Accumulating evidence has indicated that many fish populations are responding to climate change through shifts in migration time, but genetic data identifying the role of evolution in these shifts are rare. One of the first demonstrations of evolution of migration time was produced by monitoring allozyme alleles that were experimentally manipulated to genetically mark late-migrating pink salmon (Oncorhynchus gorbuscha). Here, we extend that research by using observations of the marker alleles in fry to demonstrate that these changes in migration time were caused by directional selection against the late-migrating phenotype during the oceanic phase of the salmonid life cycle. The selective event, which appeared to be driven by early vernal warming of the nearshore marine environment and consequent decreased survival of late-migrating fry relative to earlymigrating fry, decreased the late-migrating phenotype from more than 50% to approximately 10% of the total fry abundance in only one generation. These demographic changes have persisted over the subsequent 13 generations and suggest that a larger
trend toward earlier migration time in this population may reflect adaptation to warming sea-surface temperatures.