Short communication: Divergent life histories link thermal tolerance variation across Pacific salmon species and populations.

Divergent life histories among organisms are linked to variations in thermal response. Although there have been extensive studies on interpopulation variations in thermal tolerance, the ecological relevance of the tolerance to life history remains largely unexplored. We compared critical thermal maximum (CTmax) among two Pacific salmon species with different freshwater life histories, masu salmon (Oncorhynchus masou) and chum salmon (O. keta), across two local Japanese populations. CTmax was higher for masu salmon fry (26.9 °C) than for chum salmon fry (26.1 °C). Although the mean CTmax differed by 1.1 °C among masu salmon populations, the difference among chum salmon populations was much smaller, at 0.17 °C. This high interpopulation variation was well explained by variation in river temperatures during freshwater rearing. In contrast, the low interpopulation variation of chum salmon fry might reflect their immediate ocean entry after emergence from the riverbed, which leads to them experiencing similar ocean temperatures across populations. Our findings provide crucial evidence that interpopulation variation in thermal tolerance might be most pronounced for Pacific salmon species that are highly dependent on freshwater environments.