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

Highlighting the importance of rare phenotypes in population persistence, the authors show that spring-run Chinook salmon late-migrant juveniles were critical for cohort success in drought and ocean heatwave years. Combined further warming and impassable dams threaten these late migrants' survival. 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.

Automated summary

The study emphasizes the critical role of rare phenotypes in ensuring population persistence in Chinook salmon, particularly in drought and ocean heatwave years. Late-migrating juveniles are shown to be essential for cohort success, but face threats from warming temperatures and impassable dams. The loss of phenotypic diversity poses a significant impact on population persistence in the face of a warming climate, highlighting the urgent need to preserve habitat diversity for long-term species survival.