Population genetics of steelhead (<Emphasis Type="Italic">Oncorhynchus mykiss</Emphasis>) in the Klamath River

An analysis of population structure and genetic diversity was conducted on samples of Oncorhynchus mykiss (steelhead/rainbow trout) from 33 sites in the Klamath–Trinity River basin. Genotype data from 16 microsatellite loci in almost 1,700 fish revealed genetic differentiation between most sampled locations. Two pairs of samples from the same sites in different years were not significantly different, indicating stability of population structure, at least on a short time scale. Most sampling sites were genetically distinct from all other sampling sites, and there was an evidence of geographic structure within the Klamath–Trinity River basin, although populations from tributaries within the watershed (e.g. Salmon River, Scott River, Clear Creek) did not always constitute distinct genetic lineages. Population structure was evident using phylogeographic trees, isolation by distance analyses and individual assignment tests, which all found a relationship between geographic and genetic distance. Populations in the lower Klamath region, below the confluence with the Trinity River, consistently clustered together in phylogeographic analyses and had patterns of genetic diversity that suggest reduced gene flow between these sites and sites above the confluence. Finally, in an analysis that included data from other coastal California rivers, the populations closest to the mouth of the Klamath River appeared intermediate between populations from adjacent watersheds and the lineage formed by the other populations in the Klamath–Trinity basin.