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

Internationa I Counc i1 for theExploration of the Sea

C.M. 1979/ L:23Biological OceanographyCommittee. Ref. PelagicFish Cttee

Juvenile,Pacific herring Clupea harenguspallasi:Feeding in CEPEX Enclosures

.,.by

Edward D. Houdeand

Steven A. Berkeley

Rosenstiel School of Marine and Atmospheric ScienceUniversity of Miami

4600.Rickenbacker CausewayMi ami , Florida 33149 U.S.A.

Synopsis· .

Growth and foodofyoung-of-the-year herring Clupea harengus pallasiwere determined in summer 1978 from fish stocked in a 1300 mJ module aspart; of the Controlled Ecosystem Population Experiment (CEPEX) that wascarried out in Saanich Inlet, British Columbia. Stomach evacuation ratesand maximum potential rations also were estimated in experiments run withtank-held fish. Herring of 3.28-4.12 g wet weight consumed a cyclopoid,copepod Corycaeus sp., which was the dominant zooplankter iOn the module,with lesser consumption of calanoid copepods, harpacticoid copepods andlarvaceans. Growth' in, the module (wh ich had not received any nutrientadditions to h~lp maintain a high primary productivity) was slow, averagingonly 0.7% day , in c.2.ptrast to tank-he ld fish that were fed daily andwhich grew at 5.3% day • Herring in the module were feeding at about 20%of their estimated maximum capacity. This, combined with the small averagesize of prey available, accounted for the poor growth rate. A decline in

,copepod abundance that occurred in the module when herring were present wasnot solely attributed to grazing by herring, because the estimatedconsumption by the herringpopulation accounted for only 6% of the o~served

decline. Mortality of herring stocked in the module and held in 2 m, tankswas negligible during a one month period. CEPEX type enclosures can be auseful method to study the interaction of fish and zooplankton in pelagiccommunities.

Introduction

An objective of CEPEX (Controlled Ecosystem PopulatiOIi Experiment)was to advance the understanding of predator-prey interactions in. planktonecosystems. In 1978, young-of-the-year herring (Clupea harengus pallasi)

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II

were stocked in one\of the 1300 m3 experimental modules that were moored inSaanich lnlet, British Columbia. Estimates of feeding" growth anddigestion ,rates, as well as the impact of herring onthe IOOdule'szooplankto~ population were made. . i

Tbere have been numerous previous investigations of herring feeding,both on wild populations (e.g. Hardy, 1924; Savage, 1937; Rice, 1963) andon populations maintained in captivity (e.g. Battle et .11., 1936; Blaxterand Ho1liday, 1958). Most of the reports are on Atlä'iiticiherring butWailes (1936) gave .1 detailed account of feeding by larvae, juveniles andadults of Pacific herring fram populations near Vancouver Island, BritishColumbia. Blaxter 'and Holliday (1963) sumnarized much of; the know1edgeabout herring' feeding. The major items in herring diets: 'are calanoidcopepods, larvaceans, euphausiids and larval fishes. A va~iety of.otherplankton organisms are consumed and'can be important in the diet when theyare locally abundant. '.~

Tbe CEPEX conc'ept has been discussed in recent publi~~tions (e. g.Menzel and Case, 1977) as have details of module design (Case, 1978).Water columns with entr~pped plankton populations were studied initia11yto determine the impact of pollutants (Menzel, 1977; Beers et~al~, 1977).Emphasis later shifted to studies on the systems under' ~non-pollutedcond itions or in response to non-pollutant perturbations.': Growth ofjuvenile chum salmon (Oncorhynchus keta) that were stocked in modules hadbeen estimated in copper and mercury pollution ,experiments: (KoeHer andParsons, 1977; Koeller and Wallace, 1977)., lnsimilar kinds of:experimentscarried out in Loch Ewe, Scotland (Gamble et a1., 1978,: 1979) larvalherring feeding and 'growth had been studied. -rhe-;xperiments~on juvenileherring reported here provide additional information on herring feedingand insight into how vertebrate predators act on plankton commtinities~

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Methods

. Seven hundred jUvenile herring were purse seined in' Saariich lnlet on 4August 1978 and transferred to three, 2 ~ cylindrical tanks that were ona'barge anchored adjacent to the CEPEX site. Saanich lnlet'seawater, 14­180 C, was pumped through the tanks.. During the period 4-21: August therewas< 5% mortality cf herring maintained in this system. Herring were fedon Saanich lnlet zooplankton that was collected once or twice daily withplankton nets. There.was no reluctance on the part of the captiveherringto feed in the tanks ~ . ' ;.

