International Council for theExploration of the Sea

CH 1991/G:15Ref. KDemersal Fish Committee

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HOW HUCH NEPHROPS IS EATEN BY COD IN THE WESTERN IRISH SEA?

H.J. Armstrong 1, D. Smyth 2 and W. HCCurdy 1

1. Aquatic Sciences Research Division, DANI, Newforge Lane,Belfast BT9 5PX

2. Biometrics Division, DANI, Newforge Lane, Belfast BT9

ABSTRACT

Cod (Gadus morhua) and one of their main prey species, Norway lobsters(Nephrops norvegicus), are principal targets of trawl fisheries in the IrishSea (ICES Division Vlla). It is estimated that Nephrops comprised 20-40% ofthe stomach contents of cod sampled on the Nephrops grounds in the north­western Irish Sea in 1982 and 1983, representing 12-28% of the daily foodintake when the slow rate of digestion of the prawns is allowed for. Totaldaily food intake was 0.8-0.9% of body mass in spring and 1.6-1.8% during theremainder of the year. 'Some 25-34% of the stock of cod with an annual averagebiomass of 11.0 to 12.5 thousand tons of two-year-old and older fish in 1982and 1983 was assumed to occur on the Nephrops grounds, resulting in a totalannual consumption of prawns of 1.7 to 5.4 thousand tons. These estimateswere used to examine the possible impact on Nephrops of reducing the fishingmortality on cod. within the parameter space explored, yield of Nephropsdeclined by 0.8 to 2.4 thousand tons when fishing mortality on cod was reducedto 70% of the present value, assuming fishing effort on Nephrops stayed thesame. This extent of loss could, however, be substantially offset by improvedsurvival of prawns following increased predation by cod on small fish that eatjuvenile prawns.

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·.INTRODUCTION

The trawl fisheries of the Irish Sea (Division VIIa) target mainly on codGadus morhua, whiting Herlangius merlangus~ hake Herluccius merluccius and oneof the principal prey species of cod, the Norway Lobster Nephrops norvegicus.The latter species is taken by otter trawl from muddy substrata in the westernand eastern Irish Sea (Fig. 1), together with a by-catch of fish, whilst thefleets targetting on cod, whiting and hake employ either otter trawls or largepelagic trawls ,that are towed in the lower water column. Landings of cod,whiting and Nephrops from Division VIIasince 1981 are ehown-in Table 1 (Datafrom Anon. 1991a,b). The Nephrops trawlers discard substantial quantities ofsmall or unwanted fieh and small prawnä, some 25% of their total catch havingbeen discarded in 1981 and 1982 (Briggs 1985). The proportion of the prawncatch that is discarded has however declined from over 30\ in the early 1980sto less than 20\ by the end of the decade, due both to an increase in meshsize and. the retention of progressively smaller prawns by fishermen tomaintain their landings (An~n. 1991b).

Advice given by the Irish Sea and Bristol Channel working Group. of ICESfor management of the trawl fisheries in the Division VIIa has. in the past •included the observation that some over-expolitation of cod could benefit theNephrops fishery, and the combined yield (in monetary termS) of cod andNephrops, because of the predation on prawns by cod (Anon. , 1990). Thisobservation was based on the results of simulations of the biologicai andtechnica1 interactions in the cod and Nephrops fisheries reported by Branderand Bennett (1986) and Brander (1988).·

The precÜctions of multi-species models could become of increasingreüevence following steps taken in 1991, to reduce ," fishing effort in severalregions around the British Isles'by forclng vessels to'remain in harbour foreight days, in every month~ Although enforced reductlon of fishing effort hasnot yet been applied to the trawl fisheries in'the Irish Sea, the possibleeffects on the Nephrops stock of a 10nger~term increase in the blomass of codneed to be examined more thoroughly to ensure that the benefits to the codfisherY,would not be outweighed by losses of the more valu.able Nephrops in theprawn fishery. In this context, we re-examine in detail the steps involved inestimating the annual consumption of Nephrops by cod, to confirm that thelevel of predation would indeed be high enough to cause significant losses to ~the prawn fishery in the event of a reduction of fishing effort on cod. We ~inc::lude in the analysis new information on the diet and food intake of codsampled from the Northern Ireland cornmercial catches in the western Irieh Seain 1982 and 1983. A. partial ancllysis of the data collected in 1982 wasreported by Boyd (1983).

This paper is divided into four sections: (I) an examination of thecomposition of the stomach contents of cod sampled in the western Irieh sea,and of the sources of variation in these data; (II) estimation of the meandaily intake of food by individual cod; (III) estimation of the annualconsumption of Nephrops by codi and of the predation mortalities inflicted bycod duririg the years of the s~omach sampling programme, and (IV) estimation ofthe changes in biomase and yield of Nephrops that could follow a reduction infishirig effort on cod. Readers who wish to obtain a quick overview of themethods arid results should refer to the brief introductions and discussionsunder the main headings or sub-headings.

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I. ANALYSIS·QF'STOMACH CONTENTS OF COO IN THE WESTERN IRISH SEA

species composition of stomach contents

Introduction

Oetailed studies on tha diet Of cod caught pred?,?imin~ly over the muddysubstratum in the western Irish Sea where Nephrops'are abundant (Fig. 1) havebeen reported by Nagabhushanam (1965), Armstrong (1979, 1982),' Fannon andHillis (1982), Boyd (1983) and SYmonds and Elson (1983). Patterson (1983)examii'led a comparatively small nUrllber of cod caught over ci - mUddy-sandsubstratum, inshore of Armstrong's study areä. All, studies utilized volumetrieor gravimetrie estimation of the species composition of the stomach contents,with the exception of Nagabhushanam (1965) who counted the number of items inthe stomaehs • We review the findings of ,these studies and present, some newinformation. from analyses of stomach contEmtS of cod sampled from NorthernIrish landings during 1982 and 1983 •.

4It Hethods: analysis of sampIes from NI landings in 1982 and 1983

The data presented partially by Boyd (1983), and examined in more detailin the present paper; are from cod taken predominantly in ICES Divisions 36E4,37E4 and 37ES (west of the Isle of Mari, see Fig. 1), by prawn trawlers andpelagic vesselsworking over the Nephrops grounds. Commercial landirigs weresampled monthlY during 1982 and 1983 at Portavogie and Kilkeel harbours. Allsampled cod were measured, weighed, and the wet masses of different species inthe stomaehs were recorded. Fish with everted stomaehs were exe::luded fromsubsequent analyses~ The carapace lengths of Nephrops and the total lengths offish in the 'stomachs were recorded where possible~ No information wascollected onthe level of digestion Of the different items in the stomacha,but efforte were. taken to exclude items that may have beeri eaten in the trawl.

Results,and discussion

Table ,2 gives the percentage contribution of Nephrops" other reptant·decapods, :other crustacea, fish and other items (mostlymolluscs aridpolychaetes)tö the aggregate öf the stomach contents examined in each efthedifferent studies under consideration. The contribution of' whiting ia ~l1sogiven as this spaClas· ia harvestad commercially in the Irish· sea~ Theestimates forNephrops vary between 16%and 60%; the highest values, comingfrom the, stuclies of Armstrong (1979 ~ 1982) and SYmonds and Eison (19ä3),' inwhich a iarge proportion cf the trawl stations were on the muddy regions wheredensities of Neph:rops are greatest. By comparison, the predation ein whiting;which isAl~e caught.in large quantities.in prawn trawls, ia relatively lew atone to ten percent.

These data' show the large va:dation inherent in ana1yses of stomachcontents. 80th Armstrong (1979, 1982) and Fannon and Hillis (1982) showseasonality in the diet of cod, but did not investigate spatial patterns. Inthe next sub~section, we examina critically the sources of variation in thedata collected from Northern Irish landings durinq 1982 and i983.

Introduc:tion

Although the results of the different StllCÜes summarized in Table 2 show

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general-·features which can- be recogn~zed',-as '''co~-liken,. there ia nonethelessconsiderable variation in the data that may be caused by:

1. lnadequate sample sizes resultingin high variance;2. Different sizes of fish sampled;3. Different seasonal and geographie patterns of sampling;4. Different methods and timing of capture;5. Natural variability in the relative abundance, species- and

size-compositions of the prey populations.

For example, previous studies of the diet of cod in the'Irish Sea have relied. on samples from commercial or research otter trawls; However, midwatertrawling presently accounts for approximately half of the catch taken byNorthern Ireland vessels, and may take cod with different feeding habits

"compared. with those caught by otter trawi. In this, section we compare thecompositlon of the stornach contents cf eod caught by Northern Irish ottertrawlers and pelagic trawlers, allowing for the interactions between some ofthe variables given above.

l1ethod

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An analysis of variance (ANOVA) procedure was employed to investigate thesignificance of quarter (January to March, Äpril to May etc.), area of capture(ICES rectangles 37E4, 37E5 and 36E4) arid method cf capture (otter trawl andpelagic trawl) in the estimates of mean mass of total stornach contents ,percentage by mass (Pm) of Nephrops, Norway pout~ whiting, herring and spratin the stomach contents and frequencY.,cf occurrence (Fo)' of cod containirigthese prey items in each trawl sample. The average mass of cod in each samplewas included as a covariate in the analysis where appropriate. A total of 54trawl samples comprising 2i29' fiah wereanalysed~ ,To increase the independenceof the samples, the set. of. 54 samples included some in which two or moreoriginal samples were lumped together if the catch positions, date of captureand size of cod were similar. The distribution of samples by quarter, lCESrectangle and method of capture are summarized,in Table 3; ,from which it canbe seen that the otter trawl samples had good geographie but poor seasonalcoverage, whereas the pelagic samples were mostly from ICES'rectangle 36E4,but were more evenly spread acrose the year. This was 1argeiy a result of the'different patterns of fishing of each fleet. •A program developed at Queen's University, BeHast, and capable ofhandling unbcllanced data-sets, was employed to perform the ANOVA~ ThlSprogram utilizes the inverse of the variance-covariance matrix to' computesums-of-squares values for the" main factors,' interactions andcovariate/regression effects; for use in an ANOVÄ table~ Interactiori terms. arecalculated where the quality of the data are adequate.The fuH model fitted'was as follows:

Yijk ~ Y + Areai +~ethOdj + Q~ar!erk . .+ Interact~ons + b(Xijk-X) + Eijk (1)

where Yijk is the frequeney of .,occurrence or. percentage, by m~sä of a pl:'eyspecies, or average mass of stornach contents, in a cod sample taken in area i(i=1,3) by fishing method j (j=1,2) in ciuarterk (k=1;4), Y i9 the overallmean of the Yijk' Areai ia the effect of the i th level of a factor for area,Mettiodj is the effect of the ~th level of a. factor for. fishing method,Quarterk ia theeffect of ~he kt level cf a factor for qUarter of the year,Interactions are first order interactiona between area and method or method

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· and .quart.e1!'r b is the' regression coefficient of average· stomach mass onaverage.~ass of cod in the samples,. ~ljk is ;the,con~inuous indepe~~ent variateof aver~ge ~ass of cod~or .7he c~rrespond~ng Yijk' X·is. the.arithmetic mean ofthe Xijk; and Eik is the normally distributed residual variation.

