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.

种群中罕见的表型和行为经常被忽视，但它们可能对因快速变暖而受到威胁的物种发挥更大的作用。在物种范围南部边缘受威胁的春季奇努克鲑鱼产卵中，我们发现晚迁徙的幼鱼对于在以干旱和海洋热浪为特征的年份取得成功至关重要。晚期移民依赖夏季凉爽的河流温度，由于变暖和无法通行的大坝的综合影响，这种情况越来越少见。尽管晚期移民占主导地位，但其他策略在许多年中发挥了重要作用。我们的研究结果表明，表型多样性的进一步丧失将对气候变暖中的种群持久性产生重大影响。预计后期移民的热适宜河流条件将在未来迅速缩小，并将在很大程度上降级到无法通行的水坝之上。重新连接多样化的栖息地马赛克以支持表型多样性将是该物种长期存在不可或缺的一部分。