Feeding by larval cod in different water‐masses on Western Bank, Scotian Shelf

Abstract

Gut contents were obtained from 1406 cod larvae from 94 stations in seven water masses related to a gyre around Western Bank, Scotian Shelf, 22 November–16 December 1992. Initial samples were from: well‐mixed water over the bank crest (CW); a surrounding convergent FRONT; relatively cold, fresh water (CFW) largely east of CW; warmer, salty water (WSW) west of CW and FRONT. After a storm on 3–6 December, samples were from CW and CFW displaced south‐east on the bank and, after further winds 11–12 December, from CW displaced north‐west off the bank. Zooplankton biomass (300–333 μm mesh, mostly Calanus copepodids) did not differ among water masses, but larval concentrations were significantly higher in FRONT than elsewhere. The small‐copepod diets of larvae varied among water masses, partly attributable to larval growth during the sampling period. Numbers of prey in guts, and indices of fullness and digestion, varied among water masses. More reliably, after ANCOVAs significant independent variables were: overwhelmingly time of day (maximum prey numbers and fullness at ∼19:00) and larval size; water mass; weaker interactions of the above among themselves and with sample depth and date; a very weak negative turbulence‐index effect on gut prey numbers in depth‐stratified samples. After ANCOVAs, larvae from prestorm CW had significantly higher prey numbers and fullness than did those from FRONT, WSW, and CFW. Larvae in CFW were significantly fuller when sampled closer to sites of former CW after the storm. Although numbers of prey in larvae advected off the bank in CW decreased significantly, prey averaged larger, so that gut fullness did not decrease. We conclude that larvae were best fed in the `centre' of the Western Bank gyre, but not greatly affected by subsequent displacement off the bank.