Interaction between the vertical distribution of juvenile walleye pollock Theragra chalcogramma in the eastern Bering Sea, and cannibalism

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Abstract
In autumn 1984 the interaction of vertical migration of age-0 walleye pollock Theragra chalcogramma in the eastern Bering Sea, and cannibalism was examined. Patterns of vertical migration were inferred from acoustic observations, midwater trawls, and occurence of juveniles in the stomachs of adults caught in bottom trawls. Generally, most age-0 pollock were distributed in the near-surface layer at night. During the day larger individuals moved through the thermocline into the bottom layer. Variations in migratory patterns were observed and are discussed in relation to the thermocline, zooplankton abundance, growth, and size. Age-0 juveniles from 50 to 60 mm ate mostly large copepods. At 3 stations the growth of juveniles, as inferred from otolith increment widths for the 10 d prior to capture, increased with zooplankton abundance. Of 1503 adult pollock stomachs, 29% containined juvenile pollock, all of which were age-0 fish, 20 to 100 mm in length. Adult pollock contained an average of 3.0 juveniles per stomach. The size composition of juveniles in adult stomachs and the diurnal pattern of adult pollock feeding on juveniles indicated that cannibalism occurred mostly in daytime and was closely coupled with vertical distribution patterns of juveniles.