Warming climate impacts on production dynamics of southern populations of Pacific salmon in the North Pacific Ocean

This study reviews the evidence for warming climate impacts on ocean migration, growth, and survival of southern populations of Pacific salmon (Oncorhynchus spp.), in particular, Japanese chum salmon (O. keta). In this century, productivity of Asian and North American Pacific salmon declined in southern areas but increased in northern areas owing to a warming climate. Japanese chum salmon, mainly hatchery-produced fish, are roughly divided into two populations: warm-current populations (WCPs) affected by the Tsushima warm current and cold-current populations (CCPs) affected by the Oyashio cold current. Their population size and survival rate have declined in Honshu since the late 1990s and in Hokkaido since the late 2000s. The decrease in population size of CCPs is greater than that of WCPs, suggesting that WCPs are relatively adaptable to global warming. A decline in survival rate and insufficient growth of juveniles in coastal habitats due to a shortened duration of coastal stay as a consequence of global warming was identified as the factor affecting the reduced production of Japanese chum salmon. The current situation of chum salmon is approaching that of the Jomon transgression period in Japan (7-5.5 kyr before present). Therefore, it is necessary to establish sustainable conservation management for southern populations of Pacific salmon under a warming climate regime.