Changes in salmon migration timing: Potential role of microevolution

Background/Question/Methods

Changes in migration timing will be necessary for many populations to persist in a rapidly warming world.  This is particularly true for salmonid fishes because migration timing is locally adapted to environmental conditions, including thermal regimes, and closely associated with individual fitness.  In Pacific salmon, changes in migration timing could potentially be due to evolution by natural selection because this trait is heritable. Although evidence for climate-induced phenotypic change in populations is widespread, evidence documenting that these phenotypic changes are due to microevolution is exceedingly rare.  Using 30-50 years of census data for five salmonid species in a warming Alaskan stream, and genetic data spanning 16 complete generations in a pink salmon population, we sought to answer two questions:  (1) Has salmonid migration timing changed; and (2) does there appear to be genetic change for earlier migration timing in pink salmon? To address these questions we measured temporal trends in migration timing and phenotypic variation for multiple species and alternative life histories.  We also used experimental genetic data and population genetics to resolve if there has been genetic change for earlier migration timing in pink salmon.  

Results/Conclusions

We observed that migration into and out of freshwater is occurring earlier (mean = 1.7 days earlier per decade) and intra-annual variation in migration timing has decreased (mean = 10% decrease) across the majority of species and life histories.  Experimental genetic data are consistent with the hypothesis of directional selection for earlier migration timing in the odd-year pink salmon population, resulting in a substantial decrease in the late migrating phenotype.  From 1983-2011 there was a significant (P < 0.05) decrease in a genetic marker for late migration timing, but not at other neutral loci.  These results demonstrate there has been rapid microevolution for earlier migration timing in this population. Circadian rhythm genes, however, did not show any evidence for selective changes from 1993-2009.  These observations have serious implications for capture fisheries, ecosystem processes, and salmonid evolutionary and population dynamics – all of which depend, at least in part, on the temporal distribution of salmonid populations. Overall, these findings are an important empirical advancement toward understanding the process of climate-induced microevolution in wild populations.