Abstract
Understanding how pelagic fish assemblages reorganize in response to oceanographic variability is central to fisheries oceanography. We analyzed long‐term purse‐seine logbook data (1994–2022) from the East China Sea and Sea of Japan to link multivariate assemblage structure with species‐specific occurrence and habitat responses. Seasonal nonmetric multidimensional scaling (nMDS) ordinations followed by k ‐means clustering revealed pronounced temporal reorganization of assemblages, including contractions of sardine‐dominated groups and a marked northward expansion of Japanese amberjack. To explore the environmental factors underlying these shifts, we fitted generalized additive models (GAMs) for nine pelagic taxa. To evaluate robustness to fishing effort, additional CPUE‐based GAMs were also examined as a robustness check. Variable‐importance analyses showed that broad‐scale spatial variation (captured by the spatial smoother, representing geographic and oceanographic structure) was the dominant predictor of species occurrence, followed by subsurface temperature at 50 m. Thermal responses varied among taxa, including dome‐shaped responses indicating optimal temperature ranges, monotonic increases in warm‐affinity species, and negative responses in cold‐affinity species such as chub mackerel. The shelf indicator further distinguished species with contrasting affinities for coastal versus off‐shelf habitats. Predicted spatial distributions from the GAMs were broadly consistent with multivariate assemblage patterns, providing a complementary perspective on species distributions within the same dataset. These findings highlight the value of integrating assemblage‐level analyses with species‐specific occurrence‐based habitat analyses to understand climate‐driven changes in pelagic fish communities and to support ecosystem‐based, climate‐responsive fisheries management.