~ .· Results

The prey species investigated were'Nephrops, Norway pout, whlting, herrlngand sprat~ Data for both herring and sprat failed initial analyses due to thelow representation of these spaClas in the stomachs; even when they weretreated as a single prey-type. Thay ware however.examined within'a separateprey category comprising herring, sprat and Norway pout. Of the four,data-setsfinally exämined (Nephrops, wh1t:ing" Norway pout arid sprat/herring/Ncrwaypout), whiting was the only prey species' for which the main factors could befitted with no interactions. The remaining three sets had interactions forarea by method and method by quarter. The area by qUarter iriteraction andhigher order. interactions could not be ,.fitted as they produced a singularvariance-covariarice matrix. The significance cf the main effects andinteractlons are summarized in Table 4, and the'means and standard arrors are'given in 'Iables 5(a)-(c) and 'Iable '6 for the prey species Nephrops, Norwaypout; sprat/herring/Norway pout aggregate' and whiting. Ttie results of theANOVA are given below for each variable exarnlned.

Mass of stomach contents: using the model with Yljk as mean mass of stomachcontents, the covariate average mass cf cod in the sample was highlysignificarit, as, expected (F statlstic = 15.187, probability P <O~OOl), andthe January - March values were' significantly different from the values. forthe other three quarters, beirig some 50% less on average. This is reflected inthe freqUency of fishwith empty stomacha,. ,which attained 14.4% in the firstquarter; compared with values· of 4~8%, 3.1% and 6~6% in the following threeqUarters. SimUar values can be derived from the data given in Fannon andHillis (1982); their data give figures of 11.7%, 5~oi; 4.4i and 4~0%respectively for the four quarters.

predationon Nephrops: When fitted to the model without the covariate.term,the factors areai quarter and the interaction ,(method x quarter; all had asignifi~an~ ef.fect on f~eciuency of. oc~u.rrel1:ce,~f N,ephrops (FONep ) ' il1: the codstomachs (Table 5a) •. A significantly.greater proportion of cod from area 36E4had eaten Nephrops.compared with those taken,in area 37E4, and.fe\oier cod,werefeeding on prawns during the first quarter of the year~ The proportion of themasa, öf .sto~ach coritents comprläi~gprawns.(pmNep) .,was more varLable. than t~efrequency . of occurrence; and significant effects of the factors andinteractions could not be demonätrated at the operative level of sampling~Some 60 - 70\ of the cod sampled from both otter trawl arid pelagic trawlcatches taken on theNephrops grourids had been feeding on Nephrops, which madeup 32i cf the stomach contents on average.

Predation on Norway .gout:. Method of ~apture and qu~rter were significant'factors forFONOp(Table 4). The frequency for cod caught by pelaglc trawl wasapproximately twice thät for cod caught by otter trawl (Table 5b) ~ arid thevalue for october toDecember was 2 - 3 times greater ttian those for the otherquarters with tha difference being highly significant.' Although .0nlY 'theqUarter, effect was signÜicant for' pmNOP ,(Table 5b); ,lumping together ofNorway pout, herring arid sprat as a single prey type resulted in asignificantly greater proportion by mass in cod caught by pelagic trawl; the

· differerice again being a factor of twc (Tables 4 and SC).

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Predation on.whitina: ,. None of·the factars examined was significant for FOWHG(Table 4). However, the average ;mase of cod in the samples was significantwhen included as a covariate inthe analysis of PmwHG in thefull model. Themean mass of cod feeding on whiting was 5.69 kg; whilst the viÜue for cod

, feeding on Nephrops was 4.30 kg. Ä similar resultwas obtained when Norwaypout, sprat and herring were lumped together as a single prey~type, indicatingan increasing tendency for piscivorous feeding with increasing size of cod.The negative values for'pmwHG and F0wHG during th~ quarter October to December(Table 6) are a result of fitting a linear model to (in thlS' case) a non­linear process. In particular~ the incidence of feeding on whiting was minimalin smail cod, but increased sharply in larger cod.

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Discussion

The results of the ANOVA shewed a greater incidence of predation on NorwayPout, sprat and herring by cod caught.bY,midwater trawl•.However, method ofcapture did not influence. the estimates of' frequency of. cod feeding onNephrops or the proportion by mase of Nephrops in the stom~ch contents~ It isclear that over the Nephrops grounds, a high proportion of the sarnpled cod hadbeen feeding on.Nephrops irrespective of where they were eventuailY caught inthe water column. This may partly refiect the long residence time of Nephropsin the stomach. If cod move into midwater to feed on small midwater fish whenthey are' locally abundant (Brunel 1972), it would be expected that pelagiccatches of cod would tend to contain a higher freqUency of individuals withthese prey species in the stemachs. than would be the case for otter-trawlcatches which niay contain. cod that have not recently fed in midwater (asobserved by Brunel op~ cit.).

Pelagic trawlers operating in the western Irish Sea. exhibit a muchsmaller variation in catch per unit ef.effort over the year than is the casefor otter trawlers' which experience a sharp increase in' catch-rate of codduring spring (Fig. 2) •. It is therefore postulated that a more demersalbehaviour of cod during the spawning season (February to April) results in adiat comprising mainly species that occur on~' in or close to the seabed.During the second half of the year the.behaviour of the cod.is more pelagic,perhaps stimulated by' an increasadabundance of pelagic prey items. Aproportion of the flsh must migrate to the seabed at some stage during theday ~ where they .. will eat any Nephrops or other prey that .are encounteredthere~ This pattern is in general agreement with observations on the verticalmigration of cod documented by Brunel (1972) and by earlier.workers cited byhim~

size composition of Nephrops and whiting eaten by cod

. Carapace length frequencies of Nephrops in the stomachs of different age­classes of cod taken.during the 1982/1983 samp1ing programme in the Irish Seaare shown in Figure 3, compared. wlth the lEUigth frequency of commerciallandings and discards of Nephrops in 1982. Although a general increase in themean length of ingested prawns with increasing aga, of cod ia äpparent, theratio of mean whole-maas of prawn.to mean maas of cod declined from 0.4% in 1­year-old cod to 0.13% in 4-year~olds; remäining at approximately 0.1% in olderfish.

The sizes of Nephrops eatan by cod are similar to those taken by thecommercial fleet in 1982, but include smaller size-classes. Although it is

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possible. that 'a proportion of the pra~ris. eateri by cod were individualsdiscarded. from fishing , vessels ~ .the cod ..stomachs contained larger prawns onaverage than was recorded in the sampies of discards during the years of thestomach sampling programme (Fig. 3). However, given thevery large nuffibers ofprawns and juvenile white-fish discarded at sea'by Nephrops trawlers, .it isinevitable that a proportion of the discards., will beeaten by cod (and' byother fieh such a~. dogfish and rays). Hence the impact cf cod on theNephropspopulation may be 'less than 'indicated from the results of Brander' andBennett •S (1986), analysis or, in the' update of that analysiä in the presentpaper~ Thie possibility should be borne in mind in evaluating·the output ofthe cod-Nephrops model. -.

Length frequencies of whitinq in the stomachs of cod sampled ~uring 1982 .and 1983 are shown in Figure 4. Most whiting were recorded in the stomachs of60 cm+ cod, and fell.predominantly in the range. 20-36 cm total length~ AB withNephrops; the average size of the prey increased comparatively little withincreasing size of cod, probably as a result ofthe predominance of' juvenilewhiting on the Nephrops grounds durlng most months of the year .

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IntroducÜon

11. ESTlMATION OF DAILYINTAKE OF FOOO··

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Accurate estimates. of the rate of consumption of food by a predator arecrucial for the lmplementation of multi-species assessments, and are generallyobtained by applying iaboratory estimates of stomach evacuation rate to dataon stornach contemts collected in the field~ Our estimates are based on themodel of gastric evacuation derived by Jones (1974), and which haä been usedby Armstrong (1979,1982), Patterson (1983), Fannon' and Hillis (1982) andsymonds arid Elson (1983) for estimating food intake of Irish Sea fish speciesincluding cod~:

Hethod

'Oata from - the 1982/1983 samptlng programrrie. were analysed to. provideestimates of the mean masB of food in the, stomaehs ofcod of. each age- dass'during different periode of the year. The mean dally intake of food per unitbody mass was computed for each age-clasB by means cif the expression derivedby Jones (1974) for the rate of elimination of feod by haddock, cod andwhiting: .

r = 24 (2)

where r ia the rate of elimi~ation (g~d~i) ciffood fram the s~~rriach of a fiehL cm long with meanstomach content mass w resulting from continuous feedingand elimination cf food at anaffibient temperatureTo oe, and 0 ia the hourlyrate of elimination of 1 9 offood of theappropriate type from the ätomach ofa 40 cm fieh at an arbitrary temperature Tcoe.

Brö~ley (in press) has shown that Nephrops are evacuated from'ccid stomachsat approximately half the rate of fish prey, because of the. thickexoskeletons. In view of this finding~ it has been assumed in the presEmtanalysis.that avalue cif 0 ; O:07~, i:e~ half the value for fieh pr~y given byJones (1974), would be appropriate for Nephrops. If. this value is utilized topredict. the .rate . of' evacuation cif Nephrops in Bromley·s.<in. press)experiments, the average. quantity evacuated per hour over the first 48 houre

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. is . almost...identieal to' the value predicted by Bromley's. model of gastrieevaeuation.

