Opposite latitudinal biodiversity gradient in the open ocean generated by transition zones: Insights from fish distribution in the Northwest Pacific

In this study, we used five years of environmental DNA (eDNA) data to examine the geographic distribution and community structure of marine fish in the Northwestern Pacific. We identified key environmental drivers influencing fish community composition, focusing on dynamic transition zones such as coastal–offshore interfaces and the Kuroshio–Oyashio transition zone. Using eDNA samples and environmental data from 2018 to 2023, we analyzed fish community structure with high-throughput sequencing. Hierarchical clustering, non-metric multidimensional scaling (NMDS), diversity index estimation, and generalized additive models (GAMs) were employed to evaluate the impact of ecological factors on community composition and diversity patterns. Clustering showed six distinct fish communities based on the presence/absence data. Species accumulation curves indicated highest diversity in the coastal region of the Subtropical Gyre, followed by the East China Sea and the Kuroshio–Oyashio transition zone. NMDS revealed that salinity and seafloor depth strongly influenced community composition, in addition to surface temperature, chlorophyll-a, and dissolved oxygen. GAM results showed that surface temperature, seafloor depth, and vertical temperature gradients primarily drove the spatial variation in species richness, with temperature showing a nonlinear effect. Our findings revealed that fish biodiversity tends to be higher in cooler regions and shallower depths, with biodiversity hotspots occurring in coastal–offshore frontal zones and the Kuroshio–Oyashio transition zone. These hotspots formed bimodal diversity peaks relative to temperature, and a depth-related peak near the continental slope–trench transition. This study supports efforts toward sustainable marine ecosystem development and biodiversity conservation in dynamic oceanic systems.