Biological diversity is important for keeping a community stable during environmental disturbances. But recently, scientists wondered whether members of a single species similarly benefit from diversity in individual characteristics, such as when they migrate? Most juvenile salmon spend many years in freshwater before heading out to sea to develop into adults. However, the Chinook of California's Central Valley live right at the species’ southern limit. With river temperatures that are too hot and dams that block access to the cooler rivers up in the mountains, habitat is hard to come by. Instead, most Californian juvenile salmon migrate to the ocean as early as their first winter to avoid the scorching summer heat. The exception is a small group of juveniles in northern California. They delay migration like the Chinook of Washington and Canada because they have access to the only two rivers in the state with tolerable summer temperatures (Mill and Deer Creeks). A team of scientists from the USA, UK and France led by Flora Cordoleani from the University of California Santa Cruz, USA, wanted to know whether this diversity in individual migration patterns might make the population flexible to climate change?
From 2007 to 2018, Cordoleani and colleagues worked with the California Department of Fish and Wildlife to investigate the life history of adult Chinook that had returned to Mill and Deer Creeks to spawn. The scientists had the enviable job of collecting ear bones (otoliths) from the carcasses of the recently deceased. Freshwater and saltwater leave distinct chemical fingerprints in the layers laid down each year as the otoliths grow, revealing exactly when the fish departed on their ocean odyssey. George Whitman of the University of California Davis, USA, Corey Phillis of the Metropolitan Water District of Southern California, USA, and Peter Weber of Lawrence Livermore National Laboratory, USA, used 123 of these precious bones to investigate their composition and to reconstruct the daily growth rates and timing of migration for each fish. The team then grouped the fish into one of the three migration strategies based on when they left Mill and Deer Creeks. Early-migrating fish waited 2 weeks before leaving the river of their birth, whereas intermediate-migrating fish waited 3 months and late-migrating fish waited 6.5 months before departing. They then figured out how many individuals of each migration type were returning as adults each year and whether the proportion of fish using each migration strategy changed from year to year.
Correlating the fish's migration patterns with the weather, the team found that during wet years, roughly the same number of early, intermediate and late migrants returned to Mill and Deer Creeks. However, in years of extended drought and during ocean heatwaves, late migrants were essentially the only fish that could withstand the heat and return to the rivers of their birth (making up 77–100% of returning adults). California Chinook are all closely related, so this suggests that the late-migrating fish from Mill and Deer creeks can handle higher temperatures simply as a result of the environment they were raised in, rather than inheriting the ability to tolerate heat from their parents. With increased tolerance for high temperatures, these late-migrating individuals have a better chance of survival and act as the ‘life support’ for the northern California Chinook salmon during extreme climate years.
Sadly, projected increases in global temperatures will make finding comfortable river homes even harder for these fish. Alyssa FitzGerald from the University of California Santa Cruz, USA, ran computer simulations to predict future temperatures in the Central Valley river system. If average water temperatures increase by 1.0°C by 2080, even the late-migrating juveniles would lose more than half of the rivers and streams they currently inhabit. However, Cordoleani and colleagues propose a simple solution to the problem: provide access to higher elevation streams currently above dams. Removing dams could as much as triple the rivers available for all Chinook. By having diverse migration patterns, these salmon are trying their best to keep their populations afloat in the face of climate change. But California Chinook still might not be able to keep up with increasing temperatures – at least not without our help.