Expression (2) was applied to eaeh age-elass of eod to estimate the meandaily intake' of food, as a pereentage of the mean mass of fish in eaeh ageelass, and the pereentage of the daily intake eomprising Nephrops, as follows~First; the value of 0 for Nephrops was applied to expression (2) to estimatethe daily intake for' a situation. in which looi of the stomaeh, eontentseomprised Nephrops. Multiplying this figure by the observed average proportionby mass of Nephrops in the stomach contents then gave an estimate of the meandaily intake of this species( see Armstrong [1982] for' a worked example ofthis method). This procedure was repeated for the other food types, and th~'

values of daily iritake .were summed over prey-types to give the overall dailyintake of food per individual in eaehage-class of cod. For the purposes ofthe present study, the prey,items.other than Nephrops and fish were treated asan aggregate with a value of 0 = 0.12~ The relative proportions of thedifferent food items in the daily intake were then re-estimated.

As the mean mass of food in the stomachs during the first quarter of theyear was {ound to be signifieantly lower thanduring the rest of the,year (seeseetion I)i separate estimates of food intake were made for this period. Tomore accurately bracket the spawning, period, the first quarter was re­specified as the months iebruary to~Äprii.

Results

Table 7 gives the mean mass WCODj and length Lj of eod in eaeh age-class ineaeh period. of the year,the me~n mass, of ~he" stomach contents Wj and th~

number of fl.sh sampled. Table 8 g1.ves the estl.mated daily food intake rj' thetotal daily, food intake as a percen~ag~ of body mass, (lOo~rj/w~O~j)' and theproportion of t:he daily intake comprising Nephrops PN~Pj., Sea temperatur~s i llthe two periods were taken as 60 C and 110 C (Fannon and Hillis 1982). As thepercentage by mass of Nephrops in the stomachs showed considerable variationbetween and within different sampling programmes, values of 20\, 30\ and 40\were subsequently adopted to cover themost feasible'range for estimating theannual consumption of Nephrops by eod (Table' 5). Given these values, thepereentage of the daily, intake eompris~ng Nephrops" PNEPj' ~as estima~ed to be12\, 19\ and 28\ respectively, considerably less than observed in the stomaeh.eontents because of the' leng residenee time. of the prawns in the stomachseompared with faster-digesting items such as fish and natant decapods whichmake up the bulk of the rest of the diet.

The ratio of daiiy intake to body mass of cod was approximately 0.8 - 0.9\during Februaryto April, arid 1.6 -1~8\ during the remainder of the year. Thetrend withage during the latter period was a result ef the larger fish havingdisproportionately more food in their stomachs, relative to their body maSSeMean values for ratio of daily intake to body'mass in äaeh period of the yearwere obtained by weighting the.estimates for ages two to five by the survivalrate to each age-elass (Table 8).

Discussion .

Bogstad and Mehl (1990) compared the performance of six models of gastricevacuation in cod applied to data from the' Northeast Arctic eod, ineludingadaptations ef Jc:mes' (197,4)and Bremley's (in. press)' models. They estimatedthe' average daily food intake in 2-year-old arid elder ced to be 1\ of bedymass using,the Jenes model, and 1.4\ using Bromley's data. Four other models

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gave.vaiuea_of 0.7 to 1.6\. Estimates for'l-year-olds varied ·between 1.8 and6.7\ of body masSe By comparison, the estimates for 2-year-old and older codin the present study were 0.8-6~9\ of body ma~s in spring, and 1~6-1.8\ duringthe remainder of the year. The.estimate of dailyconsumption by one-year-oldcod in the Irish Sea was somewhat less than the values given by Bogstad andMehl (1990).

Values of daily intake of 0.8-1.8\ of body mass·in adult cod are.reasonablyconsistent with theenergy requirements for growth and spawning~ The annualincrease in mass of individual cod in the Irish Sea, as deduced from data onmass-at-age given in Anon. (1991a), would reqUire an average daily food int~ke·ranging from 0.7\ in six-year-old eod to 1.15\ in two-year-old eod, given theenergetic demands of metabolism; groWth and spawning arid the food conversioneffieieneies given by Jones (1978) in his atudy of the food eonsumption ofhaddock andeod~ These estimates assume that the food of eod has a mean energycontent of l~O kcal.g-1•

111. ESTlMATION OF ANNUAL CONSUHPTION OF NEPHROPS BY COD

Int"rodl1ction

Previous estimates of the annual mass of Nephrops consumed by eod in theIrish Sea are summarized by Brander and' Bennett (1986). These include 2munpubÜshed estimate by Brander, based. on the stornach analyses ~givEm inArmstrong (1979), of 3100 - 3900 t (whole weight), arid an estimate by Fannonand Hiilis (1982) of 3130 t~ Our method for estimating annual consumption ofNephrops is similar to thai: adopted by Brander and quoted in Brander aridBennett (1986).

Hethod

(5)

(3)

(4)

C~j

NCODjk

Hass of Nephrops·consumed annuallv: The annual tonnage of Nephrops eonsumedby each ,age-elass j of .eod (CNWj) during the years 1982 and,1983 ~as estimatedfrom the following expressions, where j = age-class of fish (years) and k isthe.period of the year (February to April and May to January):

K ...2. .

~ NCODjk ~ WCODjk • PGk • INTAKEjk • PNEPjk • DAYSk\<.... ,

(NCODjk NCODjk

Fcod • s . + Meod • DAYSk' I 365.jk.

Nephrops groundsj as a proportion

=

=

=

period k:·mean number of eod aged j years (mlllions)mean mass of ,eod aged j years (g)proportion of the eod stock on themean daily food intake of cod agedof.body mass

= proportion of daily intake eomprising Nephropsnumber of days in period k .number of fish aged j at.beginning of period krate of total mortality during periöd k .annual rate of fishing mortality on fully-selected.ages

= . proportion,of full fishingmortality exerted on age-

=

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where forNcod

codjkW jkPGkINTAKEjk =

PNEPjkDAYSkNcod

jk

;~~d

:\110

.. elass j during period k .= annual rate of natural mortality of eod.

'The annual consumption iJf prawns by a11 age-classes of eod, CNW, wasobtained by summing the CNWj over ages~

Consumption of Nephrops bv age-ciass of prawns: The total number of Nephropseaten annually by each age-elass of cod (CNNj) was obtained from:

where CNNjt is the number of prawns aged t years eonsumed annually by cod agedj years. •

(7)

(6) .=CNNj. -

where,WNEpcOdj is the mean mass of individual Nephrops in the stomaehs of e~daged j years sampled during 1982 and 1983~ This number was then disaggregatedinto pseudo~age elasses aceordingto proportions ,at age t; Pjt' obtained by'slicing' the length-frequeneies.of Nephrops in the stomachs of cod. (Fig. 2)at the expected lengths-at~age (see Appendix 1 for details): .

The total number of Nephrops of each 'age-elass' of prawns consumed annuallyduring 1982 and 1983, CNNt was then derived by summing over age-elasses ofcod:

CNNt = 2.. CNNjt (8)

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Estimation of mortality rates caused by predation by cod: . The numbers ofprawns in each ' age-class' consumed annually by cod (CNNt) during 1982 and1983 were incorporated into a conventional age-based eohort analysis in orderto estimate the rate of predation mortallty exerted by eod~ The mean eatehesat length of the International fleet during the years 1981-1984 waredisaggregated into pseudo"age' classes as described in Appendix 1, and it wasassumed . that the average eatcli-at-' aga' (CATCHt) , including discards; wasrepresentative of the catches from,an 'average' year-elass of prawns at thattime. Although some prawns may survive after discarding, the' proportion isunknown and was arbitrarily set to zero i.e~ all prawns eaught eontribute tofishing mortality.

0'•The take of prawns by cod was treated as the catch from a separate 'fleet';

and the predation mortality M2 t exerted on eaeh age-elass t of prawns by codwas eomputed in the same way·that the fishing mortality rates werecomputed;The residual natural mortality on prawns from sources other than eod (Ml) wasarbitrarÜy set at 0.1, hut the sensitivity of the results to a value ofMl=0.2 was also examined. The cohort, ani:llysis was run iteratively until thefishing mortality on 7-year-old and older prawns was equivalent to the averagevalue for 5- and 6-year-olds; assuming the prawns these age';'classes tc:i beequally catchable by the fleet. The selectivity of cod predation on-differentages of prawns was then estimated by comparing the age-frequency of prawns inthe stomachs of cod of eaeh age-class with' the numbers at age in thepopulation.

To allow moderate extrapolation of the results to examine theeffect onthe Nephrops stock of redueing the fishing mortalityon cod, the estimates ofM2 t were divided by the product ofthe annual mean biomass (in thousands oftons) of two-year-old and elder cod and: the mean proportion of the stoek

..•

11

occurring on- ·the Nephrops 'grounds (PG),. to ·give MPBt , the. rriortality exertedper uriit biomass of that fraction of the stock feeding.cn Nephrops~ .

Vaiües of input parameters

Cod population parameters: The average numbers of cod in each age-class atthe begirining of the years 1982 ~nd 1983 (NcOdj,l)' obtalned from an update ofthe Virtual Population Analysis (V~A) applied . ,by the ICES Irish Sea andarietol Channel working Group to catch-at-age data from ICES Division Vlla(Anon. 1991), are given in Table 9 for an assumed natural. mortality rate ofMcod = 0.2 yr-1~ Numbers were also estimated for Mcod=O.l to examinasensitivity of annual consumption of prawns to this parameter~ Ail age classesfrom three years and older ware assumed to be sUbject to the full level offishing mortality (i~e. aSYmptotlc selectivity curve)~

. ,l-I:ea.n mass~s ..atage;, W~~Djk; i?~he periode Februa~y to April,and May tcJanuary, were as estimated from the samples collected in Northern Irelandduring thä period of. the s~?mac!'J. äamplingp7,og7amll1e~ThepropC)"rtion~:Sjk ofthe full fishing mortality exerted on each age class of cod in each period ofthe year were specified. to siinulate "the highly. Eleasonil1 natureof the fisheryduring 1982 and 1983, when half of the aimual catch was taken during thespring 'run' of cod (Agnew 1988). The full rates of fishing mortality Fcod

estimated by means of VPA for the years 1982 and 1983 ware 0.925 yr-1 forcod . . codM =0.2 and 1.029 for M =0.1.

predation parameters: . The proportions-at-age Pjt of N:p!Jroßs' ineach ~ge....classof cod are given in Table ?, as are the mean masses W EP j of Nephropsin the stomachs of cod aged j years.

