Ecology: Late-migrating salmon survive hottest, driest years

Californian Chinook salmon with a rare, late-migration strategy are more likely to survive in years of drought and ocean heatwaves than their fellow population members, according to a study published in Nature Climate Change. These findings suggest that as environmental conditions continue to shift owing to climate change, conservation strategies are needed to support population diversity and ensure the long-term resilience of this threatened species.

Population diversity is an important way for species to buffer themselves against natural or human-induced disturbances, including those brought about by climate change. For fish like salmon, whose juveniles undertake an extensive migration from freshwater to salt water as part of their life cycle, flexibility in migration timing may be particularly important.

Flora Cordoleani and colleagues use ratios of strontium isotopes deposited in the ear bones of spring-run Chinook salmon (Oncorhynchus tshawytscha) as positional geographic markers to reconstruct the life history of 123 adults that returned to two streams in the Central Valley of California between 2007 and 2018. The authors show that late migrants — which remain in the freshwater streams in which they were born over the summer instead of migrating to the ocean in the first winter or spring — are critical for species survival in years of drought and ocean heatwaves.

As late migrants rely on suitably cool rivers — which are predicted to shrink rapidly in future years under climate change and are, furthermore, largely found above impassable dams — the authors emphasise the need to consider habitat connectivity to support population diversity. This will be integral for the long-term persistence of this species, they conclude.