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On 15 August 100 juvenile herring of mean weight 3.28 g,were stockedin the CEPE~module. i The 10-m diameter plastic cylinder was 17 m deep andheld 1300 m of water. Populations of zooplankton in the module weremonitored during the course of the herring experiment based on~catches fromvertical tows of a 202 pm mesh plankton net. Four sampies of'herring wereremoved fram the module by.purse seine during the experiment~to ~eterminefood ,habits and growth. Tbe experiment wästerminated~on 15 'September.

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. , Stomach anaiyse~ were carried out on herring from the barge tanks andfram the CEPEX module' to determine the kinds ancl amounts of food that wereconsumed. Contents cf both the caecal and pyloric stomachs were removed,

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identified and then, dry weighed. The regression relationship betweenweight of food in a full stomach (from herring in the barge tanks that werefed to satiation) and weight of herring was determined, from Whichestimates of stomach fullness could be estimated for herring that weresampled from the module.

Digestion time was estimated by serially sacrificing herring that hadbeen fed to satiation and then transferred to a tank with no food. Thedecrease in stomach contents with time was used to fit an exponentialregression of weight specific stomach contents (g/g.fish) on hours afterfeeding. The stomach evacuation rate (a) was estimated from theregression. The ration for herring in any sampie could then be determinedby applying the formula given by Eggers (1977),

R = 24 S a

where R = weight specific ration (g/g.fish)

a

S = 'mean weight specific stomach contents (g/g.fish)

= evacuation rate (h-1)

This expression is appropriate for fishes that feed more or lesscontinuously throughout the day, which is approximately correct for theherring used in this study.

Consumption by the population in the module was the product of numberof individuals and ration. It was assumed that no herring mortality, otherthan that due to sampling, occurred. Observations in the module by diverssupported this assumption, but because all herring were not recovered at,the end of the experiment it could not be confirmed.

Food selection by herring in the IOOdule was determined by applyingIvlev's (1961) index of electivity, in which the proportion of an item thatwas present in the stomach, (r i) was compared to the proportion that waspresent in the module (Pi)'

E =

Positive values of E indicate selection for an item while negative valuesindicate selection against the item.

Results

Growth

The herring that were held 1n the barge tanks and fed zooplankton grew

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rapidly. A. sampie of 142 Hsh on 4 August had a mean weight 'of L 86 g and amean standard length of. 55.1mm.' A sampie of 99 fish on 15:August had amean weight of 3.28' g'and amean length of 64.4 mm. The herring had:grown9.3 mm and 1.42 g. :The instantaneous growth coeffici~~t for weight w~s g =.0516, indicating a' specific growth rate· of 5.3% day. ~:;' ~

Herring .stoeked in the module' grew slow1y (Table 1);' ',Mean weight of,100 herring when stocked was 3.28 g on 15 August. A sampie of 10 herring on15 September had a' mean weight of 4.12 g. The mean' weight'_Tcrease of

, 0.84 g is equivalent to a speeific growth rate of on1y 0.7% day • Beeauseof small sampie sizes and relatively high standard errors,' it was notpossib1e to conelude that a statistieally significant increase in weightoeeurred during the' 31 day period. lt is apparent that some growth didoecur. The largest,speeimens observed in any of our sampies were obtainedon 15 September (82-90 mm SL) at the end of the experiment~;

Food

To determine the kinds of organisms 'and amounts that could beeonsumed, 20 herring, 63.0 mm mean length, 3.30 g mean ,weight, were fed tosatiation on 15 August from a cateh of zooplankton made in' a 505 ~m meshplankton net towed in the upper 100 m of Saanieh lnlet. Themean numbersof organisms per herring, from stomaeh analyses were: ;

.,

.,Copepods (mostly Calanus sp.)