COd (Üstdbutioiu Tha proportion cf .the' stock occurriiig cm the Nephropsgroundsi,PGk' was considered from two aspects: first, the average fraction ofthe Division Vlla stock·that occurs on these grounds throughout the year, andsecond, the seasonality in this fraction~ The average fraction was deduced byBrander and Bennett (1986) to be 25\, from data on catch per unit of effort ofthe England arid Wales otter-trawl fleet eetimated for different groups of ICESstatistical rectangles in the Irish Sea. This method assumes equalcatchabilities of. cod in all areas, which is unlikeiy • The catch ofcod in1986 fromthe Nephrops grounds inthe western Irish Sea was 34\ of the totalcatch from Division Vlla, and in the absence of any other. information,consumption of Nephrops was bracketed by values of PG of 0.25 and 0.34 •.

Seasonal values for PG were inferred from the qUarterly catch-rates of theNorthern Irish otter trawl and pelagic trawl fleets in. ICES Divisions 36E4,37E4 arid 37E5. Figure 2 shows the mean catch per unit of effort (cpue) of theotter trawlers over the years 1982 tc 1990 and of the pelagic trawlers ovarthe years 1984 to.1989. Although both data-sets show highest values during thefirst quarter, the seasonal pattern in thepelagic, fishery ia much lesspronounced. Bearing in mind that some seasonal depletion of the cod biomasswill occur as a result of intensive fishing, the pelagic catch~rates show thatasubstantlal portion of the cod population.maY remain on the Nephrops groundsthroughout the year; but becomemore pelagic in behaviour after spawriing~reducing their availability to otter-trawiers.

A. range of values cf pGk,for F~bruary toApril arid May to January wereutilized, representing the following scenarios: 1). PGi ,:,,' PG2 = PG (noemigration); 2) PGl = 2.4S*PG, PG2= O.4S*PG (same seasonal trend as observed

12

in otter' trawl cpue); 3) ·PG1="1.36*PG, PG2 = 0.88*PG~(sarne·seasonal trend asobserved in pelagic trawl cpue); 4) PG1 = 1~88*PG, PG2 = 0.72*PG (intermediatetrend). The seasonal change in biomase resulting from mortality and growthhasbeen allowed for in computing these values. Equivalent estimates were derivedfor PG = 0.34.

Results

Table 10 summarizes the estimates of annual consumption ofNephrops for thedifferent combinations of parameters and assumptions outiined above. Theestimates ranged widely from 1720 to 5380 tons (whole mass), and varied mostin response to the v~riables PG and PNEP, as expecte~~ The different chciice~

. . cod cod· .of natural mortality of cod (H =0.1 and H =0.2) and seasonal'pattern of PGhad only a second-order effect on the estimates. The. parameter valuesMcod=O. 2, PNEP=O .19 and PG=O ~ 25were t~ken as the most likely estimates, andwill be referred to as the ~base CaSe", for which the estimate of consumptionof prawns was 3050 tons~

The estim~tes of biomass of two-year-old and older prawns during the years1981-1984, obtained from the'cohort analysis, ranged from 17000 to 24700 tons.A detailed summary of the' output of the cohort analysis for the "base case"parameters is given in Table 11, together' with estim~tes of selectivitycoefficientsSELjt derived from the ratio of ~umbers of pr~wns of age t in codof age j (CNNjt),to the annual mean numbers. of prawns of age t in thepopulation. The rates of fishing mortality were generally much larger than themortality rates inflicted by the cod, particular1y in the older age-classes ofprawn. The highest mortality rates from both sources were inflicted on prawnsof three to four years of age (carapace lengths 24-30 mm).

Discussion

The variation in our,estimates of annual consumption of prawns by cod (1700to 5400 tons, Table 10) is probablyconservatlve because parameters such asproportion of the stock occurring on the Nephrops grounds are poorlyestimated, and the variance in estimates of annual consumption of food are notreflected. The most likely value of consumption i8 approximately 3000 tons peryear, about one third of the mass of Nephrops landed annually by theInternational fleet (Table 1). Previous estimates of consumption of Nephropsby cod in the Irish Sea (Fannon and Hillis 1982, Brander and Bennett 1986)fall within the range of values in Table 10 and are close to our "base case"estimate~

These estimates refer to the years 1982 and 1983, and cannot be used forexamining the effect on the Nephrops stock of a long-term reduction in fishingmortality on cod without first considering the dynamics of the predator-preyinteraction under investigation, 'and the most likely values of cod andNephrops recruitment· in future years. These are· addressed in Section IV(below) •

IV~ THE EFFECT ON NEPHROPS OF A REDUCEDFISHING HORTALITY ON COD

Introduction

In this section we estimate the average biomass and age-structure of thepopulations of· cod and Nephrops at the current levels of fishing mortaiity on

..•

•

---- ---- -----------------

13

each stock,.. .and· thein determine· the. changes in yield. arici-,revenue that couldfollow.a.reduction of the.fishing mortality in either or both of the directedfisheries.· We focus on a 30\ reduction ~f fishing mortality on cod as being afeasible initial target given present values in the region of F=l~Ö for thisspecies. To compute revenues, we have adopted the 1990 values of the landingsof cod arid prawns into Northern Ireland of El185 and Ei402 per ton whole massrespectively. The following analysis is based orily on the extra predationmortality caused by an increase in the cod stöck; the possible implications ofchanges in abundance of other predators on Nephrops are considered later.

In section III; estimates were made of the predation mortality exerted oneach age-class of Nephrops per thousand tons ,of cod in 1982 and 1983, wher~

cod biomass referred to the annual average biomasä oftwo-year-old and oldercod occurring on the Nephrops grounds. If it is assumed that codwill exert aconstant' predation mortality over the variations in stock size consideredhere, the future effects on the Nephrops stock of a change in cod blomass canbe ea~iiy computed. A constant predation mörtality implies that cod will eatmore prawns relative to their own biomasswhen prawns ,are more abundant; whichia a reasonable supposition 'as long as the changes in biomass arecomparativelysmall.

Method

In each of 'the runs 1-19 in Table 10, a further cohort analysis was carriedout on, the mean catches of Nephrops in pseudo age-:classes during the years1987-1990, as derived from data given' in Anon. (1991b). In, this case, thepredatien mortalities M2t were specified as the product of the·estimates ofmortality per unit biomase of cod (MPBt) carried over from the iriitial cohort,analysis of _the 1981-1984 prawn data, the most recent, estimates cf meanbiomass oftwo-year-old and elder cOd for the years '1987-1990, and theoperative value of PG. The estimates of cod biomass were ebtained, from ,anupdate, of the VPA results giVE!n in Anon~ (1991a) with McOd=0.2, 'and anequivalEmt run carried out at MC?d=O.L The estimates of ,mean biomass cif codwere 12500t and 10200t respectively~,This analysis provided new estimates.ofNephrops recruitment arid pattern" of ,fishing mortality at age appropriate tothe present circumstances of the fishery~

since we were interested J.n the gains or losses in yieid that wouid be,incurred on average following a change in fishing effort, average biemass andyields ,cf Nephropsat the.

ccurre!nt rate of fishing mor,tality were 'next computed

from the new estimates of Nephrops recruitment and fishing ,mortalities~, withthe predatien mortalities specifiedfrom the initial'estimates of MPBt 'and the

. average, biomass of ,cod. The biomass ',and yield' cif, cod wereestimated withrecruitment set at the geometric mean of the annual VPA estimates from 1972 tci1989. The lattervalues were 7.189x106 and 5.142x106 O-year-old cod atMcod=O.2 and Mcod=O~l respectivelY.

Having set up this equiÜbrlum äituation, the fishing mortaÜties in thedirected cod and Nephrops fisheries were reduced and the biomasses, yields andcombined revenues were computed for the riew stable aga distributioris~ The

• • , . . • ,'. 0' • ',. • • ~ .... 'ilo 'I' . '. ,. , . _ \.

proportion of the mass .of Nephrops discarded. was set at 20\ in line withrecent practices, .and for consistencywith the'cohort,analyses, discärds weregiven zero survival. It was assumed that 20\ of the fishirig rnortality on, codwould be generated by the directed Nephrops fishery, sinee 20% cif the annualcatch.of cod· iS,taken, by prawn trawlers (this value was also. adopted byBrander and Bennett 1986 in their.cod-Nephrops model). Following ,an initial

14

exploration-cf the effect that varying the fishing mortality on cod could haveon the yield of Nephrops, three scenarios were examined in detail:

1) Fishing mortality on cod reduced by 30% in the cod fishery;2) Fishing mortality on Nephrops reduced by 30\ in the Nephrops

fishery with no adjustment in the cod fishery; and3) Fishing mortality reduced in both directed fisheries to give

an overall reduction in cod F to 70\ of present value.

An additional run of the "base case" model (run nQ. 20) was carried outwith the Nephrops recruitment set at the average of the values estimated for1981-1984 and 1987-1990~

Results

The results of the cohort analysis of 1987-1989 prawn catches are given iriTable 12 for 'the "base ,case" parameters (PG=0.25, PNEP=0~19i ,Mcod=0.2). Bycomparison with the results of theequivalent cohort analysis of the 1981-1984 •data (Table 11), the relative fishing mortil1ity on 2-year-old prawns haadeclined, probably reflecting the increase in mesh-size from 60mm' to 7OInm idespite fishermen retaining more of the small prawris that are caught~ The.estimated number of O-year-old prawns was 70\ of the estimate for the years1981-1984, whilst the biomass of 2-year-old and older prawns was 88\ of the1981-1984 value~

Table 13 gives the leng-term change in ,yie~d of ~ephrops for the base-casemodel~ and for the ,"worst" and "best" scenarios (run mimbers 3, 9 and 12respectively iri Table 10), for o. different values of .. fishing 'mortality on codrelative to the present situation. It can be seen that even when veryconservative estimates of predation rate are adopted, the yield of prawnscould b'e quite sensitive to the rate ,of ~ishing on coc:i, ,assuming no otherchanges in the system affecting mortality of prawns.

