Feeding selectivity of larval walleye pollock Theragra chalcogramma in the oceanic domain of the Bering Sea

Abstract

Feeding of larval walleye pollock was examined with respect to density and distribution of micro-and mesozooplanktonic prey (<500 pm) at 6 depths in the oceanic domain of the Bering Sea to determine if first-feeding larvae select among available prey and to assess their foraging environment in comparison to other locations where walleye pollock spawn. At 30 m depth, walleye pollock larvae and copepod nauplii occurred at maximum densities of 106.6 ind. 100 m-3 and 26.3 ind. 1-', respectively. First-feeding walleye pollock larvae (3.5 to 6.5 mm) fed exclusively on copepod nauplii and copepod eggs. Among copepod nauplii, larvae selected for Metridia sp. and Microcalanus sp. but against Oithona similis, even though the latter was the most abundant prey taxon in the study area. Of the nauplii ingested by larval walleye pollock, there was evidence of selection for larger nauplii within Metridia sp., Microcalanus sp., and 0. sirnilis. In addition, larvae preferred Stages I11 to V of calanoid nauplii Larvae at 30 m depth had the highest incidence of feeding (97.5%) and number of prey items (4.6 larva-') ingested. Although the 30 m depth stratum prov~ded best physical and foraging conditions, the overall low percentage of feeding larvae and low numbers of prey consumed suggest that foraging conditions for larval walleye pollock at the time of sampling were below saturation feeding levels.