Winter distribution of young walleye pollock Theragra chalcogramma investigated with quantitative echosounder on the Pacific coast of eastern Hokkaido, Japan

Abstract

  To understand winter distribution patterns of young walleye pollock Theragra chalcogramma and its determinants, we conducted an echo integration survey on the Pacific coast of eastern Hokkaido (the Doto area) in late February of 1998. Pollock aggregations were mainly located from the shelf edge to the continental slope region (150-400 m) with temperature ranging from 2 to 4°C. In contrast, the fish echoes were not present in the continental shelf region, which was dominated by the low temperature (<2°C) and low salinity (<33.0) Coastal Oyashio Water. Walleye pollock showed diel vertical migration patterns, involving staying near the bottom during the day and rising to the mid-water area at night. Our results show the fish's (1) offshore-ward movements and (2) concentrations in deeper water in winter. These observed pollock distribution changes were probably enhanced in marine environmental dynamics, caused by intrusion of the Coastal Oyashio Water into the shelf region. It is suggested that such distribution changes affect young pollock survival and are important for determining year class strength.