- Table 14 shows the expected change in yleld of Nephrops for the threescenarios examined in detail, urider the conditions of the nineteen runs of themodel listed ili Table 10 and the 20th run in which future recruitment ofNephrops was set at the mean of the 1981-1984 and 1987-1989 estimates~ Tables ~15-17 give,the equivalent changes in Nephrops (2-yr~old+)' biomase, revenue of'"the combined yield of cod and Nephrops from bothfisheries~ and cod (2-yr­old+) biomass'and yield.

The only circumstance where an increase in yield of Nephrops fol10wed areduction in fishing mortality in either fisherywas in run nQ~12 where therewas a combinatlon of lew natural mortality on cod, lOw proportion of Nephropsin the diet, only 25\ of the cod stock occurring on the Nephrops grounds andfishing mortality on Nephrops was also reduced by 30\. In all ethercircumstances examined, the, preserit fishing mortality on Nephrops wasestimated to be at or below the level giving maximum yield-per-recruit, withthe.result that reducing the fishing rnortality on prawns would lead to lowercatches in the long term. (Fig~ 5) ~ It earl. be seen from Figure' 5 that for the"base case" parameters,' reducing the fishing mortality on cod pushes the apexof the Nephrops yield-per-recruitcurve farther to the' :dght, so that lossesin yield of Nephrops' could only, be offset by fishing harder on prawns (toremovethe extra cod)~ or, presumably, by allowing greater escapement of amallprawns. This mirrors the findings of Brander and Benriett (1986), who

·'-l

15

incorporated- similar .parameter values· to . those of our, "base case" into theircod-Nephrops model~

In olir "base-case;; model, r~ducirig. fishing mortality on cod by :30% resultedin a loss of some 1500 to 2000 tons of Nephrops yieid in the directed Nephropsfishery, rep~esenting a loss of E2 ~ 0 to E3 ~ 0 million in revenue in thisfishery~ A relatively small gain of 200 to 300 tons of cod (less than EO.5million) wouid accrueto prawn fishermen as a result of the increase in thecod stock (Tabie 17). The increaae in the cod stock is expected,to deplete thebiomass of Nephrops by 2600 tons from the equilibrium level of 19000 tons inthe "base case", if the fishlng mortality on Nephropsremains unchanged (Tabie15). Reducing Nephrops fishing mortality' allows the biomass of prawns t~increase; but as explained earlier, does not allow an increase in yield,although the costs of fishing would go down.

When the combined revenue from the two fisheries ia considered (Tabie 16),most of tha modei runs predicted an overall loss following a 30% reduction infiahirig mortality on cod, because ofthe large decline in valuable landings of

e" prawns ~ However,·· 15-20% of the sets of .. parameters examined predicted anincrease in overall revenue of up to EO. 8 million l:iecause the gains in codoutweighed the losses of Nephrops.

Discussion

•

Model predictions: If the fäte of the prawn fishery alone is considered; theresults summarized' in Tables 14 to 17· show that this sector of the U~K~

fishing . industry, based largely in Northern Ireland arid the Republic ofIreland, could suffer a decline in landings in the range 800 to' 2400 tfollowing ci 30% reduction in fishing effort in the cod fishery, if fishingeffort on Nephrops is not allowed to increase~ If the reduction ~n fishingeffort includes the prawn fishery, the landirigs would decline furthar,although costs would also decrease~ Whilstcod fishermen would benefit iri the10ng run from a reduction in fishing effort, a sudden reduction.bY 30\ wouldresult'in a ahort-term reduction iri yield until catch rates,improve (ignoringat present the irifluence of recruitment fluctuations on yield). It is likelythat prawn fishermen would only improve their lot by fishing more selectively;allowing more young prawns to escape the trawl, although thay would still becompeting with cod for the same resource. . '

Additional sources of uncertainty: How reliable are these findings? Althougha large riuIDber of coIDbinations of parameters have been examined, the dynamicsof the populations and thei%.-. interactions (1. e. how they will change overtime) are'poorly understood. The follo~ing would,result in cod having a moresevare effect on the prawn stock than we have predicted:

the prawn"as, .,"~

prawns1) . Cod forage' harder arid maiIitain their· intake ofbiomass deciines;

2) Cod recruitmentincreases with lncreasing cod biomass;

3) Nephrops recruitment deciines with declinlng biomass of prawns; .

The following would result in cOd having a leBS severe effect than predicted:

4)..

COd Bwitch to feedirig on other. prey when the availablity of prawnsdeclines;

16

5) Cod recruitment declines'with further increases in'cod biomass;

6) Nephrops recruitment or. growth . rates increase as the prawn biomassdeclines;

'.

7)

8)

9)

The range of the cod stock expands as the biomass of cod increases, sotl1at the 'proportion of the stock occurring over the Nephrops groundsdec1ines;

Some of the other species eaten by cod also prey on Nephrops and wou1ddec1ine in abundance fo11owing an increase in cod;

A proportion of the Nephrops eaten. by cod are discarded prawns thatwou1d have died anyway;

Estimates of recruitment are high1y variable in the case of cod(Anon.,1991a) and have on1y been obtainedas averages in the oase of Nephrops~

However, over the range of spawning biomass of each species exp10red in oursimulations, it is doubtfu1 that an effect of spawning biomass on recruitmentcould be detected. Also, at the high rates of fishinq mortality exerted onNephrops, there may be littlescope for increasing growth rate ... as acompensatory mechanism. As far as predation on discarded prawns is concerned,all that can be stated is that it undoubtedly occurs to some extent, but wouldbe difficult to quantify except by taggingdiscarded prawns and carrying outan iritensive stomach-sampling programme at the same time~ A1though it is rareto find only the carapace of" a tailed prawn in a coci stornach, the livediscards, blinded by exposure to bright day1ight, wou1d: be easy prey. Thiswou1d reduce the effective consumption of prawnsin our analysis to the 10wer­end of the range giveri in Tab1e 10, or even to below this.

It is not expected· that variations in the commercia1 catch of otherpredators on large prawns would.have'much effect.on the Nephrops stock; theother abundant predator is the lesser spotted dogfish~ but these are discardedand being very resilient probably have a high survival rate.-

Validating the model A 30\ reduction in fishing mortality on'cod could resu1tin a 15\ decline in Nephrops biomass, whi1st an additional reduction of effort •in the prawn fishery by' 30\ cou1d' result in a smaller increase in biamass.Given the imprecision of indices of abundance such as catch per unit effort~

and the effects of recruitment fluctuations, it is un1ike1y that such smallrelative changes cou1d bedetected and confident1y attributed to predation bycod. Hence the 1ikelihood of validating our model with empirica1 evidencewou1d be very sliin. Large changes. in cod biomase resu1ting .. from - recruitmentvariations cou1d cause a measurab1e response in the Nephrops stock, butmonitoring of blomass, recruitment, growth rate and predation by cod wou1dhave to be carried out at an appropriate intensity to disentangle the variouscauses and effects.

Knock-on effects in the food web All of the aspects of cod and Nephropsdynamics 1isted above are poorly understood a.t present. However, it ispossib1e to compute the magnitude of. an effect that wou1d negate thepredictions of the cod-Nephrops model. Asan example~ we take the case ofpredation on juvenile Nephrops by poor cod (Trisopterus minutus), which arealso eaten by cod. In the cohort analyses descibed ear1ier; - the naturalmortality of Nephrops from sources other than cod was for simplicity assumedto be Ml=0.1 or Ml=0.2 on all age classes of cod. However, if a larger and

--- -------------------------

17

more, realistic value (e ~ g. Mi>o. 5) iä; adopted ,for O-year-old pra~n.s whenestimating present population size by cohort analysis (with the "base case"parameters), a.nd if, this value is subsequently reduced such that,. 600xl06

additional O-year-old prawns survive in the future (simulatinq the depletionof a predator such as'poor cod), this improvement in surviva.i wouldbe enoughto allow the comrm:!rcial' catch of Nephrops to remaiIi the same after a 30\reduction in fishinq mortality 'on cod: Could tha increasedpredation on poorcod under these circumstances riüeclse, enough younq prawns to' maintain thecommercial' ylelds? -, "

Data given by Armstrong (1979; 1982) showed that poor cod longer than 10cm sampled throughout theyear ovar .~he muddy substrata off the ,Iale of Manhad a' daily food consumption equivalent to 'some 3% of their body mass, aridthat O-group prawns with ci mean live' mass of 0.2 q comprised an average of3.5\ of their ~ail~food in~ake duri~g the years 1976-1979: Lower eitimateswere qiven by Symonds and Elson (1983), but most of their sampling took placein autumn whereas Ärmstrong (op. cit~) found, the incidenee of predation onNephrops to be· highest in the first six months' of the year; An annualcorisumption of 600x106 O-qroup prawns waighing 0:2 q eaeh would under thesecireumstanees require just over 300 tons cf peor ced~

We have estLmated that a 30% reduction in fishinq morta~~ty on eod ceuldresuit in a 6400t increase iri the average biomäss of two-year-old and oldercod. These fish would have a total annual consumption of 37000 t of food, atleast 25% cf which eould ooeur ovar the Nephrops grounds. On average~ poer codcomprised 2i of the mass, ofstomach contents of eod äaInpled frem landings intoNorthern Ireland in 1982 and 1983, whieh, allcwinq for relative rates ofdigestion of prawns arid fish in thestomachs, probably represents , some' 3% ofthe daily food intake, or 280 ',tons consumed annually: Thus~ ,inereasedpredation' on poer cod followinq a6400t groWth of the cod stock could releaseenough juvenile prawns to offset the additional consumption of older prawns bycod:

•These calculatlons are speculative, being, based on considerably

informationthan i9 availabie on predation by cod on Nephrops, arid theof poer eod, must at present be regarded as second-order untilunderstanding of the trophic dyncimics 'cf the north-western Irishachieved.