Amph ipods '( Paraphemis to sp.)EuphausHdsChaetognathsInseeta (Diptera)

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149.211. 93.00.05

1 0 •10.. . ~ ~

Copepods were the most eommon item in the plankton tows and in the herringdiet. The,mean drYI weight per partially digested food itemiwas 120 J.Ig.The juvenile herring had no difficulty eapturing large, prey su'eh asamphipods and euphausiids, although' these items may. not. be readilyavailable to herring juveniles in the surface waters of Saanichlnlet.

Foods eaten by herring in the module are listedin Table 2. Thepredominant food item was a small cyelopoid eopepod, possibly Corycaeusanglieus, although the speeific status of this copepod has recently beenquestioned (Gibson and Griee, 1978). on 21 August most of the Corycaeusconsumed were adults. On 30 August and 6 September most were'copepodites.Other organisms were uncommon constituents of the diet. Abundances of the

I '.zooplankton in the, module, based on 202 ~m mesh net collections, areillustrated in Figure 1. lt is c1ear from this figure thatlCorycaeus wasthe dominant organism in the module, as weIl as in the herring diet. Italso is evident that there was a drastic decline in copepod abundanee inthe module that was eoincident with the stocking of the he'rring. It istempting to speculate that herring predation caused the; decline ineyclopoid copepods, but similar declines oceurred in two othermodules thathad no herring. All modules in 1978 had dense populations of ctenophores(Pleurobrachia bachei and Bolinopsis infundibulum) and their predationprobably was more influential on copepod abundance' ,than was' predation by

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herring.

Digestion Time

Tbe data and the fitted regression line are given.in Figure 2. Estimatedpercent digestion, 100 [l-exp( -at}] , is:

Digestion, based on stomach evacuation rates of barge' tank herring,was' estimated to be > 95% completed at 15 h after an item was consumed.The exponential regression of weight specific stomach contents (g/g.fish)on hours after herring were fed was

638695

>98

2r = .85

Percent digestion

t hours after herring were fed

St = 0.01424 exp (-0. 1976t}

St = weight specific stomach contents

-0.1976 = a, the evacuation rate

The prey selection analysis indicated that Corycaeus and harpacticoid .copepods were positively selected by herring in the module and that otheravailable prey were negatively selected (Table 3). On the last day of theexperiment (15 September) calanoid copepods and larvaceans (Oikopleuradioica) were positively selected, possibly because divers had been in themodule and may have altered the usual feeding behavior of the herring onthat day. Tbe apparent negative selection of calanoid copepods andlarvaceans might be a result of differing vertical distributions betweenherring and these prey items, rather than simple avoidance of theseorganisms by the herring . Tb is seems probable because calanoids andlarvaceans are often dominant items in the' diet of herring in nature.Harpacticoid copepods were not numerically important in the diet but'theirpositive selection by herring indicates that feeding near the walls of themodule may take place.

Stomach contents of 18 juvenile rockfish (Sebastes spp.), 28-40 mmSL, which were collected from the module when herring were sampled, alsowere examined. Tbe rockfish originated from larvae entrapped in the watercolumn when the module was raised. Only 12 of the 18 rockfish had food inthe stomaeh. Eight rockfish had eaten Corycaeus, but'in relatively smallnumbers compared to herring. Amphipods (caprellid and gammarid),harpacticoid copepods and dipterid insects were relatively common inrockfish stomachs. Rockfish typically had at least one large prey itemamong the stomach contents (e.g. insect, amphipod, euphausiid). Rockfishdid feed on water column plankton, but a large part of their dietcame fromorganisms that were associated with the module wall.

where

Hrs after feeding

5101520

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Stomach Fullness

Eighteen herring, 2.0-5.0 g wet weight, were fed to satiation and then'sacrificed.·· The dryweight of the stomach contents (mg) i was plottedagainst the wet weight of the her ring (g) (Figure 3). The linear'regres­sion is,

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F=5.92+5.55 W 2·r. = .64.

where F = estimated dry weight of contents of a full stomach (mg)

W = wet weight of herring (g)

This relationship was useful to estimate the percent stomachlfullness ofherring that' were sampled from the module. The dry weights of stomachcontents of herring sampled from the module were expressed aspercents ofestimated full stomach contents (Table 4). Herring from the,module neverhad full stomachsj the percent fullness ranged from 9.10 to 131.90, with amean value of 19.14%. The variability in observed percent fullness was not

- related to time of day that the herring were sampled.

