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