" ,REFERENCES

poorereffeetbatter

Sea is'

AGNEW, D.J~ 1988: Evidenee for the existence of two populationsof Irish Sea cod (Gadus morhua L.) from consideration of growth rates.ICES C:M. 1988/G:65. (mimeo.)

ANON •. 1991 a. Report of,the Irlshsea 'and Brietel Channe1 workingGroup~ ,ICES C:M. 1991/Assess: 1; (mimeo.)

ANON. 1991 b. Report of the Workinq Group on Nephrops stocks;ICES C~M. 1991/Assess:11: (mimeo:)

ARMSTRONG, M.J~ 1979. The feeding eeology of a demersal fishassemblage ovar a muddy substrate off the west coast of theläie of Man:Ph.D. Thesis, Uriiversity of Liverpool. p~182

18

ARMSTRONG"M.J. 1982. The predator-prey,relationships of IrishSea poor-cod (Trisopterus minutus L.), pouting (Trisopterus luscus L.),and cod (Gadus morhuä L.). J. Cons. int. Explor. Mer, 40:135-152.

BOGSTAD, B. and S. MEHL. 199Ö. The consumption rate of NortheastArctic cod :- a comparison of gastric evacuation models. ICES C.M.1990/G:22. (mimeo.)

BOYD, R. 1983. The feeding of cod in the north-west Irish Sea.ICES C.M. 1983/G:6. (mimeo.)

BRANDER, K.M. and D.S. BENNETT. 1986. Interactions between Norway ~

lobster (Nephrops norvegicus) and cod (Gadus morhua) and their fisheriesin the Irish Sea. In: G.S.-Jamieson and N. Bourne (ed.). North PacificWorkshop on stock assessment and management of invertebrates. Can. Spec.Publ. Fish. Aqua~. Sei. 92: 269-281 •.

.' 1

BRANDER, K. 1988. Multispecies fisheries of the Irish Sea. In:J.A. Gulland (ed.). Fish.Population Dynamies (2nd edition): the'.Implications for Management. John Wiley and Sons. pp.303-328.

BRIGGS, R. 1985. Catch composition in the Northern IrelandNephrops fishery. Fisheries Research, 3: 47-60.

••BROMLEY, P.J. in press. Gastric evacuation in cod.(Gadus morhua L.). Rapp. P.­

v. Reun. Cons. int. Explor. Mer.

BRUNEL, P. 1972. The Gaspe cod ecosystem in the Gulf of St~

Lawrence. III. The daily and seasonal vertical migrations of cod(Gadusmorhua L.) in 1960:-62. Naturaliste Can. 99(4):287-357. !

FANNON, E. and J.P. HILLIS 1982. Studies on cod prey with specialattention to Nephrops norvegicus. ICES C.M. 1982/G:51. (mimeo.)

. .JONES, R. 1974. The.rate of elimination of food from the stomaehs of haddock

Helanogrammus aeglefinus, cod Gadus morhua and whiting Herlangiusmerlangus. J. Cons. int. Explor. Mer, 35: 225-243.

JONES, R. 1978. Estimates of the food consumption of haddock (Helanogrammusaeglefinus) and cod (Gadus morhua). J. Cons. int. Explor. Mer, 38:18-27.

NAGABHUSHANAM, A.K. 1965. On the biology of the commoner gadoids in Manxwaters. J. mar. biol. Ass. U.K. 45:615-657.

PATTERSON, K.R. 1983. Some observations on the ecology of the fishes of amuddy sand ground in the Irish ~ea. Ph~D. Thesis, University ofLiverpool. p. 180

SYMONDS, D.J. and J.M. ELSON. 1983. The food'of selected fish species onNephrops grounds in the western Irish Sea. lCES C.M~ 1983/K:8. (mirneo.),

TULLEY, 0., HILLIS, J.P. and D. McMULLAN. 1989~ Fitting normalcurves to polymodal length freqUencies to assess growth in Nephrops.ICES C.M. 1989/K:132.

•

•

19

Table 1. Total international landings, for human consumption, of Nephrops(western Irish Sea stock), whiting and cod (metric tons). (Data are as used byleES Working Groups, Anon. 1991a,b).-----------------------------------Year Nephrops whiting cod

1981 7 339 17 029 14 9071982 9 234 17 219 13 3811983 8 898 10 508 10 0151984 7 070 11 561 8 3831985 6 430 15 952 10 483 '.

1986 8 747 10 086 9 8521987 9 257 10 697 12 8941988 8 254 9 955 14 1681989 8 065 11 139 12 104------------------------------------

Table 2. Percentage composition of the stomach contents of cod caught in thewestern Irish Sea, as reported by different authors. Data from all lengthclasses and months of capture are lumped in each case. NB = Nagabhushanam(1965); SE = symonds and Elson (1983); AR = Armstrong (1979, 1982), PS =present study; FH = Fannon and Hillis (1982); KP = Patterson (1983). (* = noinformation)

, composition, by author

Food type NB SE AR PS FH KP---------------------------------------------------------------Nephrops 301 58 60 30 16 20

ether reptant

}55decapods 162 12 12

}753

ether crustacea 25 12 6

• ether items 12 1 4 7 17

Fish (all species) 16 16 17 43 22 6

whiting .* 5 1 10 9 *----------------------------------------------------------------

1 includes some Alphaeus2 "other decapods" in original paper

20

Table 3. Number of trawl samples included in the ANOVA to detect significanceof quarter, area of capture (lCES rectangle) and method of capture in theestimates of species composition of the diet of Irish Sea cod sampled in 1982and 1983.

. 1

Method = otter trawl

AREA

Method = Pelagic trawl

AREA'.

QUARTER

Jan-Mar

Apr-Jun

Jul-Sep

Oct-Dec

37E4

4

2

1

o

36E4

12

2

1

1

37E5

1

4

2.

1

37E4

2

o

o

o

36E4

6

5

4

5

37E5

o

o

o

1 •Table 4. significance of main effects, interactions and covariate (mean fishmass) in the analysis of variance carried out on stomach content datacollected from samples of lrish Sea cod in 1982 and 1983 (see text fordetails). Fo = frequency of occurrence; Pm = percentage by maSSe Levels ofsignificance: NS = not significant; * = significant with P < 0.05; ** = P <0.01; *** = P < 0.001. Dash indicates that interaction or covariate terms werenot fitted. SP/HR/NP = sprat+herring+Norway pout.

Main effects Interactions

Variable Area Method Quarter Area Method CovariateX Math. X Quart.

------------------------------------------------------------- •Stomach mass NS NS * NS NS ***Fo Nephrops * NS ** NS ***Pm Nephrops NS NS NS NS NSFo Norw. pout NS * *** NS NSPm Norw. pout NS NS ** NS NSFa SP/HR/Np· NS NS NS NS NS *Pm SP/HR/NP NS ** * NS NSFa whiting NS NS NSPm whiting NS NS NS *------------------------------------------------------------

?.l

Table. 5. Frequency of occurrence (Fo) and percentage by mass (Pm) of preyspecies in the stomach contents of Irish Sea cod sampled from Northern Irelandotter trawlers (OTT) and pelagic trawlers (PLT) in 1982 and 1983: ANOVAestimates of mean and standard error of mean (in parenthesis) for main effectsand interactions. See Table 4 for significance tests.

(a) Nephrops as prey

Table 5a Fo Nephrops Pm Nephrops

EFFECTS OTT PLT Mean OTT PLT Mean

Jan-Mar 46.1 45.9 46.0 37.2 17.7 27.5( 5.0) ( 8.9) ( 5.1) ( 5.9) (10.4) ( 6.0)

Apr-Jun 76.4 52.2 64.3 58.7 18.2 38.5( 6.3) (11. 0) ( 6.3) ( 7.4) (13.0) ( 7.5)

• Jul-Sep 55.4 100.5 78.0 49.9 37.6 43.7( 8.7) (11. 7) ( 7.3) (10.3) (13.8) ( 8.6)

Oct-Dec 72.5 79.8 76.2 19.3 22.4 20.8(12.5) ( 9.0) ( 7.7) (14.7) (10.7) ( 9.1)

Area 37E4 65.5 86.7 76.1 42.8 27.8 35.3( 7.5) (13~8) ( 7.8) ( 8.8) (16.2) ( 9.2)

Area 37E5 70.0 52.3 67.4 53.9 21.6 37.7( 6.5) (18.5) ( 9.8) ( 7.7) (21.8) (11. 6)

Area 36E4 52.3 57.4 54.8 27.1 22.6 24.8( 5.9) ( 3.9) ( 3.5) ( 7.0) ( 4.6) ( 4.2)

---------------------------------------------------~------------MEAN 62.6 Ei9.6 66.1 41.3 24.0 32.7

( 4.3) . ( 7.7) ( 4.4) ( 5.1) ( 9.0) ( 5.2)----------------------------------------------------------------

•

Table 5 centd..

(b) Nerway peut as prey---------------------------------------------------------------Table Sb Fe Nerway peut Pm Nerway peut

---------------------------------------------------------------EFFECTS OTT PLT Mean OTT PLT Mean

---------------------------------------------------------------Jan-Mar 9.3 33.5 21.4 5.8 20.9 13.3

5.8) ( 8.7) ( 5.2) 5.1) ( 7.7) ( 4.6)

Apr-Jun 7.5 31.4 19.5 1.0 21.7 11.48.4) (10.6) ( 6.8) 7.4) ( 9.4) ( 6.0)

Jul-Sep 11.6 34.2 22.9 10.5 17.1 13.8(10.2) (11. 3) ( 7.6) ( 9.0) ( 9.9) ( 6.7)

Oct-Dec 49.6 63.4 56.5 34.8 41.0 37.9(12.1) ( 8.8) ( 7.5) (10.7) ( 7.7) ( 6.6) •Area 37E4 14.9 59.6 37.3 9.1 35.3 22.2( 8.3) (13.4) ( 7.9) 7.3) (11. 8) ( 7.0)

Area 37E5 27.9 29.3 28.6 '17.3 19.2 18.2( 7.0) (17.8) ( 9.6) ( 6.1) (15.7) ( 8.4)

Area 36E4 15.6 33.1 24.3 12.7 21.0 16.9( 7.3) ( 3.8) ( 4.1) ( 6.5) ( 3.4) ( 3.7)

------------------------------------------------------------MEAN 19.5 40.6 30.0 13.0 25.2 19.1

( 4.7) ( 7.4) ( 4.4) ( 4.1) ( 6.5) ( 3.8)------------------------------------------------------------

•

.1

Tab1e 5 contd.