Rations ..

Rations can be expressed either as the total daily consumption by anindividual of mean weight (column 7, Table 4) or on a weight specific basis(column 8, Table 4)., The column 7 estimate of R results from substitutingthe column 4 estimate of mean weight of stomach contents , S, into theequation, R = 24 S Cl, where Cl = .1976, the stomach evacuation rate.' Thecolumn' 8 estimate resul ts from substituting the column 6 estimate of meanweight specific stomach content~, S, into the equation. .

,Rations for individuals of mean estimated weight ranged from 0.00797

to 0.03457 g. On a weight specific basis the rations ranged :from 0.00313to 0.01005 g/g. fish. The mean ration' for a herring over the entireexperiment was 0.02149 g, Which corresponds to a weight specific ration of0.00644 g/g.fish.

Daily consumption by the population was estimated by expanding theindividual ration estimates to the number of herring estimated to be in themodule. The estimates ranged from 0.7173 to 3.0630 g dry weight (Table 5).The mean daily consumption by the population was 1.9089 g. Most of theprey was Corycaeus. Mean wet weight of an adult Corycaeus is approximately70].Jg (Koeller and Parsons, 1977) or approximately 14].Jg dry weight.Therefore, the herring were consuming, on average, an estiinated 136,350prey day- 1 in the module. This number may be a low estimate ,on 30 Augustand 6 September, whem most prey. were Corycaeus copepodites,' which ·wouldhave dry weights much less than 14 ].Jg ..

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Maximum Potential Consumption

Mean stomach fullness for herring sampled from the module was only"19.14% indicating that the herring were consuming prey at far below theirmaximum possible rate. The mean dry weight of herring stocked and sampledfrom the module during the 31 day experiment, was 3.30 g~ At 100% stomachfullness a_herring of that weight would have mean weight specific st~mach

contents, S = 0.00734 g/g.fish. Recall that a = .1976. Then, R = 24 S a =0.03482 g/g.fish. For herring of 3.28 to 4.12 g, the sampie mean weightsof herring at the beginning and end of the experiment, the .!!!flximumestimated daily consumption is 0.11422-0.13651 g. At 14 ~g, prey ,themaximum consumption ranges from 8159 to 9751~rey. Those m~iimumivalues

correspond to 5.67 to 6.77 prey captures min for 24 h day . It seemsunlikely that herring could capture prey of that size at those rates for apro10nged period, but if prey in the module weighed > 14 ~g herringprobably could consume the maximum ration.

On a dry weight to dry weight basis (assumi~g herring dry weight = 0.2wet weight), individuals. consuming_rt the maximum potential rate couldconsume 17% of their body weight day . The actual mean consumption, ba~Id

on 19.14% stomach fullness, was estimated to be 3% body weight .day .Little growth occurred at that ration, indicating that 3% may be near themaintenance ration for juvenile herring.

Impact of Herring on Module Zooplankton

The observed decline in the module zooplankton population almostcertainly was not caused only by herring predation. The estimatedconsumptionrates, estimated maximum potential consumption rates and thestomach analyses themselves do not lead us to believe that the herring werethe major factor in causing the zooplankton decline, but that invertebratepredators or unevaluated factors were more important caus_~ of zooplanktonmortalisy. The copepod population declined from 13,873 m on 14 August to1367 m on 21 August, during the initial week that herring were in themodule. This represents a loss of 16.3 million copepods in the .!!!pduleduring seven days, an average decrease of 2.3 million copepods day • Apopulation of 100 herring weighing 3.28 g per indi~idual at maxiE!ymconsumption rate could consume only 0.8 million preyday at 14 ~g prey .This would account for only 35% of the observed daily loss. . In fact,herring in the module were not consuming P.!.IY at maximum potential "rates,but consumed an estimated 136,350 prey day ,which is 6% of the observeddaily losss. If mean prey weight were only 5 ~g, the loss of zooplanktonattributable to herring would still be only 16% of the observed total dailydecline. Also, if there was some mortality of herring, other than samplingmortality, the actual consumption by herring in the module was even lessthan our estimated consumption ..

