The Distribution of Walleye Pollock, Theragra chalcogramma, and Conditions of its Habitat off the Southern Kuril Islands

AbstractData on the spatial distribution of the Walleye Pollock, Theragra chalcogramma, and the conditions of the habitat off the southern Kuril Islands in the spawning season of 2015 and the feeding season of 2016 are provided in this report. In the spring, most walleye pollock are distributed more evenly over the shelf and continental slope to a depth of 900 m; in the warm season, the area of aggregation is reduced and fish move beyond the shelf zone and concentrate off the Pacific side of Iturup Island in a narrow depth range of 200–270 m. In spring, walleye pollock are found at a water temperature of 0.1–1.5°C; in summer they are found at 1.3–2.9°C. Off the Pacific side of the Kuril Islands, walleye pollock aggregations are recorded from shelf and intermediate waters in the spawning season and from intermediate waters over the slope in the feeding season. On the Sea of Okhotsk side, walleye pollock are caught at the boundary of the Sea of Okhotsk area of mixed waters in spring and over greater depths in the upper layer of intermediate waters in summer. On the Pacific side of Iturup Island, variations in depths and horizons of walleye pollock habitat are minimal; in the Sea of Okhotsk waters and along the Minor Kuril Ridge fish descend to lower layers of the epipelagic zone and to the mesopelagic zone in summer. Seasonal redistribution of walleye pollock and changes in the size–age structure of their major aggregations were also observed.