by Emil Morhardt
Chinook (king) salmon deposit their eggs in the cobbles of Pacific Coast streambeds, where they, and the subsequent developing juveniles spend months before heading out to sea. Some of these streams can get quite warm by salmon standards, and more than one major run has been heavily depleted by temperatures higher than 24 °C which seems to be generally lethal to this species. As global warming progresses we can expect more salmon streams to reach this temperature, so an important question is whether these fish have either the developmental plasticity or the genetic variability that will allow them to adapt. Muñoz et al. (2015) crossbred 16 wild chinook salmon caught at Canada’s Quinsam River Hatchery so as to get 64 different genotypes, then reared half of them at the hatchery ambient water temperatures and the other half at temperatures 4 °C higher. They then looked to see if cardiac function, the apparent limiting factor in high temperature mortality, was shifted to higher temperatures. The fish reared at the higher temperatures were capable of higher maximum heart rates, suggesting that they would be able to withstand higher temperatures than those reared at the existing ambient temperatures, but both groups suffered from fatal cardiac arrhythmia at an average of 24 °C although some of the individuals tested remained unarrhythemic at nearly 26 °C. It thus appears unlikely that these Canadian runs, at least, have the genetic capability to adapt to temperatures any higher than 26 °C. Based on projections of West Coast stream temperatures in the coming decades, it looks as though many streams will lose their salmon, and under the maximum predicted warming, almost all of them will by 2100.
The authors note that wild Atlantic salmon (of the genus Salmo rather than the genus Oncorhynchus studied here) can increase the temperature at which arrhythmia occurs through this sort of acclimation, and that sea-run brown trout (also in the genus Salmo) have greater developmental plasticity than those that never go to sea, suggesting that the greater environmental heterogeneity they experience may have allowed greater genetic heterogeneity as well. Perhaps more southerly runs of Chinook salmon that experience greater environmental heterogeneity as well, say from California’s Central Valley, may be similarly endowed with greater developmental plasticity. At least we can hope.
Muñoz, N.J., Farrell, A.P., Heath, J.W., Neff, B.D., 2014. Adaptive potential of a Pacific salmon challenged by climate change. Nature Climate Change DOI: 10.1038/NCLIMATE2473.