Discusssion

Juvenile herrin&-.jn the module consumed about 3% (dry weight basis> oftheir body weight day • Same individuals grew weIl, but the population as

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, ". \ : .. .a whole d1d not grow on the diet that consisted primarily of the cyclop01dcopepod Corycaeus sp. The herring consumed only about one-fifth of theirestimated maximum potential ration. The module that receiv~d the ,herring ,was not typical of a11 CEPEX enclosures. It had recevied no nutrientadditions throughout the course of the. summer experiment. . Its primaryproduction rate was! low, it had only a sma11 population of large calanoidcopepods and invertebrate predators that consumed copepods:were abundant.Koe11er and Pars.Q.fs: (1977) found that chum salmon of 2-9 g wet:wc:ight couldgrow at 4.0% day 1n CEPEX modules if large Calanus was the dom1nant food,but nogrowth was observed when small Corycaeus predominated, in' theirdiet. In our single experiment Corycaeus was the o~ly copep~d·that.was~~e

only copepod that was Common in the module and herr1ng ate thousands day ,yet little growth was achieved on this small prey item. If, larger copepods

,had been more available, they would have contributed more to ;the diet andbeÜer growth likely would' have occurred, as was observed ror, the, chumsalmon fry. 'Herring held on the barge tanks were offered food' only 'once ortwice daily that consisted of large calanoid cope.Epds, amphipods and

',,' , euphausiids. Those herring grew'at a rate of 5.3% day and 'possibly would'have grown' faster if food had been offered at more frequent intervals .'

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. ' Herring in nat~re appare~tlY are opportunists. Diets aiejvariable bu'tcalanoid copepods: usually' predominate (e.g. Savage,' 193,7>', ! Otherimportant food can include larvaceans, fish larvae and euphausiids.· Bothcalanoid copepods ~md larvaceans, were present in' small numbers in- themodule andwere consumed by the herring, but both of those prey items werenegatively. selected. It:, i8 possible that the herring ,and those 'preyinhabited differentlparts of the module, making'the prey mostly una~ailable

to the herring: ' The predominant food of juvenile herring thatwas reportedin British Columbia waters was calanoid copepods, but a variety of otherprey was eaten; including the cyclopoid copepod Corycaeus (Wailes, 1936).In nature the calanoids must be readily available to the herring, but inthe module they were negatively selected on three of the, four samplingdates. ,i:', '

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Large enclosures offer good opportunities to study how planktivorousfishes interact with other components of the plankton community.' Previous

'CEPEX experiments with chum,salmon (Koeller and Parsons, 1977; Koeller and: Wallace, 1977) demonstrated, that quite different responses,; in terms of'salmon growth~ can :be obtained that are related to kinds,of zooplanktonpresent in the modules. The single experiment reported here indicated thatjuvenile herring can be used in such systems, that thei~'mortality isnegligible and that important fish-zooplankton interactions 'can beevaluated. Gamble et al. (1978) have successfully followed development ofcohorts of herring:la;;ae in Loch Ewe. modules. They found: that larvalherring growth differed among modules in reponse to differing zooplankton:populations. We did not have an opportunity to 'repeat our juvenile herringexperiment but we believe that their growth also would differ appreciablydepending on thezooplankton community that develops. Careful monitoiingof the entrapped zoOplankton populations, which are self-sustäining inthemodules, and ,concurrent sampling of the fish provide an excellent means to

- "study ecological and nutritional factors related to herring biology'~,

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Acknowledgments

This research was funded by National Science Foundation Grant OCE77­27225 through the Office for the International Decade of Ocean Exploration.Special thanks go to CEPEX staff membersoJ •. Barwell-Clarke, R. Brown andF. Whitney for their excellent technical assistance.

References

Battle, H.I., Huntsman, A.G., Jeffers, A.M~, Jeffers, "G.W., Johnson, W.H.and McNairn, N.A. 1936. Fatness, digestion and food of Passamaquoddyyoung herring. J. Biol. Bd. Can., 2: 401-429.

