,j-

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Not 10 hc ci/ed witllOut prior reference to the authoT

International Council for theExploration ofthe Sea

CM 1997/GG:06 ,Trophic Relationships (GG),

ECOLOGY OF BALTIC COO FEEOTNG AND ITS PLACE IN THE TROPHIC SYSTEM, OF THE BALTIC SEA

by

Patokina F.A., Kalinina N.A.. .'

Atlantic Scientifie Research Institute ofMarine Fisheries and Oceanography(AtlantNIRO) , 5Dm,Donskoy St., Kaliningrad 236000 Russia

ABSTRACT

Food eomposition of the Eastern Baltie eod population and its spatial and temporalvariations at difTcrcnt stages of ontogenesis was researched during spring 1992-1997 in theRussian Fcderation zone. Interannual variations of eod feeding werc eonsidered using average ,long-tenn published data from 1963 to 1990 und original data for 1992-,1997. Diurnalrythmic5 of eod fecding 'was rcvca1ed. Thc analysis of eod food relations was earried out atvarious stages of ontogenesis. A single instantaneous food eonsumption (SIC)by eodamounted to 246't in 1995~ 283 t in 1996 and 142 t in 1997. In spring 1996 eod eonsumed 60thous. t of food. About 70% of food consisted ofsprat and Baltie herring. '

TNTRODUCTION

, • At present eod is abundant commereial species of the' Baltie Sea from near-bottomfish eommunity. Adult eod population is predator - ehthyophage, consuming such importantcommercial species ofthe Ballie Sea as sprat and Baltic herring..

, .'. The purpose ofthis work is to identifY faetors forming the bulk of eod biomass on the

basis of study of it5 feeding eeology and to spceify eod position in the trophic chain of neritie

zone in the basin studied.

MATERIAL AND METHODS

. SampIes offish food eot'lected at RV "AtlantNIRO" in Mareh-April1997 (1470 ind),as well as data on eod feedingin spring 1992-1996 were uscd as material in our rcsearehes.Trawlings werc pcrformed by the bottom trawl HAKE-4m ",ith minimum fish patch of 10

. mm. Sampling and processing were rerformed using standard methods(Shorygin, 1952). CodsampIes processing was carricd out by means of individual quantitative weight method.Single instantaneous food eonsumption was estimated with the following eqtiation:

bookeyeThünen

2.

B.W foodP =" ", >-, .... '•• ~here. - W'f1sh \

" ~ • I I ":4: ~. +

W fOOd - weight offood eonsumed by fish researched :'>~: /',.. --. ,_.-_.-W tish - weight of fish researched

B - fish biomass

Food consumption by cod was estimated using annual energetie budgets of various agegroups (Shatunovsk-y• .1988). '

, The Eastern Baltie eod population distributes in~ zone. In spring eod distributed indeep-water pari ofthe area. mainly rit the depths of 50 m and more. Juvenile fish prefcr lessdepths. Cod Jives in the near-bottom ]ayer and'migrates for food into the \vater eoJumn. wheremain food items distribute. Fishes of 30-34 em in Jenb:rth with gonades at 2-3 maturity stagespredominated in catche~. In the coastal areasjuveniJe fish of5-]5 em in length was caught.

RESULTS OF RESEARCHES

Variatio~s of eod food with growth .

Food availability at early development stages is one of fish ab~danee stabilizatio'neomponents. therefore' it is important to study strueture'of1'000 relations at different stages ofontogenesis. Cod larvae at early stages of development aie stenophages and eonsume mainly'nauplii and first copepodites stages of cOPepods, species composition of which depends onzooplankton development peculiarities in the area. Further, as juveniles grow up, they settle to'the bottom. Setteled juvenile fishes ap'pear inadopted to feeding in new eonditions, wherefood supply is low. It results in significant mortality during the first month of their life nearthe bottom and in inerease offood spectrum (Nikolskiy. 1953, 1?68; Gmuman et al.. 1988),Enormous losses due to larvae and juvenile fishes ritortality may be eonsidered as an elementofabundance regulation mechanism (Cushing,' ]975). . ..

