 1969). The increase in specific gravity at this stage was probably due to breakdown in osmoregulation and the

following increase of salt in the body fluid leading to osmotic equiliribrim

- 11 -

with the surrounding sea water. At _~ ^r th~Sf ,sta-ge- ithe cod larvae had most

,' •• 1, \ "j ~ __ t.." ^{',!. -_} . , ' , , - • ~

"'-,>'=.-_" r.~ .. ~h" __ ~LT.C'.~~_,, .. ""~_~.""~-.... .-,=_

probably passed the PNR and had obviously reached a state of irreversible starvation, because three days later all larvae were dead.

il-<,d' (i 1'-,1 2 ^{[\!,' { ,",.- ,IJ r} I:,;l(n'!l:;JT-T,~, J),

r;

^{T .H3TXA~Hi}

If nQ! ,;fQQ~rbeeomes available the- b)(l la:J3Va~?m1iisrtp(rEfJguinla:bl't"l-~e£:dPI:he ^PNR

ei!g'h't---1;~"-t';~-l~eda y~'

after hatching ( at 5⁰ C). The PNRf)

1

s

]l;ii-''6

baldly clo se to the twelfth day after hatching, because there was a significant drop in

. . . . . . . . -._. _., n ^,040 r -,'/ ^T,[

r

~r:';+T"< \ rH sWlmmmg actlvlty asso.clated",wlth feedlhgr!be%.:av~m!r u{om ab'oU1:' 60 "p-er-'d~ITt-

on the eleventh day to about 40 pe'r' :ceri'f bn.ltheEL1h~.t.};{fnHcda¥Xi.~, '-IT

, t ~., ^{(,' '; )} j !.' f \ rt,fi fl ,t, (J .h::;; /;ol-i. ,~'~ ~ \ff

(d: ;'1 ^{.I', ,,;'> j l} " A'l' <J '( 'h'

c,

^-[IT

SUMMAS Y:I ^{j , '. i} ,! '\ ^{I , ,','} " ^h"'C) ". ^-;-1':1' . '._" " I " •. \,_L P-"IT)' _'J{..lL . . <;rT.T'>] r:" Li'f.l.fI ^".oi I') ,.,,~ d ^{I > L} T \./.J.':J.~ ~\ (T'''1'~[ . .e-\,'._}..~. '" r i·l- nf .g8\·,(::'\',1 crq ^{o r \} t't V ';. i~-)! ' ) , ; j ,f' ~ -tf h \ £.1'.' 51n_fTJcHi 10 o:oj-j,(;V'U-;:)S gni:''fffb

• • • -,' ' , . - . , I T Lh" \ ::~'~l T '[:1"<'I'v,oIH •

I .. , <P92?--:',J~r\yae ~wlllg" <?11:¹"theJ..r yol~\C;J,"~($:~x~~teJ:), 1j1flOWea

'a

'steauY"l'nC1:ease ^III specific gravity towa.rds the end of the 'Yblikl\~adf iJfa~b,'

Du:?ih

g incipient starvation there was a progressive decrease when the larvae became

~K.}lly'.1i:lQr-ibundand sank to thebotto'm. "!.lA, fe-w~day~'-llatei-

:£1'1

l~rv~"JTOU-I

wC?l!' ~.:-:,<:l~a,d . ^{f '_~ J '~I;} f-,n'<:J'C-}i- ~

, 'f'.:, \~ -.., .. \" I ,'. !~, ^t -.! :,-,-, ' /

2. The change in activity was closely related to the oh\&n;ges in;!:~pecific gravity.

fJ ()(' i j" i,:~h: ;:"_1 -: \" 'I' .\.:" ^{~,d: CH} H[WUFHJ:),ul;' ,,():,~JJ. ,T, ,'1'):1.0[,1-1 . 3, Cod larya~ responded ,to changes ih,light-iht~miit)i?Qire to'twb~jdays after

hatching. During the period of high activity the larvae did not show a;gt,i(V,~ y:.e17~ic~l( mig:r,;ation towatt'd-s ,the Lsutfao.e '\wlI~n IH~:6Vld~creftsed!,2.D:i?,IkI

b8 ~.He,~ 'wdaptio:-n to l~h()u~!3 light. There'-:was'; a'LfeductiOri,;iw~'ci~tivity in darkness an d the position_ 6f the larvak~:in the, water

c'diumb

was dependent upon their buoyancy.

:t fl;'J . J ~) 'lJ . h '~:I} ; , , ) . 't (. !" 1', ,-I ~ I,~.' If' £.0 l:l -:) .. ~~ CJ , f. '\' ",.. ~ c~r

:

\.1 0-(1

4. The til'1}!l~~~I_;r.~~ch'ithe PNR wasasSes'sed ff'orh:itHtf clfal~ges;

Hi

buoyancy and activity and was determined to occur-'

clO's'e

to'

Hie

eleVenth day after hatching or three days after yolk re sorption (at soC).

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,f REFERENCES

-I,

(\

I rJ ^{I' ; ,;}

»/

BLAXTER, J. H. S. & HEMPEL, G. 1961. Biologische Beobachtungen bei del' Aufzucht von Heringsbrut. Helgoland. wiss. Meeresunters/, : ⁱ

7: 260 - 283. ^{" { '~ (}

\"i~

r ~ , I " " i ' 1\

BLAXTER, J. H. S. 1969. Development: Eggs and, ,larvae. .;>!, \"

--' LL~.-; .,i " ,<', '

In: Fish Physiology 3, 177 -: 252. ^{>'1 { ' I ! } ;;:,:}

W. S. Hoar and D. J. Randall (eds.).

New York - London: Academic press, 485 pp.

BLAXTER, J. H. S. and EHRLICH, K. F, 1974. Changes in behaviout.: i/ :!\

during starvation of h.erring and plaice larvae. pp 575- 788, in . Blaxter, J. H. S. (ed) The. early lite history of fish. Springer-

l.'< '/: . ^.1,',

i ,.I! Verlag, Berlin, 765 pp ..

HJO~T, J. 1914. Fluctuations in the great fisheries of northern Europe viewed in the light of biological research. Rapport Proc.ess Verbeaux Reunions Conseil Perm. Intern. Exploration Mer,

; ( I : :.t.Q.: 1 - 22.8. ^{! (}

HJOR T, J. 1926.

:,-!,: fishes.

Fluctuations in the year clas ses of important food

;r

Conseil Intern. Exploration Mer',:ill,:!-Si- 38.

). ^{• t} .l '.' . ~, I

LASKE~ •. ,R •.• FEDER, ,HIM., THEILACKER, G.H.'. &,;MAYj <R.C;19!70.

\ ' , } " , ' -.' '- .'

, i Feeding, growth and survival of :g;ngraulis mordax ·la;t'vaereared .in.t:p.e laboratory. Mar. biol., 5: .345.-,353.

, i i i

SOLEMDAL, P. 1971. Prespawning flounder s tranferred to different

'1 . . 1 • !3alinities and the effects on their eggs. Vie et milieu, '.i ^{) \ .}

;- supp!., 22(1): 409 - 423.