River-sea migration patterns of juvenile Japanese seabass influence migrations in subsequent stages

The Japanese seabass Lateolabrax japonicus, an important commercial fishery species, exhibits partial migration, where some individuals migrate upstream into rivers, and others inhabit estuarine and coastal areas within the same population. Although this migratory behavior has been documented in juvenile seabass, the prevalence of river use and the ecological significance of partial migration in subsequent developmental stages remain unclear. To understand the river use of subadult and adult Japanese seabass, we estimated the river use history using otolith Sr/Ca ratios from individuals captured in both marine and riverine environments. Based on hierarchical clustering with Dynamic Time Warping, Sr/Ca ratios at age 0 were clustered into three habitat use patterns: coastal resident, estuarine resident, and river migrant. Groups that used coastal and riverine habitats at age 0 tended to use coastal and riverine habitats, respectively, from spring to fall of age 1 and above, with overlapping distributions during winter. Subadult and adult river use patterns were linked to juvenile river use, indicating a carryover effect. Additionally, seasonal migration between the sea and rivers became less active with age, and the high growth sustained by river use also declined with age, suggesting that the benefits of river use are less for older (subadult and adult) seabass. This study highlights that juvenile habitat use experience drives subadult and adult partial migration of Japanese seabass, with individuals seeking environments adapted from past experiences. Our findings indicate that the seabass population consists of individuals adapted to various environments, from the sea to the rivers, maximizing the use of biological productivity.