Quantitative Relationships between Autumn Catches of Swordtip Squid ( Uroteuthis edulis ) and Oceanic Conditions to the East of Tsushima Islands, Japan

Although the behavior of swordtip squid (Uroteuthis edulis) caught in the southern Japan Sea and eastern channel of the Tsushima Strait in autumn have been estimated, it needed to be improved in scientific and fishery benefits, which were a harvest scenario and the reason for serious poor catches in the autumn of 2019. This study aimed to identify oceanic conditions affecting autumn catches using a numerical ocean model. To determine the effects of eddies that occurred in the waters to the east of the Tsushima Islands, Lagrangian tracers were released and the number of tracers remaining in the waters, where the squid of the autumn migrating group are supposed to grow in summer, were counted regularly. Quantitative results in the tracer experiments supported and improved the hypothesis of the migratory behavior of the autumn group. The regression expression was obtained with 0.851 of correlation coefficient between the catch per unit effort (CPUE) in autumn and the index composed from the number of the remaining tracers in summer and the CPUE of May. In the summer of 2019, a U-shaped current occurred from the north to the east of the islands, which perhaps accounted for the poor catches in the autumn.