Genetic divergence of sympatric resident and anadromous forms of Oncorhynchus mykiss in the Walla Walla River, U.S.A.

Genetic structure (six microsatellites) and gene flow were examined among anadromous (steelhead; n  = 326), resident (rainbow trout; n  = 52), and mixed ( n  = 407) Oncorhynchus mykiss life‐history forms collected from the major drainages the mainstem Walla Walla River and the Touchet River within the Walla Walla River sub‐basin, Washington, U.S.A. Genetic structure was detected between the two major drainages. Exact tests, F _ST, multi‐locus assignment tests and a neighbour‐joining dendrogram revealed genetic divergence between sympatric reference populations of adult steelhead and resident rainbow trout in the mainstem Walla Walla River, but not in the Touchet River. Tests of Hardy‐Weinberg equilibrium indicated anadromous and resident O. mykiss reference populations were in equilibrium, but many of the mixed life‐history collections were out of equilibrium. Populations out of equilibrium in the mainstem Walla Walla River appeared to be admixtures as confirmed by multi‐locus assignment tests. This is evidence of genetic divergence probably resulting from assortative mating between life‐history forms or out‐of‐basin stocking practices.