Genetic population structure of chinook salmon, oncorhynchus tshawytscha, in the Pacific Northwest

Abstract

Variation at 25 polymorphic protein coding loci was examined for 86 populations of chinook salmon, Oncorhynchus tshawytscha, ranging from the Babine River Columbia to the Sacramento River in California. Substantial differences in allele frequencies identified patterns of genetic variability over the geographic range of the study. Nine major genetically defined regions were formulated. Populations sampled within a region tended to be genetically distinct from each other although they exhibited the general patterns of variability that defined the region. Within a region there was little distinction among populations returning to spawn at different times. The persistence of these geographic patterns in the face of natural opportunities for introgression, and sometimes massive transplantations, suggests that genetically adapted groups within regions have resisted large-scale introgression from other regions. Repopulation of deglaciated areas in the Fraser River, Georgia Strait, and Puget Sound apparently occurred from multiple sources; most likely sources included Columbia River populations and northern refuges rather than from the large coastal group of populations. Patterns of genetic distribution of chinook salmon differed from those of other anadromous salmonids studied within this region. A conservative policy for stock transfers was suggested based on distinct genetic differences observed both between and within regions.