Feeding habits of cod, capelin, and herring in Balsfjorden, northern Norway, July-August 1978: The importance of euphausiids

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Abstract
Midwater and bottom trawls were used at two stations (120 and 180 m depth) in Balsfjorden during the summer of 1978 to study the feeding of fishes on euphausiids, one of the major causes of 120 kHz sound scattering. The principal food of cod consisted of capelin (Mallotus villosus), euphausiids (Thysanoessa raschii, T. inermis, and Meganyctiphanes norvegica), and shrimp (Pandalus borealis). At the 120 m station, herring and small cod fed mainly on euphausiids. Because of diel vertical migrations, euphausiids are presumably close to the bottom during the day at this depth, and hence very susceptible to predation by both pelagic and benthic fishes. During the darkest period of the 24-hr day cod caught in midwater (50–70 m depth) had appreciable quantities of fresh euphausiids in their stomachs. At the 180 m station euphausiids were relatively unimportant in the food of cod caught in bottom trawls but were numerous in cod caught in midwater trawls at depths of the 120 kHz sound scattering layer. The scattering layer did not impinge on the bottom at this station. It migrated from depths of about 150 m by day into the upper 100 m at night; both capelin and cod were associated with the layer. Although copepods were the most numerous type of prey in capelin stomachs from the 180 m station, euphausiids predominated volumetrically.