Characteristics of juvenile chum salmon carrying out inverse migrations after ocean entry from rivers along the Pacific coast of Hokkaido, Japan

After ocean entry, juvenile Japanese chum salmon migrate northeastward to the Sea of Okhotsk. However, some juveniles originating in rivers along the Pacific coast of Hokkaido were reported to migrate in the opposite direction (westward). We compiled historical data to determine which river‐origin juveniles migrate westward and to what extent. Then, the ocean‐entry conditions and growth rates of 398 juveniles from two rivers along the Pacific coast of Hokkaido were estimated with otolith daily‐increment analysis. Many juveniles migrated >100 km westward from multiple natal river mouths. Juveniles collected to the west of their natal rivers tended to enter the ocean earlier than those collected to the east, and many did so before sea surface temperatures reached 5°C (the lower limit of the empirically favorable temperature range). In the west, many small juveniles were collected soon after ocean entry, suggesting that they migrated passively with the Coastal Oyashio, a westward cold current. By contrast, juveniles collected in the east tended to grow faster in length and have a lower body condition factor at capture, possibly due to growth‐dependent mortality during their migration against the flow of the Coastal Oyashio. Moreover, many juveniles in the west were exposed to temperatures near 13°C (the upper limit of favorable temperatures) at capture. In recent years, there has been a shortening of the favorable water‐temperature period and a drastic decline in adult chum returns in the region. There is hence concern that the likelihood of abortive migrations for juveniles transported westward will increase.