Vertical Distribution and Prey of Walleye Pollock in the Northern Japan Sea

Abstract

Vertical distributions of adult walleye pollock Theragra chalcogramma and oceanographic conditions were examined in the northern Japan Sea. In the daytime, most walleye pollock were distributed from 150 to 250m depth in April and from 400 to 500m depth in October. Between 150 and 500m depth, temperatures in October (0.4-8.8°C) were similar to those in April (0.5-5.4°C), salinity levels were fairly uniform in both months (34.0-34.2 PSU), and dissolved oxygen concentrations were 4.6-6.1ml/l in April and 4.7-6.4ml/l in October. The main foods of walleye pollock in April were the amphipod Themisto japonica, the euphausiid Thysanoessa longipes, and the chaetognath Sagitta elegans. Daytime weighted mean depths of T. longipes were significantly greater in October than in April. Biomass of T. longipes in the habitat of walleye pollock was significantly greater than other layers in both months. The seasonal change in vertical distribution of walleye pollock is presumably related to food availability of T. longipes.