(c) Sprat+herring+Norway pout as prey--------------------------------------------------------------Tab1e Sc Fo SPR/HER/NOP Pm SPR/HER/NOP--------------------------------------------------------------EFFECTS OTT PLT Mean OTT PLT Mean--------------------------------------------------------------Jan-Mar 25.7 42.4 34.1 11.8 35.7 23.8

( 9.1) (15.6) ( 8.6) ( 4.5) ( 7.4) ( 4.3)

Apr-Jun 15.0 62.8 38.9 3.9 39.8 21.8 ".

(11.2) (17.4) (10.4) 7.4) ( 9.4) ( 6.0)

Jul-Sep 25.2 38.6 31.9 10.4 24.8 17.6(17.8) (18.5) (12.6) ( 8.5) ( 9.5) ( 6.4)

Oct-Dec 52.4 84.6 68.5 34.9 53.3 44.1

• (19.7) (14.4) (12.2) (10.1) ( 7.4) ( 6.3)

Area 37E4 26.6 76.8 51.7 11.1 57.2 34.1(12.9) (22.2) (12.6) ( 6.4) (11. 3) ( 6.5)

Area 37E5 38.1 52.3 45.2 19.7 31.4 25.6(11. 7) (29.1) (15.7) ( 5.8) (15.0) ( 8.0)

Area 36E4 24.0 42.3 33.2 15.0 26.6 20.8(10.8) ( 6.7) ( 6.1) ( 5.5) ( 3.2) ( 3.2)

--------------------------------------------------------------MEAN 29.6 57.1 43.3 15.3 38.4 26.8

( 7.7) (12.2) ( 7.2) ( 3.7) ( 6.2) ( 3.6)--------------------------------------------------------------

Table 6 •. Fz:.e.queney of oeeurrenee (Fo)· and pereentage by mass (Pm) of whitingin the stomaeh eontents of Irish Sea eod sampled from Northern Ireland ottertrawlers (OTT) and pelagie trawlers (PLT) in 1982 and 1983: ANOVA estimates ofmean and standard error of mean (in parenthesis) for main effeets only. SeeTable 4 for significanee tests.

. 1

EFFECTS

Fo whiting

Mean ( SE )

Pm whiting

Mean ( SE )

7.8 (14.3)

-1.3 ( 9.7)

Jan-Mar

Apr-Jun

Jul-Sep

Oet-Dee

9.2

13.2

2.9

-1.2

3.6)

4.6)

7.3)

4.8)

22.8

25.9

6.7)

9.5)

•Area 37E4

Area 37ES

Area 36E4

1.6

6.1

10.5

7.3)

5.9)

2.7)

3.4 (14.7)

9.5 (11.2)

28.4 ( 5.1)

MEAN (OTT)

MEAN (PLT)

GRAND MEAN

8.0

4.1

6.1

4.2)

4.4)

20.9

6.6

13.7

8.6)

7.7)

5.8)

e~ •Table 7. Mean fish mass W , length Land mean mass of stomaeh eontents W ofeod sampled during two periode of the year during 1982 and 1983. N is thenumber of eod sampled.

February - April

Age weod (g) L (em) w (g) N

May - January

weod (g) L (em) w (g) N

1

2

3

4

5

123

1140

3160

5920

8220

20 2.14

43 12.80

63 30.91

77 89.67

85- 143.00

157

322

327

278

102

1000

2090

3880

5730

7370

40

55

67

77

82

11.86

36.38

87.92

170.1

277 .4

123

584

429

321

98

:<5

Table 8 •. The. estimated· total daily food intake (r j ) , . total daily food intakeas a percentage of body mass, (rj/body mass), and proportion of daily intakecomprising Nephrops, (PNEPj), for different values of percentage contributionof Nephrops, pmNEP to the stomach contents of cod during (a) February - Apriland (b) May to January 1982 and 1983. Mean values for r/body mass wereobtained by weighting the values for ages 2-5 by the survival rate to each ageand rounding to one decimal place.

(a) February - April (T = 6°C)

PMNEP = 20% PMNEP = 30% PMNEP = 40%

Age rj(g)

rj/bodYmass (%)

PNEPj(%)

rj/bodymass (%)

PNEPj(%)

rj/bodymass (%)

PNEPj(%)

•1

2

3

4

5

1.55 1.26

10.48 0.92

26.83 0.85

57.99 0~98

82.54 1.0

o

12

12

12

12

1. 55 1.26

10.24 0.90

26.23 0.83

56.71 0.96

80.72 0.98

o

19

19

19

19

1.55 1.26

9.19 0.81

23.53 0.74

50.86 0.86

72.40 0.88

o

28

28

28

28

Mean (2-5) 0.9 0.9 0.8

(b) May - January (T = 11°C)

PMNEP = 20% 30% PMNEP = 40%

Age rj(g)

rj/bodymass (%)

PNEPj(%)

rj/bodymass (%)

PNEPj(%)

rj/bodymass (%)

PNEPj(%)

•1

2

13.88 1.39

36.30 1. 74

12

12

13.37 1.34

34.98 1.67

19

19

12.00 1.20

31.38 1.50

28

28

3 71.81 1.85

4 118.20 2.06

5 161.65 2.19

Mean (2-5) 1.8

12 69.20 1.78

12 113.91 1.99

12 155.78 2.11

1.8

19

19

19

62.07 1.60

102.17 1.78

139.73 1.89

1.6

28

28

28

Table 9.· Data for cod employed in estimating eonsumption of Nephrops in 1982and 1983: NCOd~,l= numbers at age j (millions of fish) at beginning of periodFebruary - Apr~l, obtained as averages of VPA estimates for 1982 and 1983 fromruns of the VPA done at natural mortality rates Mcod=0.2 and Mcod=O.l; WCOdj,k= mean mass of cod in 9 during periods February-April (k=l) and May-January(k=2); Sj k = proportion of full fishing mortality exerted on each age elassduring ea~h period k of the year; WNEpCOdj = mean whole mass of individualNephrops in cod stomachs; Pjt = frequeneies of prawns aged t years (seeAppendix 1) in cod stomaehs during 1982 and 1983.

Cod age-class j-----------------------------------------------------------------Variable 0 1 2 3 4 5 6 7+-----------------------------------------------------------------Ncod , 1 4.419 3.618 3.114 2.031 0.933 0.234 0.075 0.037(Mco~:.0.2)Ncod , 1 3.299 2.985 2.783 1.827 0.842 0.211 0.067 0.033(Mco~:.O.l)wcod 120 1140 3160 5920 8220 9840 12500 •j,l

WCODj,2 1000 2090 3880 5730 7370 8230 10170

Sj,l 0.0 0.0 0.22 0.50 0.50 0.50 0.50 0.50

Sj,2 0.0 0.2 0.64 0.50 0.50 0.50 0.50 0.50

WNEpcod , 4.0 5.6 6.8 7.8 8.8 9.6 10.5J

Pj,l 0.21 0.22 0.15 0.08 0.05 0.05 0.05Pj,2 0.58 0.51 0.45 0.47 0.43 0.43 0.43Pj,3 0.21 0.20 0.28 0.30 0.32 0.33 0.33Pj,4 0.05 0.07 0.10 0.14 0.13 0.13Pj,5 0.01 0.02 0.03 0.04 0.03 0.03

Pj,6 0.01 0.01 0.01 0.01 0.01Pj,7+ 0.01 0.01 0.01 0.01 0.01-----------------------------------------------------------------

•

"27

Table 10. Estimates of mass of western Irish Sea Nephrops consumed annually(CNW, in thousands of tons) by cod, for different assumptions regardingnatural mortality of cod (Mcod ), natural mortality of Nephrops from sourcesother than cod (NQnep ), proportion of the daily food intake comprisingNephrops (PNEP), mean proportion of the cod stock occurring on the Nephropsgrounds (Pe;) , and seasonal values of PG (PG1' PG2). Associated estimates ofbiomass of 2-year-old and oider prawns (BNEP, in thousands of tons) and numberof O-year-old prawns (RNEP, in millions) during the early 1980s are alsogiven. Run number 3 is considered the most likely scenario and is treated asthe "base case".

"

Run Mcod M1nep PNEP PG BNEP RNEP

•123*4

0.20.20.20.2

0.10.10.10.1

0.190.190.190.19

0.250.250.250.25

0.250.340.470.62

0.250.22 .0.180.12

3.313.193.052.75

19.319.219.018.5

2320229022602190

5 0.2 0.1 0.19 0.34 .0.64 0.24 4.10 20.6 2480

•

67

89

1011

1213

1415

1617

1819

0.20.2

0.20.2

0.10.1

0.10.1

0.10.1

0.20.2

0.10.1

0.10.1

0.10.1

0.10.1

0.10.1

0.10.1

0.20.2

0.20.2

0.120.12

0.280.28

0.190.19

0.120.12

0.280.28

0.190.19

0.190.19

0.250.34

0.250.34

0.250.34

0.250.34

0.250.34

0.250.34

0.250.34

0.470.64

0.470.64

0.470.64

0.470.64

0.470.64

0.470.64

0.470.64

0.180.24

0.180.24

0.180.24

0.180.24

0.180.24

0.180.24

0.180.24

1.932.60

4.015.38

2.723.65

1. 722.31

3.564.78

3.054.10

2.723.65

17.218.3

20.523.0

18.419.9

17.017.8

19.821.8

22.724.7

22.023.9

20302170

24602740

21902380

19902110

23602610

31703470

30703340

Table 11. Detailed results of the cohort analysis carried out on mean catchesof prawns in pseudo age-classes during the years 1981-1984 (CATCHt, millions),incorporating in the analysis the numbers of prawns consumed annually by cod

. (CNNt , millions) in the "base case" analysis. NNEPt = number of prawnsattaining each age t (millions); FNEPt = fishing mortality on prawns at age t;Ml= natural mortality due to causes other than cod; M2 t = predation mortalityexerted by cod; MPBt = M2 t divided by the mean biomass of two-year-old andolder cod during the year. Also shown are the predation selectivities (SELjt)computed from the ratio of numbers of prawns of age t consumed by cod aged jyears divided by the mean number of prawns of age t during the year (meannumber = CATCHt/FNEPt), and normalized against the maximum value for each age~

class of cod.

