Pacific salmon migrations through warmer water
Salmon populations migrate through all of the major river systems on the west coast in an ordered sequence. Each population has a characteristic run timing that has evolved over thousands of years to ensure they can reach their spawning grounds in time to produce the next generation. Thus spring Chinook salmon that spawn in August enter the Columbia River earliest in the spring. The sockeye salmon that spawn two months later enter the mainstem in July, and the steelhead populations that migrate the following spring enter in summer through fall.
Each population has its own adaptation to the thermal regime it generally encounters. Summer steelhead, for example, have plenty of time before they spawn, and they often use cool-water tributaries to avoid high mainstem temperatures. However, Snake River spring/summer Chinook and sockeye travel directly to their spawning grounds, and delays to their migration because of high temperature tend to increase their mortality.
Assuming that other factors are similar to today, such as fishing and juvenile transportation, we projected salmon survival for the 2040s using temperature projections from global climate models (right panel1). Our models allowed the populations to shift their run timing in a plastic response. Our results suggest minimal effects on spring Chinook populations, moderate effects on summer Chinook populations (4-15% declines in survival), and severe impacts on Snake River sockeye (80% declines).
Endangered Snake River sockeye are the most sensitive salmon population in the Pacific Northwest to warm river temperatures, especially in the mainstem rivers. This population migrates at the warmest time of year through some of the hottest rivers in the region. Declining summer flows exacerbate the risk to sockeye, especially in the free-flowing Salmon River. Snake River sockeye are also among the most sensitive populations to climate change because of other human impacts on them, especially their population history of captive rearing, which is likely part of why they suffer more from juvenile transportation and higher temperature than other salmon populations. Across the same river reach, from Bonneville to McNary, Snake River sockeye survival is significantly lower than upper Columbia River sockeye, even when they encounter the same temperatures.
Climate change is happening faster than average rates of evolution. Still, strong selection on traits with a large genetic component can cause rapid evolution, as we see in various species3.
Evolution of migration timing
Salmon are exquisite at sensing their environment and modifying their behavior in response. We, therefore, expect many changes in salmon behavior immediately in response to climate change. However, in some cases, for populations to persist they will need to change more than that. Salmon are incredibly variable genetically across populations in migration and spawn timing, and these traits are expected to evolve faster than other traits.
Columbia River sockeye have adapted to rising water temperatures by migrating earlier in the summer. From the 1950s (dotted line) to the 2010s (solid line), Columbia River sockeye shifted their migration 11 days earlier2. We believe this happened partly because of changes in flow management which caused a volitional change in behavior. But it was also because, as temperatures have increased over this period, later migrants became more likely to encounter high temperatures and die. Therefore, later migrants were more common in the next generation. For populations to evolve like this, they need to be relatively large and heterogeneous. This is one reason why preserving large, diverse, wild populations is vital for natural adaptation to climate change.
- Crozier, L. G., J. E. Siegel, L. E. Wiesebron, E. M. Trujillo, B. J. Burke, B. P. Sandford, and D. L. Widener. 2020. Snake River sockeye and Chinook salmon in a changing climate: implications for upstream migration survival during recent extreme and future climates. Plos One 15(9):e0238886. https://doi.org/10.1371/journal.pone.0238886.
- Crozier, L. G., M. D. Scheuerell, and R. W. Zabel. 2011. Using time series analysis to characterize evolutionary and plastic responses to environmental change: A case study of a shift toward earlier migration date in sockeye salmon. American Naturalist 178(6):755-773.
- Crozier, L. G., and J. A. Hutchings. 2014. Plastic and evolutionary responses to climate change in fish. Evolutionary Applications 7(1):68-87.