Long-term change in the distribution of Japanese sardine in the Sea of Japan during population fluctuations

The distribution of small pelagic fish species changes in association with fluctuating stock biomass. We analyzed the spatio-temporal dynamics of Japanese sardine Sardinops melanostictus inhabiting the Sea of Japan during periods of stock biomass fluctuation using generalized additive models (GAMs). Logbook data from drift gill net surveys and large-and medium-type purse seine fisheries, conducted from 1970 to 1999, were used. A 2-step data analysis was performed: (1) for the presence or absence of sardine, and (2) for the catch per unit effort (CPUE) where fish were present. The explanatory variables in the GAMs were year, longitude/latitude, sea surface temperature, and fishing method. The best model based on Akaike's information criterion showed that the likelihood of high sardine presence was limited to areas around the Oki and Tsushima Islands in the early 1970s. However, these areas expanded south along the coast and offshore to the north until the mid-1980s. Subsequently, sardine presence contracted to the initial distribution areas during the mid-1990s. Over the 30 yr period, the spatial distribution of the CPUE changed minimally for both fishing methods. Compared with the trajectory of stock biomass, the potential distribution area began expanding during the initial phase of population expansion, and started contracting before biomass peaked. There was a time lag between the peaks of distribution area and biomass. In conclusion, we discuss how changes to oceanographic conditions in response to a climate regime shift may influence the distribution of Japanese sardine in the Sea of Japan.