Trend to eod food eomposition vari~liions with grO\\-1h, observed in the previous years,is apparent also' during the period studicd. Food of fish 01' up to 20 em in lcngth and above 30em differred very signifieantly. therefore we eonsidered feeding of the abO\'e groupsseparately. Feeding of fishes of 20-30 em in length also was studied individually(intermediate group).

Cod ofup to 20 em in length (as a ruJe,juveniJe)feeds upon mysids (A(~'si... miX/tl) andbenthos (polycl~aete. sporiges. mesidotea).· Prey size excecded no 2.5 eIn (exc1udingPolyehaete). Degree of stornach fullness was rather Jow - 44 prooeeimils. .

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

eod of 20-30 em is eh'aracterized by the start of rep~oduetiv~ products maturation:Mysides and benthos proportion in its feeding dcereases and fish appcarcs as a food item (upto 40%) mainly sprat and hcrring. Prey size is from 6' to 20 em. Degrce of stomaeh fullncssincrcascs to 66 prodceimils. Data on eod fceding for scveral years show that during the latest

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"years significant charge of food Items sj>ectnim occurcs towards -decrease of berithos" proportion rind increasc of fish proportion (Fig.I)"Mysides still constitute thc bulk of small

cod ration. ho\\'ever. in the 'pcriod from 1995 to 1997 thc latter proPortion dccreases slightly.. , , .

. M;ature eod ~boye 30'em fecds mainly.o~ fi;h (up t~ 87%) among whieh sprat ~nd. herring domin~te. In eod stomaehs other fish speCies of seeondary irriportanee are also fourid.such as red hake. oeean pout. flounders. sculpins. Juvenile fishes rriairily sculpins constituteinsignifieant proportion. No youngcod waS observed in stomaehs. In ICES Working Group

· niaterials it \\'as noted that in the Eristerri ~altic Sea ,cannibalisni was" of low imrortanee alsoin other seasons. Degree of stomaeh fullness of timt eod was t\viec higher that of'small .juvenile ead (Fig. 1).

. ThiJs. cad is a typicaf plankio~hage at'early stagcs~ theri it transfers into benth~phagcarid into prcdator -iehthyophage as it grows up. Change of eod feeding pattern oeeures duringentire life cycle and. as a rule. corrcsponds to biolInterannual variations of cO

· inereased. in 1995-1997 eomparatively early eod transfer irito ,fish eonsumption' occurred.,,,hieh is likely to be rehited \vith ap(>earan6e of large amourits of fisli food items due to high

"spmt abundance in strorig year-classes of 1994-1995 and deereaSe, of gro\\1h herring rate()bserved dunrig th,e latest years'and resulted in small slowly grO\Ving individllllis withratherhight abUridance. Two year old herring of 12-14 cm ,vere obseived duc to slow growth rate,\vhieh results in the latter aceessibility to eod together with sprat Adult ead fonns the bulk of ..iis biomaSs consurriing fish (mainly sprai and herring). Minimum proportion cifthese fishes in "eod ration \Ws observed in early 1980s.,Uzars (1984) tbinks it· is relaicd 10 deereaSe of spi"atand herring biomass at suffieiently high biomass of eod d,:,rjng above "mentioned years. lri late1980-s cod biomass decreased sharply as a result of stagnation events iri hydrological regimeand high fishing rate~whieh caused increase of sprnt biomass in early 1990s. In 1993-1994one of the most strong adveetions on the North Sea ,vater masses occtirred in the Baltie Sea

· for thc latest years. \vhieh resulted in significani improvement of eod reproduction eoriditioris.Year-c1asses appeared. abundailce of whieh at the age Of 2 years was 'by 1.4-1.8 times liigherthan minimum abimdrinee duririg stägnation years. In 1994-1995 strong year-classes ofsprat

