Abstract
Salmon life histories are finely tuned to local environmental conditions, which are intimately linked to climate. We summarize the likely impacts of climate change on the physical environment of salmon in the Pacific Northwest and discuss the potential evolutionary consequences of these changes, with particular reference to Columbia River Basin spring/summer Chinook (Oncorhynchus tshawytscha) and sockeye (Oncorhynchus nerka) salmon. We discuss the possible evolutionary responses in migration and spawning date egg and juvenile growth and development rates, thermal tolerance, and disease resistance. We know little about ocean migration pathways, so cannot confidently suggest the potential changes in this life stage. Climate change might produce conflicting selection pressures in different life stages, which will interact with plastic (i.e. nongenetic) changes in various ways. To clarify these interactions, we present a conceptual model of how changing environmental conditions shift phenotypic optima and, through plastic responses, phenotype distributions, affecting the force of selection. Our predictions are tentative because we lack data on the strength of selection, heritability, and ecological and genetic linkages among many of the traits discussed here. Despite the challenges involved in experimental manipulation of species with complex life histories, such research is essential for full appreciation of the biological effects of climate change.
Highlights
Climate change is transforming the fitness landscapes of millions of species at a rapid rate, but we have little understanding of the evolutionary consequences
Evolutionary responses to climate change are important because nongenetic responses, such as shifts of range edges and plastic phenotypic change, might not be sufficient for the persistence of many populations (Sala et al 2000; Thomas et al 2004), and because strong selection increases the risk of extinction in small populations (Burger and Lynch 1995)
Considerable uncertainty attends the prediction of evolutionary responses to climate warming (Holt 1990), even for a short-lived organism with a simple life cycle that is amenable to experiment (Etterson and Shaw 2001)
Summary
Climate change is transforming the fitness landscapes of millions of species at a rapid rate, but we have little understanding of the evolutionary consequences. Selection on spawning date depends on at least three changes that are uncertain: (i) the advance in optimal emergence timing, (ii) a plastic change in spawning date owing to warmer waters, and (iii) potential costs of longer delays between migration and spawning With regard to this last effect, advancing upstream migration dates and higher summer temperatures increase the length of time in freshwater during which energy stores are depleted, and cool-water refugia might contract, increasing prespawning mortality. This means that selection might not favor a delay in spawning date, but rather slower, embryo development rates. It does draw attention to a particular case where the plastic response in one stage might be unfavorable for the subsequent life stage
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