Food habits of walleye pollock inhabiting the mesopelagic zone in the Northern Japan Sea in spring and autumn

Abstract

The diet and feeding intensity of walleye pollock Theragra dialcogramma were investigated in April and October 1993, and in April 1994. Specimens ranging from 262 to 456 mm in standard length were sampled with a mid-water trawl off Hiyama Subprefecture and Aomori Prefecture in the northern Japan Sea. The main food items were large-sized Zooplankton. In April, pollock fed chiefly on euphausiids (Thysanoessa longipes), chaetognaths, and hyperiid amphipods (Themisto japonica). In October, the main food organisms were hyperiid amphipods (T. japonica). Stomach-content weight indices indicated that the feeding intensity of pollock was stronger in April (after spawning) than in October (before spawning). The numerical compositions of food categories in April differed among sampling stations. Such differences were not observed in October. It seems likely that the distinct variation of diets in April is ascribed to geographic differences in Zooplankton composition in the mesopelagic zone.