Sea of Okhotsk warming impacts adult return abundance of southwestern marginal Chum salmon populations over four decades

The Sea of Okhotsk (OK) is a crucial transition zone for Chum salmon Oncorhynchus keta from Japanese (JP), Korean (KR), and Russian (RS) populations. Population-specific return-abundance patterns diverged in the early 2000s, when OK warmed rapidly. Previous studies focused on coastal early-life factors cannot fully explain shifts in populations returning to the rapidly warming East/Japan Sea. This study analyzed how environmental changes influenced the return-abundance patterns of JP, RS, and KR, focusing on differences before and after 2000. Key drivers shifted over time: KR, the southwestern marginal population, became increasingly dependent on marine environmental conditions; the drivers of JP return abundance shifted from environmental to hatchery-driven factors; and RS return abundance increased due to favorable conditions and greater releases. During the return stage, key environmental variables for all populations shifted from the Soya to the Tatar route, suggesting northward migration in response to warming. OK warming contributed to reduced JP return abundance across early-life and return stages, whereas RS return abundance increased under more favorable conditions. The increase in RS return abundance increase helped maintain KR return abundance post-2000. This study highlights those population-specific responses to a shared warming signal and helps management strategies, improving forecasts of climate-change impacts on Chum salmon.