Beers, J.R., Reeve, M.R. and Grice, G.D. 1977. Controlled ecosystempollution experiment: effect of mercury on enclosed water columns.IV. Zooplankton population dynamics and production. Mar. Sci.Commun., 3: 355-394'0"•

Blaxter, J.H.S. and Holliday, F.G.T. 1958. Herring (Clupea harengus L.) .in aquaria. H. Feeding. Mar. Res. (Scotland), .No. 6, 22 pp.

Blaxter, J.H.S. and Holliday, F.G.T. 1963. The behavior and physiology ofherring and other clupeids. Adv. Mar. Bioi., 1: 261-293.

Case, J.N. 1978. The engineering aspects of capturing a marineenvironment, CEPEX and others. Rapp. P.-v. Reun. Cons. int. Explor.Mer, 173: 49-58.

Eggers, D.M. 1977. Factors in interpreting data obtained by diel samplingof fish stomachs. J. Fish. Res. Bd. Can., 34: 290-294.

Gamble, J.C., MacLachlan, P.M., Nicoll, N.T. and Baxter, I.G. 1978.Morphological changes during larval development of herring (Clupeaharengus L.) reared in large plastic enclosures. ICES CM 1978/L:34,7 pp. plus tables and figures (mimeo).

Gamble, J.C., MacLachlan, P., Nicoll, N.T. and Baxter, I.G. 1979. Growthand feeding of Atlantic herring larvae reared in large plasticenclosures. ICES ELH Symp./I:l, 18 pp. plus tables and figures(mimeo).

Gibson, V.R. and Grice, G.D. 1978. The developmental stages of a speciesof Corycaeus (Copepoda: Cyclopoida) from Saanich Inlet, BritishColumbia. Canadian J. Zooi., 56: 66-74.

Hardy, A.C. 1924. The herring in relation to its animate environment. PartI. The food and feeding habits of the herring with special referenceto the east coast of England. Fish. Invest. Lond., Sero II, 7(3),53 pp.

Ivlev, V.S. 1961. Experimental ecology of the feeding of fishes. YaleUniv. Press, New Haven. 302 pp.

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Koe II er ,P. and Parsons, T. S. 1977.(Oneorhynehus ~): eontrolledBu11. Mar. Sei., 27: 114-118.

The growth of young sa lmonidseeosystem pollution experiment.

Koeller, P. andexperiment:of juvenile395-406.

Wallaee, G. T. 1977., Controlled eeosystem pollutioneffeet of mereury on enelosed water eolumns. V. Growth

ehum salmon (Qneorhynehus keta). Mar. Sei. Commun., 3:

Menzel, D.W. 1977. Summary of experimental results: eontrolled eeosystempollution experiment. Bull. Mar. Sei., 27: 142-145.

Menzel, D.W. and Case, J. 1977. Coneept and'design: eontrolled eeosystempollution experiment. Bu1l. Mar. Sei., 27: 1-7.

Riee, A.L. 1963. The food of the Irish Sea herring in 1961 and 1962. J.Cons. int. Explor. Mer, 28: 188-200.

Savage, R.E. 1937. The food of North Sea herring 1930-1934.Invest. Lond., Sero 11, 15(5), 60 pp.

Fish.

Wailes, G.H.waters.

1936. Food of Clupea pallasii in southern British ColumbiaJ. Biol. Bd. Can., 1: 477-486.

.,

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• ,Table 1.

,.Lengths and weights of herring from CEPEX module, summer 1978.

Table 2. Summary of data fram stomaeh analyses on juvenile herring aampled from CEPEX module, summer 1978. The mean number of eaehclass of organiam ~ 1 standard error is given and the pereentage eontribution of each e1ass is presented.

