1Graduate School of Fisheries Science, Hokkaido University, Kokkaido 041-8611, Japan
Density of the fish shoal and vertebral counts of Pacific cod at various locations in the Sea of Japan and in the Pacific Ocean off the Tohoku District, and in the Pacific Ocean off Hokkaido were studied. Off the Pacific coast, there were three high density areas of fish shoal, i.e., the waters off Sanriku in the Tohoku district, the waters off Cape Esan and the waters off Kushiro in Hokkaido. In the Sea of Japan, there were two high density areas of fish shoal, i.e., the waters off Aomori and Akita prefectures and the waters off Yamagata prefecture. In addition, factor analysis of the monthly catch per seine net showed that the fish in the eastern waters off Cape Erimo, those in the western waters off Cape Erimo, and those in the waters off Sanriku each have peculiar trends of change in the CPUE (catch/net) among monthly catches. Furthermore, the statistical analysis of the vertebral counts showed that the Pacific cod from the eastern waters off Cape Erimo had significantly higher vertebral counts than thePacific cod in other areas. The results of the present research and the information on spawning grounds previously reported, suggest that there are three subpopulations of Pacific cod in the present research area in the PacificOcean, i.e., a subpopulation in the eastern waters off Cape Erimo, a subpopulation off Cape Esan and a subpopulation off Sanriku District. Further studies are needed to determine the locations of subpopulations of this species in the Sea of Japan.