Climate-driven fitness decline in Japanese chum salmon reshapes North Pacific chum salmon biogeography

Abstract Japanese chum salmon supported by one of the world’s largest hatchery programs have experienced severe declines in marine survival and egg size. To investigate the underlying mechanisms, we analyzed a 21-year time series (1999–2019) of reproductive traits of age-4 chum salmon from 13 rivers together with climate and salmon abundance data using a bootstrap-supported Bayesian network. Here, we assumed that environmental variables can affect the chum salmon populations, but not vice versa, and that there could be maternal effect on reproductive traits, but not the other way around. These constraints enabled us to infer the causal links that shaped the biogeography of North Pacific chum salmon. Global warming caused a decline in Japanese chum salmon abundance, resulting in the increase of the competing Russian chum, which in turn decreased the female body size, fecundity, and egg size of Japanese chum. These findings suggest that climate-driven warming may have exposed genetic effects of hatchery practices, contributing to fitness decline in Japanese chum salmon and the ecological reorganization of chum salmon populations in the North Pacific.