. and herrillg also appeared. however. herling grO\..1h rate was very io\v and spawriing biomassof the latter deereased. since slo\v geowirig fishes of 2 year old (age 01' mass maturaiion 01'herring) appeared not ready to spa\\nirig neither in spring 1996, nor in spring 1997. Evidently.ablindaht fish focid items. such as sprat ofstrong year-c1asses 1994-1995 ami slO\'viy grO\vinghemng \vere one of the major reasons of eod biomass inerease in 1995-1996. Ho'..'ever, sueheod spawniiig biomass inereaSe was a shon-time everit (1995-1996) and in 1997 the trend todecrease appeäred.

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4 .

, Ratio of sprat and hemng in cod stomachs varies' by years ami refkcts directly theirstock size. According to Zalachowski. 1956)' from 1977 to 1982 whcn hcrring biomass washigh and that of sprat was relatively low. herring proportion in cOd teeding was higher thansprat. Aeeording to our datil in spring '1992-1996 sprat biomass was at a high level. whilefrom 1997 slight decrease had been observed. It reflected .upon cod food eomposition. Whilesprat predominated, in eod stormichs from 1992 to 1996. proportion. of sprat and hemngbeeame alm05t equal in 1997 (Fig. 3).

, ". . Thus. adult cod feeding pattern reflects directly prey stocks state. \vhich in its turn is ,inversely related to predator (cod) stock size. .

Cod f~edingvariations relative diStribution deptb

SmaÜ fish below 20 cm distributed at the depths up to 50 m. feeds mainly on Mysides "while at the depth up to 75 m benthos (Polychaete) predominates' in its ration. Fish of.intermediate Jength group up to 50 m feeds mainJy on Mysides. With depth increase fish andbenthos proportion in eod feeding inereased whiJe that ofMysides deereased. Large eod at a))depths sampled consumed fish. At the depths up to 75 m herring predominated in eodstomaehs. With the depths increase. sprat proportion increaSed whiiedegree ofeod stomachfullness decreased with dept~" The ~atter event waS characteristic ofall cod lenh'1hs '(Fig. 4).

. .~cal va'riations or cod feeding

Increase of cod feeding intensity increase was cJeai-ly observed in coastaJ areas whereherring and Mesidotea (Fig. 5) predominated in eod stomachs. In oO:shore areas cod 'consumed mainlY,sprat arid Polychaete (Fig. 6). Food cornposition 01' eod depends onmajor food. items distribution and overlapping of· their. distribution areas.

Cod feeding "ririations In relatio~ to genades ~aturit}' stage

The highcst fecding intensity was observcd in fecding fish with ·gonadL.'S at 3-4 'stages' of maturity. Prespawning and spawning eod eontinue feeding, however Jessintensively. Juvenile fish food eomposition was represented mainly by Mysides. •Secondary food items were represented by Polychaelt::s. In SlUrllachs or eod with gonadesat 2 stage of maturity fish food items (mainly sprat) (431}.;,) and Mesidotea appeared. Fishwith gonadcs at 3-5 stagl.'S consumcd mainly sprat.·· herring and to thc kss cx1cnt.Mcsidotea (Fig. 7).

Thus, in eod being a consumer of highly eoloriOlis food, no sienifican1 dccrcase offeeding intensi1y in relati~n to gonades maturity was observed. .

Differences of females and maleS feeding

.' Males and femaJes food composition was similar, .however, males consumcd lcssfish than fcmalcs. while benthos and Mysides proportion in males food is highcr than infemaJes (Fig. 8). Females fecding intensity was higher tha1 01' males. For fish below 20 cmthe above difi'erence was rather significant (almost twice). -

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5

Dit.ir~al ,r)·thmics of cod fcCding

During a day onc peak of fccding at 2.00 p.m. \~as observed tor tish bdo\v 20 em. 'Small fish fecding on benthos arid Mysides. :.ifC gra;drig prcdutofS. Satuf~tion of thescfish oceures gradually (Fig. 9). Cod above 20 em starts to "ced on fish, howc\'cr,invertcbratcs ~till are rathcr importarit iri its ration. Fecding aetivity was obscrvcd at8.00. 11.00 a.m. and 6 p.m. when eod consumcd invertebratt.'S. I\t 8.00a.m. and 6.00 p.m.

