Threatened salmon rely on a rare life history strategy in a warming landscape

Rare phenotypes and behaviours within a population are often overlooked, yet they may serve heightened role for species imperilled by rapid warming. In threatened spring-run Chinook salmon spawning at the southern edge of range, we show late-migrating juveniles critical to cohort success in years characterized droughts ocean heatwaves. Late migrants rely on cool river temperatures over summer, increasingly rare due combined effects warming impassable dams. Despite dominance late migrants, other strategies played an important many years. Our results suggest that further loss phenotypic diversity will have impacts persistence climate. Predicted thermally suitable conditions shrink rapidly future be largely relegated above Reconnecting diverse habitat mosaics support integral long-term this species. Highlighting importance persistence, authors late-migrant were drought heatwave Combined dams threaten these migrants’ survival.