Fish Food Organisms

Mean Mean Mean numberNumber I eng th weight Cyphonautes Brachiopod Biva1ve organisms

Date of fish (mm) (g) Coryeaeus Harpac tieoids Ca1anoids Oikopleura larvae larvae 1arvae per herring

21 Aug 10 65.6 3.40 1742.9 .:!: 227.1 5.8.:!: 1.2 1,9 .:!: 0.7 4.4 .:!: 2.0 1.6 .:!: 0.5 34.9 ~ 8.9 .14.7 ~ 2.9 1806.2

96.50% 0.28% 0.11% 0.24% 0.09% 1.93% 0.81%

30 Aug 7 62.0 2.28 1179.1 .:!: 90.9 25.3 ~ 3.3 0.3 .:!: 0.3 0.4 .:!: 0.3 0.0 .:!: 0 1.6.:!: 0.7 1.1 ~ 0.9 1207.8

97.62% 2.09% 0.02% 0.03% 0.00% 0.10% 0.09%

6 Sep 6 69.5 3.44 4951.8.:!: 988.6 31.2 ~ 6.6 9.7 .:!: 5.2 78.0 .:!: 27.9 30.7.:!: 19.7 0.0 ~ 0 0.0 ~ 0 5101.4

97.07% 0.61% 0.19% 1.53% 0.60% 0.00% 0.00%

15 Sep 10 71.8 4.12 44.6 .:!: 10.5 1.6 ~ 0.7 0.9 .:!: 0.5 6.5 .:!: 2.6 0.1 + 0.1 0.0 .:!: 0 0.0 + 0 53.783.05% 2.98% 1.68% 12.10% 0.19% 0.00 0.00

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Table 3. Ivlev electivity indices for organisms consumed by juvenile herring from CEPEX module on four 1978 dates. (r ...proportion of item in diet; Pi .. proportion of item in the plankton).

1

Sampling Date Foad Organism

Herring Zooplankton Corycaeus Calanoids Harpacticoids Larvaceans Other zooplankton

r. Pi E r. Pi E r. p. E r. Pi E r. p. E1 1 1 1 1 1 1

21 Aug 21 Aug .9650 .5248 +.30 .0011 .0047 -.62 .0032 .0047 -.19 .0024 .0047 -.32 .0283 .4599 -.88

30 Aug 30 Aug .9762 .2324 +.62 .0002 .1816 -.99 .0209 .0048 +.63 .0003 .0242 -.98 .0022 .5520 -.99

6 Sep 5 Sep .9707 .2052 +.65 .0019 .0054 -.48 .0061 .0024 +.44 .0153 .0262 -.26 .0060 .7605 -.988 Sep

15 Sep. 12 Sep .8305 .1857 +.63 .•0168 .0038 +.63 .0298 .0030 +.82 .1210 .0108 +.84 .0019 .7962 -.99

Kean +.55 -.36 +.42 -.18 -.96

Table 4. Herring weight and stomach content weight relationships for eEPEX herring juveniles sampled in August andSeptember, 1978.

Number Mean dry weight Dry weight of Weight specificof Mean wet weight stomach contents Estimated percent stomach contents Ration ration

Date herring fish (g) (mg) stomach fullness (g/g.fish) (g) (g/g.fish)

21 Aug 10 3.40 .6.46 26.18 0.00190 0.03063 0.00901

30 Aug 7 2.28 1.68 9.10 0.00074 0.00797 0.00350

6 Sep 6 3.44 7.29 31.90 0.00212 0.03457 0.01005

15 Sep 10 4.12 2.70 9.38. 0.00066 0.01280 0.00313

Muns 3,29 4.53 19.14 0.00136 0.02149 0.00644

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Tab1e 5. Estimated weights of food consumed per day by individua1s andby the population of herring in the CEPEX module.

Individual Herring Population dai1yMean weight ration population consumption

Date fish {g} {g} size {g}

21 Aug 3.40 0.03063 100 3.0630

30 Aug 2.28 0.00797 90 0.7173

6 Sep 3.44 0.03457 83 2.8695

15 Sep 4.12 0.01280 77 0.9856

Means 3.29 0.02149 1.9089

x

/ ----.X CORYCAEUS

x

......

-.\-./\.'\

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149

SAMPLING DATE

Figure 1. Abundance estimates of four kinds of zooplankton in CEPEXmoduleCEE4 from 9 August to 12 September 1978.

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HOURS AFTER FEEOING

•Figur. 1. The exponential regreuion of weight speeific atoaach contenta

on houra after herring were fed, from wich Q, the atcaach evauationrate. end dige8tion tilDe wen estilDaUd .

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FISH WEIGHT (gi

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Figure 3. 111e relatioD8hip betveen dry veight of food in • full ata..ch.nd wet veight of juvenile berring.