Electrophoretic Evidence for a Genetic Admixture of Native and Nonnative Rainbow Trout in the Yakima River, Washington

Allele frequencies for wild rainbow trout Salmo gairdneri from the upper Yakima River and two tributary creeks were intermediate between those for introduced hatchery populations and those for inland populations native to other areas of the Columbia River basin. Previously published results had demonstrated a widespread geographic consistency in allele frequencies at two loci among both anadromous and nonanadromous populations in the Columbia River drainage east of the Cascade Mountains. The intermediate allele frequencies at these two loci for rainbow trout from the upper Yakima River, therefore, suggest that these populations represent genetic admixtures of native and nonnative stocks. Allele frequencies at several other loci plus field surveys of spawning fish further suggest that nonanadromous rainbow trout from domesticated hatchery strains, rather than hatchery‐reared steelhead (anadromous S. gairdneri), have been responsible for the suspected introgression of nonnative genes into the Yakima River populations. We hypothesize that nonanadromous rainbow trout of hatchery origin may have survived and reproduced in relatively large numbers in the upper Yakima River because of major declines in the abundance of native steelhead and of two indigenous species of Pacific salmon Oncorhynchus sp.