, Mysidt.'S prcdomiriatcd, and at 11 a.m. - Mysid\.'S and Mesidotca;' At 2.00 p.Ili. during .major feeding. eorl consumed mainly sprat. eod abovc 30 cm ,consumed herring arid,Mesidotea at 8.00 a.m. and sprat, herring and Mesidotea at 10.00 a.m. During aday-timepeak', sprat, hcrring and fvtysides predominatcd in eod stomachs. In the c\'criirig spratpredominated while other flsh species werc of less importance. '

\',

, One fceding'peak was 'observed iri diurnal ~ythmics of feeding' of small cod. \vhile'large eod had 3 peaks: in the morning, day-tüne and evening. Mo'st intensive feeding wasobscrved in day-time. . , '

'Total ~ood cOnSUmption arid SIe of cod

, Food consumption' by, cod in spring period, cstimated usin'g, encrgctic.'budjcts(Shatunovskiy: 191)8) amounted to 60 thous.t in 1996.,Spr.lt and 'herring eonstituted

" above 70%of the above value which eorresponded to -29% 01' sprat biomass und 25% of :herring biorri~ss. . , , ., ' '

. S"iriglc instantaneous food eonsumption (SIC) provides the general idca on foorl, umount in eod stomaehs at the moment of study. Jt amounted to 246 t in 1995, 283 t in

1996 whieh eorresponded approximately to 1% of eod biomass as estimated on the basisofthe trawling survey resu~ts. In 1997 SIC decreased to 142 t. '

. . ",. .' ~Food relationS of cod' ,

Scheme'01' eod food relations deseribcd in 1996 (Patokina•. 1996) was rathergeneral and, accounted no variations' of :food relation structur~ in the 'proeess'ofontogenesis. Analysis 01'. eod feeäing at different stages 01' ontogen~is allowed to'replenish the picture of trophicnetwork in the Baltic Sea. Cod larvae and juvenilesfeeding on Copepodes: fonn rather simple'chain of grazing type (Cushing's classifieation,

, 1975) whiehconsists 01' 2-3,links. . '

Thetrophic ehain -01' cod below 20 em; utilized to ereat~ own biomass of,zooplanktophages, detrüophages and benthophagcs bccomes longer and complcx.' Theabove ehain. f!lay be classified as adetrital one. Cod of 20-30 m fonns its biomasseonsuming benthos and nectobenthos and fishes. lts fceding type may be spccificd as amixed detrital-grazing one., " ' '.'

Cod above 30 cm feeds on fish; mai~ly, pelagi'e ones'(sprat arid herririg)., Othe~ .organisms 'are of secoridary importanee in its ration.' Feeding type is agrazing onc (Fig. ",10). " . . . - ~,,' : . .' ". . ."

.' .'. j

. Thus, eod oeeupies an important placc in the Ballie Sea trophi~ nct\l;ork und is an,upper link. of one of the major feeding chains where such important elements ofbioeenosis, as sprat and herring are mcorporated.t • . •

. - ,'.

"' ..

• <

6

CONCLUSION

Four ecological groups may be specified on the; basi;~ of cod food composition: ~larvae and juvenile eod are planktophages, juvenile 'cod bdow 20 cm - bemhophagt'S andneetophages, eod of 20-30 cm· ~ is intermediate group transferring from feeding on .invertebrates to fish, adult mature eod abovc 30 em - obligatory ichthyophagcs. . .

