Long-term comparison between the Japanese sardine Sardinops melanostictus stock level and simulated zooplankton density around the Kuroshio region

Abstract

Sardine species are known for multi-decadal-scale stock fluctuations. The Japanese sardine Sardinops melanostictus is one such species that is distributed near the Kuroshio western boundary current system. Like other sardines, food availability in the larval stage is thought to be an important factor in Japanese sardine stock fluctuations. Previous studies have suggested the significance of the feeding environment, particularly in the winter and spring seasons, within the Kuroshio axis area. However, collecting zooplankton, the primary food for larvae, along the strong current is challenging, and there is a lack of long-term observational data on zooplankton in this area. Therefore, in this study, we estimated zooplankton density in the Kuroshio axis area from 1967 to 2018 using a coupled physical-biological lower trophic level ecosystem model and compared it with the logarithm of recruitment per spawning stock biomass (LNRPS) and logarithm of recruitment per egg (LNRPE) of the Japanese sardine. Significant correlations were found between the zooplankton density and LNRPS from 1967 to 2004 and between zooplankton density and LNRPE from 1979 to 1996. Since, in this study, Granger causality was established between zooplankton density and LNRPS before 2005 and between zooplankton density and LNRPE before 1996, in these periods, namely 1967-2004 and 1979-1996, there is a high likelihood that zooplankton density influenced the interannual variation in recruitment. On the other hand, we found no relationship between recent food availability and LNRPS/LNRPE. This aspect remains a subject for future research.