Diversity of Coastal Cutthroat Trout across Their Distributional Range
Thomas H. Williams, Kitty E. Griswold, Ernest R. Keeley, Kenneth P. Currens, and Gordon H. Reeves
Abstract.—We examined patterns of dispersal and colonization after Cordilleran glaciations, population connectivity, levels of genetic diversity, and potential impacts of anthropogenic changes to Coastal Cutthroat Trout Oncorhynchus clarkii clarkii. Populations were mostly small with restricted dispersals but exchanged one to two migrants per generation on average. Genetic differences among local populations of Coastal Cutthroat Trout accounted for approximately three-fourths of the total genetic variation among groups, with differences among different geographical groups accounting for the rest. Because of this, hierarchical geographical population structure was difficult to detect except at small geographical scales that reflected local dispersal and gene flow or at broad geographical scales that reflected divergence associated with long-term isolation during Cordilleran glacial advances. Evolutionary processes such as gene flow and genetic drift reflected in isolation by distance occurred at distances up to 600–700 km but mostly lesser distances, whereas divergence associated with Pleistocene glaciation occurred at 1,900 km or greater. Glacial refugia existed south of the Salish Sea along the Washington, Oregon, and California coasts; in the Haida Gwaii or Alexander Archipelago; and possibly near the central coast of British Columbia near Bella Coola. Throughout the range, hybridization with Rainbow Trout O. mykiss or steelhead (anadromous Rainbow Trout) appears to occur naturally at low levels, but releases of hatchery-produced O. mykiss can lead to higher levels of hybridization and rarely hybrid swarms. Degraded habitat may contribute to hybridization, but most anthropogenic habitat alterations reduce habitat quantity and quality and disrupt opportunities for dispersal, contributing to declines in abundance, population connectivity, and genetic diversity.