Zooplankton prey selection by juvenile fish in Nova Scotian Shelf basins

The stomach contents of fish <100 mm standard length (SL)and fish >100 mm SL collected during June and October intwo deep basins on the Scotian Shelf, showed that juvenile co(and silver hake fed primarily on the two most abundant speciesof zooplankton in the basins: Calanus finmarchicus and Meganyctiphanesnorvegica. These zooplankton were abundant in the basin: duringthe 2 months, forming high concentrations at depths below 200m during the day. In June fish (<100 mm SL) concentratedin the upper 50 m during both day and night. Cod and wolffislcollected between 195 and 240 m had a large percentage of C.finmarchicusin their stomachs suggesting that these fish exploited the highconcentrations of C.finmarchicus below 200 m depth There wasno evidence that Calanus hyperboreus was eaten by any juvenilefish species. Calanus finmarchicus was the most common preyof cod feeding in the top 50 m, but the percentages o C.finmarchicusstages IV and V were lower in the stomachs than in the watercolumn. Cod showed ; strong preference for M.norvegica, withthis prey species having a modified Ivlev's index of 1 indicatingthat cod were highly selective for this species. There was noevidence that C.hyperboreus. stage IV was eaten by any of thespecies offish. Silver hake and dogfish (>100 mm SL) bothfeed oi M.norvegica and C.finmarchicus in the deep regions ofthe basins. In October, silver hake was the most common juvenilefish in the basins, feeding primarily on small stages of M.norvegica.The populations of fish (>100 mm SL) in the basins were mainlyspecies known to feed heavily or euphausiids. The two basinsare unique regions of the shelf because of the large populationsof Calanus copepods and M.norvegica they contain all the yeararound, thereby providing an atrractive feeding ground for manyspecies of fish, particularly silver hake.