Pseudo age-class (t) of prawns

o 1 2 3 4 5 6 7+----------------------------------------------------------------CATCHt 0 25 583 399 121 35 13 15 •CNNt 0 72 224 1],9 33 8 4 3

NNEPt 2261 2046 1759 824 253 82 33 14

FNEPt 0.0 0.013 0.476 0.833 0.804 0.650 0.584 0.616

MI 0.1 0.1 0:1 .0.1 0.1 0.1 0.1 0.1

M2 t 0.0 0.038 0.183 0.248 0.219 0.148 0.180 0.080

MPBt 0.0 0.012 0.059 0.079 0.070 0.047 0.058 0.026

SEL1,t 0.23 1.00 0.92SEL2,t 0.28 1.00 1.00 0.80 0.04SEL3,t 0.13 0.64 1.00 0.91 0.63 0.78SEL4,2 0.06 0.58 0.94 1.00 0.83 0.68 0.62SELS,t 0.03 0.38 0.71 1.00 0.79 0.49 0.0SEL6+,t 0.03 0.41 0.79 1.00 0.64 0.52 0.48----------------------------------------------------------------

2')

Table 12. As.Table 11 for the'''base case" parameters, but incorporating datafor the years 1987-1990, and inputting the predation mortalities per unitbiomass of cod that were estimated from the eohort analysis of 1981-1984 prawneatehes (see text). (The M2t are the same in Table 11 and 12 beeause theannual mean biomass of 2-yr-old and older eod at Mcod=0.2 was the same in bothperiods.)

Pseudo age-elass (t) of prawns----------------------------------------------------------------

0 1 2 3. 4 5 6 7+----------------------------------------------------------------CATCHt 0 23 259 308 108 29 12 14

NNEPt 1598 1447 1239 709 241 83 39 19

FNEPt 0.0 0.017 0.275 0.728 0.743 0.500 0.430 0.460

M1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1

• M2 t 0.0 0.038 0.183 0.248 0.219 0.148 0.180 0.080----------------------------------------------------------------

Table 13. Expeeted long-term ehanges in yield of Nephrops (delta.Y, inthousands of tons) following different proportional adjustments to fishingmortality of eod (delta. Feod) in the eod-directed fishery. Values for the"base" case, "worst" case and "best" case scenarios (run numbers 3, 9 and 12in Table 10) are given

delta.Y

delta.Fcod "Base case" "Worst ease" "Best ease"(run 3) (run 9) (run 12)

0.5 -2970 -4290 -21600.6 -2220 -3300 -15600.7 -1560 -2370 -10600.8 - 973 -1510 - 6400.9 - 456 - 720 - 2901.0 0 0 01.1 400 650 2501.2 750 1240 4601.3 1070 1780 6401.4 1350 2260 8001.5 1590 2700 940

Table 14. Expected changes in yield of Nephrops (delta.Y, in thousands oftons) resulting from (a) a 30\ reduction in fishing mortality in the directedcod fishery only, (b) a 30\ reduction in fishing mortality of Nephrops in thedirected Nephrops fishery, and (c) an equivalent reduction in both fisheriesresulting in an overall 30\ reduction in fishing mortality of cod. Numbersrefer to the run number of the appropriate model (see text and Table 10).

delta.Y

o to +0.4

o to -0.4

(a) (b)

12

1,2,3,4,5,6,7,810,11,14,15,16,20

(c)'.

-0.4 to -0.8 9,13,17,18,19 •-0.8 to -1.2 6,12,20

-1.2 to -1.6 3,4,7,10,13,16,18 6,12,20

-1.6 to -2.0 1,2,5,8,11,14,17,19 7,13

-2.0 to -2.4 9,15 2,3,4,1016,18

-2.4 to -2.8 1,14,19

-2.8 to -3.2 15

-3.2 to -3.6 9

Tabie 15. As Table 14, but showing expected changes in biomass (delta.B) oftwo-year-old and oider Nephrops (in thousands of tons).

-------------------------------------------------------------------

31

deita.B ( a) (b) (c)

+7 to +8 20

+6 to +7 1,2,3,4,5,6,7,8,910,12,13,16,17,18,

+5 to +6

+4 to +5

+3 to +4 6,12

+2 to +3 4,7,13

+1 to +2 1,2,3,10,16,18,20

0 to +1 5,8,11,14,17,19

0 to -1

-1 to -2 6,12

-2 to -3 1,2,3,4,7,10,13,16,18

-3 to -4 5,8,11,14,17,19,20

-4 to -5 9,15

9,15

-------------------------------------------------------------------

32.

Table 16. As Table 14, but showing the change in revenue from the eombined eodand N~phrops landings in millions of E sterling.

delta. Y (E)

+0.8 to +1.2

( a) (b) (e)

12

+0.4 to +0.8 12 6,12

o to +0.4 10,13,18 4,7,10,13 6,13

0 to -0.4 6,7,11,14,16,19 1,2,3,8,11,14,18,20

-0.4 to -0.8 1,2,3,4,15,17 5,15,16,19

-0.8 to -1.2 5,8,20 9,17

-1.2 to -1.6 9

-1.6 to -2.0

-2.0 to -2.4

-2.4 to -2.8

10,18

4,7,19

2,3,11,14,16

1,15,20

5,8,17

9

Table 17. As Table 14, but giving the ehanges in biomass of 2-yr-old and oldereod and total yield of eod in the two fisheries (thousands of tons) for runsof the model carried out at Mcod = 0.2 and 0.1 respeetively.

0.2

0.1

Yield

+0.98

+1.48

(a)

Biomass

+4.68

+4.84

Yield

+0.22

+0.32

(b)

Biomass

+0.92

+0.92

Yield

+1.26

+1.96

(e)

Biomass

+6.41

+6.71

33

APPENDIX

slieing of Nephrops length frequeneies into pseudo age-elasses

Information on growth of Nephrops eontained in Tulley et al. and Anon.(l991b) were examined and, in the ease of the data for males given in Tulletet al. (op. eit.), were re-analysed to give a eonsistent pattern of divergentgrowth for males and females. Carapaee lengths at attainment of age were asgiven below. Length frequeneies in eommereial eatehes of male and femaleprawns were "slieed" into pseudo age elasses aeeording to the lengths-at-age.The data for males and females were then pooled aeeording to age for input tocohort analysis. The Nephrops in cod stomaehs sampled during 1982 and 1983were sexed, and so the same procedure could be adopted. For the purposes ofthe cod-Nephrops model, sexes were not treated separately and the averagemass-at-age of male and female prawns at attainment of age was used forcomputing biomasses. The mean mass-at-age of prawns in the commercial catchwas assumed to correspond to the expected lengths at 1.5, 2.5 years of ageetc.

Carapace lengths (mm) Mass (g)

AGE (years) Male Female (male+female)

1 10 10 0.57

2 17 17 2.92

3 24 24 8.45

4 30 27 14.37

5 35 30 21.56

6 39 32 28.33

7 41 34 40.0 ("plus" group)

. 1

Mud and sandy mud

~ muddy sand

36ES

o

54

Figure 1. Map of the north-western Irish Sea showing ICES statisticalrectangles. Also shown are sediment types as identified by theBritish Geological Survey.

ReLQt~ve catch per ~t effort3r-----=-....::....::...:....::.-~------------------__,

-v- PeLagLc - .~ - Otter

•

---------)(- - - - - - - - - - - * -

~

'\'\

2 ~ '\'\

'\'\

'\

1~'\--v-------_. ~

'\.5 t-

15 ~

25

OL---------------------------'2

Quarter cf ,yecr3 1

Figure 2. Quarterly catch per unit effort in the Northern Irish otter trawland pelagic trawl fisheries, expressed relative to the annualaverage value. Quarter 1 is February to April.

50-59 em cod

o60-69 em eod

6 10 14 18 22 26 30 34 38 42Total length (em)

o

o

90-99 cm eod

5

70-79 em cod

o

5

5

5

5

o

10

10

10

10

10

Numbers

Fig. 4. Numbers of whiting of different lengths in the stomachsof cod during 1982 and 1983, by length elass of cod.

1-yr-old eod

6-yr-old + cod

1982 commercial catch(---- discards)

2-yr-old eod

4-yr-old eod

3-yr-old eod

5-yr-old eod

,-.I \

I \I

JJ

JI

I/

"o

\1'1M

\

20

10

0\

20

10

0\

20

10

0\

20

10

0\

20

10

0\

20

10

0\

20

10

6 10 14 18 22 26 30 34 38 42 46 50 54Carapaee length (mm)

. ~

Fig. 3.of

for

Carapaee length frequeneies of Nephrops in the stomaehsdifferent age-elasses of cod durinq 1982 and 1983. Data1982 eommercial eatches are also4ljown.

12 YLeLd (lhousand tons)

•

10

8

6

"1

2

I7-'_'-'-'v-.-.-:::.:~:=:.:::.=~:.=-=:.-.~--=:-

,,~.~ ~-~

*-,/ .... '

./ ./"/' /." /

I /

/1//

//?/

'I

OIA--------------.....L.- ---.J

o 2 .1 .6 .8 1 1.2ReLatLve change Ln fLsloLng mort.aLLt.~

L6

Figure S. Average long-term catch (including discards) of Nephrops fordifferent levels of fishing mortality relative to present values.Key to symbols: 0----0 = natural mortality includes componentgenerated by predation by cod, but with fishing mortality on codas presently estimated; )(.- - -x = fishing mortality on cod reducedby 30%; V_·_·V = all sources of natural mortality on Nephropsheld constant at M=O.3 yr- l •