Diel vertical migration and interaction of zooplankton and juvenile walleye pollock (Theragra chalcogramma) at a frontal region near the Pribilof Islands, Bering Sea

The diel vertical migration of age-0-walleye pollock (Theragra chalcogramma Pallas) and their principal prey organisms were examined at a productive frontal region in the Bering Sea using 38 kHz acoustic measurements and net samples. Small copepods dominated the catch of depth-stratified plankton tows. Two copepod species (Calanus marshallae and Metridia pacifica), euphausiids and chaetognaths, exhibited strong diel vertical migrations, although the magnitude and timing of the migrations varied among taxa. Age-0 pollock dominated midwater trawl catches (92% by number) that targeted layers of strong acoustic backscatter. Distributions of target strengths (TS) recorded within the layers corresponded well with predicted values based on empirical length/TS relationships for age-0 walleye pollock. Juvenile pollock in these layers migrated from daytime maxima at 40 m depths to less than 20m at night. The proportion of large copepods, euphausiids, and chaetognaths in the diet of juvenile pollock increased with increasing fish size, but prey composition did not change significantly throughout the diel period. (C) 2000 International Council for the Exploration of the Sea.