, Long-term researches show, that adult cod feeding pattern retkets· direetlyconditions of its prey stocks (mainly sprat and herring)' whieh 'in their turn inversely'related to predator stock size. With the depth increase (>50 m) fish (mainly sprat) andbenthos (Polychaeta) proportion, as weil as degree of stornach fullnt.'SS inerease. At

, shallower depths «50 m) and in coastal areas herring andMesidotea predominate in eodration. \Vith reproductive 'produets maturation, eod feeding intensity'and eonsumptionof highly colorious food inerease, Prespawning· and spawning eod eüminues 't~ding,however, lcss intensively, '. ..., .

Apparent diurnal rythmics is evident in small juvenile eod feeding with maximumat 2.00 p.m. Feeding rythmies of eod above 20 em, transferring to fish eonsumption, is ••related to majorprey behaviour. Ouring rcscarches 3 pC41k's of t~eding aetivity wereobserved in morning, day-time and cvening.

11

Fish eonsumption by eod within RF zone during ~prihg 1996 amou~ted to 6'0thous.1. Sprat eonstitutes about 70% ofthat value. .

. SIe of eod food, items within RF; zone in" 1997 am~untcd' to I·H t which issignifieantly less, than in the previous years: It is related to deen~ase of its minimumbiomass. .

, eod is an upper link of the Baltie Sea trophic chain. In the proccss ol' ontogcnesis' .its food relations vary and may be dcseribed convcntionally (using gerlcrally adoptedtCf!llinology) as 4 types of trophie chaines which play an important role In thc Baltic Seatrophie nCt\vork, since ineorporate abundant commereial fish spccies. As a ruk, foodniehcs of main eod ecologieal groups differ signifieantly. Most' overlap 01' these niches .was observed in intermediate group (20-30 m)., .. , ,

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.REFERENCES

Cushing 0.11.1975. Marine Ecology and Fishcriesl/Cambrigde ünivcrsity Pn.'Ss-.1975.-P.198-215. "

Grauman G.R, Lisivenko L.N.", Sidrevits L.L. 1988. Pcculiarities ofthe Baltic fish larvaefceding//Feeding of marine fishes and utilization of foragc rcsourccs as elements offishery predictionffhes.reports All-Union Conference.-Murmansk.-1988.-P.60-61.

Nikolskiy G.V. 1953. On regu1arities offreshwater fishes food relaiionsl/Collectcdpapers.Outline of general ichthyologica1 problems/Academy of Sciences ofUSSR.-1953. -

Nikolskiy G.V., Belianina T.N., Ponomarenko I.Ya., Sysoieva T.K.1968. Same data onvariability of indications and food composition ofjuvenile cod in ßarentsSeal/Materials on cod ecology in the North Atlantic.-l\1.Nauka.-1968.P.l 8-25.

Patokina F.A. 1996. Feeding peculiarities and interrelations of abundance 01' fishes [rom. the Baltic Sea.llICES C.M.l996/J:5. .

Shatunovskiy MJ. 1988. Comparative analysis of annuaI energetic budjets in various agcgroups of North Sea and Baltic Sea codllFeeding of marine fishcs and utilizationofforage resources as an element offishery predictionIThes.report of All-UnionConference.-Murmansk.-1988.-P. 5-7.

Shorygin A.A. 1952. Feeding and food relations of Caspian Sea fishesl/M.-Pischepromizdat.-1952. 268p. .

Uzars D. 1994. Feeding of cod (Gadus morhua callarias L.) in thc Central BaItic inrelation to environmental changesl/ICES Mar.Sci.Symp.. 198:P.6 I2-623.

Zalachowski W. 1986. An attempt to estimate food.biomass annually by the cod (Gadusmorhua L) pop~lation in the Baltic, based on studies in 1977-198111Acta ichtioI. etpiscat.-1986.-Szczecin.-P.3-23.

80

70

60

50

%40

30

20

10

0 -----

Feeding variation with growth of cod.

100

90

BO

70 )(..60~.:50 ::..40..:

'5·30....

