Effects of oceanographic environment on the distribution and migration of Pacific saury (Cololabis saira) during main fishing season.

The Pacific saury (Cololabis saira) is one of the most commercially important pelagic fishes in Asia-Pacific countries. The oceanographic environment, especially the Oyashio Current, significantly affects the distribution of Pacific saury, and may lead to variations in their migration route and the formation of fishing grounds in Japanese coastal region and the high seas. In this study, six oceanographic factors, sea surface temperature (SST), sea surface chlorophyll-a concentration (SSC), sea surface salinity (SSS), sea surface height (SSH), mixed layer depth (MLD), and eddy kinetic energy (EKE), were associated with the monthly catch per unit effort 1 (monthly CPUE, ton/vessel) and the monthly CPUE (ton/day) of Pacific saury from Chinese fishing vessels during the optimal fishing periods (September-November) in 2014-2017. The gradient forest analysis showed that the performance of monthly CPUE was higher than monthly CPUE and SST was the most important oceanographic factor influencing monthly CPUE, followed by EKE. The generalized additive model indicated that SST, SSH, and EKE negatively affected monthly CPUE, whereas SSC, SSS, and MLD induced dome-shaped increases in monthly CPUE. The distributions of fishing locations are likely to form along Offshore Oyashio current and meanders, especially in October and November. Synchronous trends in the relationship between the intrusion area of the Oyashio and relative abundance variation index suggest that an increase in the intrusion area of the Oyashio causes more Pacific saury to migrate to the Japanese coastal region, and vice versa. These findings extend our understanding of the effects of the oceanographic environment on Pacific saury.