Abstract:
Climate change is a formidable challenge for fish and wildlife conservation because it will directly influence the ecology and evolution of wild populations. Though climate-induced temporal trends in phenological events are common in many populations, there remains considerable uncertainty in the patterns, mechanisms, and consequences of phenological shifts. To address this, and clarify how climate change has impacted salmonid migration timing and microevolution in a warming (0.34�C per decade) Alaskan stream, long-term demographic and genetic data were used to answer these questions: how has migration timing changed in multiple salmonid species; what sources of variation influence migration timing; are changes in migration timing a result of microevolution; and does migration timing and change in migration timing influence intra-population genetic variation? For most salmonid species, life stages, and life histories, freshwater temperature influenced migration timing, migration events occurred earlier in time (mean = 1.7 days earlier per decade), and there was decreasing phenotypic variation in migration timing (mean 10% decrease). Nonetheless, there were disparate shifts in migration timing for alternative life history strategies indicative of biocomplexity. Population abundances have been stable during these phenotypic changes (lambda ≈ 1.0), but adult salmon availability as a nutrient resource in freshwater has decreased by up to 30 days since 1971. Experimental genetic data spanning 16 generations in the odd-year pink salmon population demonstrate that earlier migration timing is partly due to genetic changes resulting from selection against late-migrating fish and a three-fold decrease in this phenotype. However, circadian rhythm genes hypothesized to influence migration timing in Pacific salmon showed no evidence of inter-generational selective change. Migration timing itself influences the distribution of genetic variation within pink salmon, as there were genetic differences between early- and late-migrating fish. Despite shifts in migration timing, genetic structure and the genetic effective population size were both stable across years, indicating that in the absence of demographic change patterns of genetic diversity are resilient to climate change. These findings indicate that climate change has significantly influenced the ecology and evolution of salmon populations, which will have important consequences for the numerous species, including humans, who depend on this resource.
