1 Not to be oited without prior reference to the authors nternational Council for the GM 919ß..: 33 Exploration of.the S Shefish Committee. Ref: Demersal Fish Committee Observations on the feeding o:f young 90d GadUfi ljorhua L.. on Nephrops norve,goicus (L.) in" the' Firth of Forth" by j.a.m.., Kinnear and K. Livingstone DAFS Marine Laboratory, Victoria Road Aberdeen, Scotland,, Abstract This paper presents the results of a preliminary invesi;igat'ion into the feeding habits of young cod Gadus morhua L. in the Firth of Forth, Scotland. Cod. smaller than 40 cm were found to be feeding on pa.nd.a.lid and crangonid shrimps. Larger cod. (up to 64cm) had been feeding on small fish, porturrl.d crabs and Norway lobsters, Nephrops norvegicus (L.). The Norwa3 lopsters were mainly small immature specimens Cmean ca.!'apace length 2Omm). t is tentatively suggested that some of these small Norway lobsters had. previously been discarded by fishing vessels working in the area. Rsum Cet article prsente les rsultats d'une investigation prliminaire sur les habitudes d'alimentation des jeunes moreus Gadus mqrhua L. dans le Firth of Forth, Ecosse. On a trouv des morues de moins de 40cm se nourissant de petits poissons, des crabes portlliides et des homards de Norvge, Nephrops norvegious L. Les homards de Norvge taient pour la plupar.t de petits spcimens qui n'avaient pas atteint la maturit (longeur mqyen de carapace 20mm). On propose une suggestion provisoire que certains' d'entre ces petits homards de Norvege aient prcdemment t jets par. les barques de peche qui travaillent dans la rgion. ntroduction Although a proper uriderstanding of the feeding biology of the species concerned ia es.sential to any truel evaluation, of the dynamics of 'an exploited fish population, it has taken th recentupl:!urge of interest in multi,species modelling to re,veal how little is kno about, the trophod.ynajnics cf the main exploited species. Two sorts of information are required, qualitative data' on interspecific re1ationships and quantitative data on the energy links betwee,n pretor and. t>tey- The present work is a Bll contributit>n to the., qualitative data base for the cod Gadus morhua;. L.
2 Methods t' F The 600 were l collected during a crmse by.:t'he DAFS Fisheries Resea:r;-ch Vessel tgoldseeker' in November Fi-shing was concentrated on the commercial Nephrops.fishing ounds betwee;n the Bass Rock and Fidra (56 5t'N/2o,28iLand- 56 5i.!N2041'W). All the. hauls were made within the depth range 36-38m using a' standard commerci:al" Neg,l:!.r'..2.12! tra\il. A total of 392 cod were caught, of which the stomachs of 241 were examined. The percentage size distributions of all the cod caught and those examined are shown in Table 1, from which i t can be seen that fis:g. in the 3ü-40cm size group predominated. The stomachs were dissected and their contents identified immediately after capture. Any ephro;ps present were measured wherever the remains were sufficientlycomplete for this to be possible. Results and discussion The main items of food found in the stornachs were as follows: Nererops norvegicus (L.) Y.rangon crangon (L.) Pandalus spp. Hil,POglossoi.des ;elates.soid (Fabricius) Other species present (Total numbers fund; - in brackets) were:, LJ1Pl,ffnus lupljretaeformis. (lrlalbaum) (2) Aphrodite sp. (2) Eolychaeta (unidentified) (2) The relative importance of the main species varied a.ccording to the size of the cod (Figure 1). Crangonid and pandalid shrimps were the most important i tems in the diet of the smaller fish (25-40cm) but their occu'z1ence in the stomachs fell as predator size increased, to be replaced by Non1ay lobsters, swimming crabs and small fish. Approximately 15% of all the fish examined had empty stomachs. Thirty wo per cent of all the cod. examined contained Nephr0,as Temains,. ranging from 11.8% in cod smaller than 30cm to 44% in cod of 40cm or larger (Table 2). Cod less than 50cm had no more, than one Nephrops in the stomach contents; those of 50cm or larger occasionally contained two Nephrops. but none had more than two. The size of :N;e,phro,ps found in the stomachs varied in carapace length from 8 to 30mm with a mean of 20mm compared with a mean size of 28mm for the trawl-caught specimens. ]ie;phrcw.s. in the l5-3omm carapace length range were found in cod throughout moßt of the size range, but mainly in specimens cf 40cm or larger. Nephrop" smaller than 12mm predominated in ced smaller thanaocm, which might be expected owing to their apparent preference for shrimps, which are smaller than Neghr,oJ?s. As wel as a change in diet from 2.
3 shrimp to J?hro.l?S there is a little evidence for a slight increase in the size of Nephrons eaten as cod size increases, but the general pref'!rence seems to be for the smaller NephroM (Fig 2). Simple preference y the predator may not be the entire explanation however. t 1s possibte that in this heavily-fished area the size composition of the NeJ?hrogs eatn by the cod was biassed downward by the presence of "disca.rd.e". n other worde some of the eghrogs eaten by the cod had already been caught by commercial trawlers and thrown back. Such animals are likely to be especially vulnerable to predation since apart from gross damage they may have suffered in the trawl and on deck they would lack the immediate protection of their burrows and could also be suffering from impaired vision (Loew, 1972). On the basis of the circumstantial evidence presented here it i6 not possible to say whether discarded Neghros suffer enhanced mortality due to predation over and above any mortality caused by direct fishing-indueed injury. nevertheless feel that the issue is suf'ficientlyimportant to warrant experimental investigation. Reference - Loew, E.R. Light and photoreceptor degeneration in the Norway lobster, NeRh,rogs. norv;egicus (L. Proc. R. Soc. (Ser. B) (1976). r.]e 3.
4 TABLE Size composition of codj (1) total catch, (2) sample examined for gut contents (1) Size(cm) No % (2) No %
5 -. --_... _ _.. - TABLE 2 Cod Stomach Contents (Nephros) Firth of Forth 1978 Cod Size Rap,g,e,CTl..f9,o-39 :1-<>-4,2, 50;-5; OveGf-ll Numbers of stomaehs examined Number of stomachs containing w,pe %containing Nererp,,:e,s, Total number of NepN;,ops foum rean number of JtehrOtS stomac * Size Range of 'phrogs cm None measurable 1 Mean size of Nephrogs *Excludes empty stomaehs.
6 -.G. ;,' 1' /,"'".;,tf r '1 /','l /;' "",. o,..,....,,"" " A:: -o A. o,.,,1ft- A. 0'".. ' 0 0. \, J'... \ ;..,.. y...,/. o' """oo /..7,',.....".0'./ "..0"..,,-., 4';;iO \ /'.. "---". ' l. ----:------' _-----_.. _:_- _ _ , ' :'... o' : M"''_"':";''...."... '..... _.: ",.. '. ":...
7 '" r '....,..," , "...-:"":\ ;. f l,l i.l rj 20 ".,,, \. i. \.,"" ",.,,, \ ",' '\, "'\ \, \., \, " \, ' \ '\ \ " \ \ r '':\ J!J'... '34 (mm) i f r,. f f '1 '.. ir Ä j st. '; :----::"----: '- _ _-_ _- _-_ ,. -,.... Per.centage '6:tz COltp6tchs (Cai-ac lon8\;hs b7 groups) of trawled Wl'!r.ro'PS (ccn'tinuowf lilie) end,!frlr.!...found cod 8toinaS\brokä1liite). ". M,".".... ".. :...." : : "" '-,, -... "..,. ".