Comparison of fluctuations in fish communities and trophic structures of ecosystems from three currents around Japan: synchronies and differences

Features of three marine ecosystems affected by the Tsushima (TWC), Kuroshio (KC), and Oyashio (OC) currents were analysed based on fishery, oceanographic, and climate datasets during 1955-2010. Principal component (PC) analysis for catches of 25 indicator species showed evident decadal variation patterns with a step change in the first principal component (PC1) in the late 1980s, indicating synchronies in the dominant variation mode across the ecosystems. Step changes were also detected in the mid-1970s in PC2 and PC3 in OC, and around 1970 in PCs in KC and TWC. These indicate that the most marked change across the ecosystems occurred in the late 1980s, corresponding to the late 1980s climate regime shift, but OC also responded strongly to the mid-1970s regime shift, indicating different responses to regime shifts. The generalized additive model showed the PCs associated largely with water temperature in each region as well as climate indices, indicating the importance of regional oceanographic conditions. Ecosystem indicators such as the mean trophic level showed similarities between TWC and KC but differences with OC, indicating that trophic structures in TWC and KC were largely dependent on the mid-trophic (small pelagic species) level, while on demersal species in OC.