20

10""""".,.,.,L--+ 0

30 • all fish

- Sp.spratllls c::::::::J Cl.harenglis c::cJ:l:l Pisces olh.~ Mysidaceae~ Mesidotea~ Benthos,olh. -fulness index

Fig.1.

Average long-term variations of cod food composltion.

•

•

50 T

:: f35 t30 +

I

% 25 II

20 +Ii

15 T

10 +i

5 T

60 TI

sol

I40 -!-

j

% 30 1II

20 ~!;

63-69

63-tl9

70·75

70-75

76-79

76-79

80-85

80-85

86-90

86-90

92·96

92-96

4000

RELATION BETWEEN PREYS NUMBER IN COD STOMACHS AND PREYS ANDPREDATORS BIOMASS

3500

3000

2500

~cri52000::I:...

1500

1000

500

o. -1977

•

1981

-\-

1982 1992

Fig.3.

1993 1994

•

-+- .,1995 1996

c::J HERRING BIOMASS

~ SPRATI BIOMASS

.-eoo BIOMASS

-TOTAL FOOOeONSUMPTIONBYeoo

-o-HERRINGeONSUMPTION BYeoo

-x-SPRATeONSUMPTION BYeoo

•

Variality of cod food composition and rate of feeding in relation to the area depth(march 1997).

"170

80

Local variability. of cod food composition (march 1997 r.).

80

70

60

50

% 40

30

20

10

03864 3964 3965 4064

rectangles

4065

120

100

80)(..~

.:60 \Il\Il..

.!E:;40 -20

04164 4165

_ Sp.sprattus c:::::J Cl.harengus ffiHfl Pisces,oth. E:3 Mysidacea~ Benthos,oth. -fullness index

Fig,5.

..

Cod feeding rate by rectangles.

20.0°19.0° 21.0°-l- ---'- +- --'- -4 J----,--__-i1r-_--,- 56.5°

18.0°

54.5°

56.0°

55,5°

55.0°

•. 3965

• - 50-70 %00

• - 70-100

• - >100

3864 - rectangle

•3864

" J

I 111 I )1

• •4164 4165 .• 0\ , I

I I I

- - - - - - - - -: - - - - - - ~ - -: - - - - - - - - -:I I II I I1 I I

I • 1 • II 4064 I 4065 I

I I II I I

- - - - - - - - _1- 10 - - _1- _I I II I I

1 I

I I

I • I3964o I I

I I /r 1

_________: '_ --:- ----/1:- --:--/;/

I . I

I

1

I

\ :l- - - - ~- - - -:- - - - - -:/ -

o ~ I ~

I ~j Il-

•

Fig.6.

Variability of cod food composition and rate of feeding in relationto gonades maturity stage (march 1997 r.)

100 f T12080 t 100 )(..60 + 80 "0.E

%40 + 60

.......40 E

20 t "320 -o+-- 0

2 3 4 5

mlturity stage

Fig.7..

Differences of food composition and feeding rate )n cod males and females(march 1997 r.). •

males

males

females

females

2Q.30 sm 8075 ~...70 .565 :60 ~55 :;50 - •

50

40

30%

20

10

omales females

>30 sm T95 ~+90 -g

Jasj80_ Sp.spraltes c:::::::J Cl.harengus lIII!D Pisces,oltl. c::::::::3 Mysidacea~ Benthos,oth. --fullness index

Fig.8.

Diurnal feeding rythms for cod (march 1997).

1~~I807060

% 5040 +302010o+-~-

8 9 13 1415

< 20 sm

1a

T120100

ao60

40

o

)(~..,.:........,§~_.

20-30 sm• 80 ,70 IT

60 .

Scheme of trophic relations for the different size of cod

ICopepodita I

'IPhytoPlancton 1-E---7'r.--~L".-':""""":"_~'---

" .MORHUA,LARVAE)--------.----'

Fig. 10

. 11Sp.sprattus