Vulnerability of Atlantic salmon to climate warming across the life cycle

Climate change is inducing more frequent and severe heat waves than organisms have encountered in recent evolutionary history. Predicting the consequences of extreme temperatures for biodiversity depends on understanding how thermal tolerance varies within species, yet this is limited by a lack of empirical data across all life stages, particularly those predicted to be most vulnerable: embryos and the reproductive stage. We measured acute upper thermal tolerance limits across all life stages (embryo to spawning adult) and both sexes of Atlantic salmon (Salmo salar) acclimated to a common temperature. Thermal tolerance was lowest in embryos, followed by spawning adults, and highest in intermediate life stages, with variation of ~6°C across the life cycle, and no sex differences for any life stage. However, comparing tolerance to freshwater habitat temperature records revealed that juveniles and adults on their spawning migration have the smallest thermal safety margins and therefore highest vulnerability to temperature extremes, while early life stages have the largest thermal safety margins. Our findings indicate that further warming of rivers poses a severe risk to river-bound juvenile and migrating spawner life stages of Atlantic salmon, suggesting targeted conservation actions may become necessary for the persistence of some populations.