Mass Occurrence of Walleye Pollock Gadus chalcogrammus (Gadidae) in the Surface Layer of the Oyashio Basin (Western North Pacific): Possible Benefit from Abundant Copepods

A total of 2999 individuals of walleye pollock Gadus chalcogrammus was caught by a surface trawling net covering the 0—ca. 30 m depth layer during May 2022, at two sites, 36 and 172 km off the southern coast of Hokkaido Island. The fact that no fish was caught at a station further south suggested that the pollock originated from the coastal area of Hokkaido Island rather than Honshu Island. Although the body length of the fish from both sites was similar, ranging ca. 35–45 cm standard length, fish condition differed significantly with fish from the southern (offshore) station having stouter bodies. Stomach contents analysis revealed that the fish from the southern station ingested 2.1 times more prey (measured as stomach contents index) consisting mainly of copepods (86% in dry weight) than those from the northern station. Because the southern station was in a more advanced condition of phytoplankton blooming, it has been suggested that the walleye pollock migrated south to utilize the abundant copepods in the surface layer during the blooming.