Genetic Integrity and Microgeographic Population Structure of Westslope Cutthroat Trout, Oncorhynchus clarki lewisi, in the Pend Oreille Basin in Washington

Five microsatellite DNA loci (Ots-101*,Ots-107*,Oki-10*, Ogo-3*, and FGT-3*) were screened to evaluate the genetic characteristics and population structure for cutthroat trout from eight tributaries of the Pend Oreille River in northeastern Washington and to compare these collections with two hatchery stocks of westslope cutthroat trout, Oncorhynchus clarki lewisi, Yellowstone cutthroat trout, Oncorhynchus clarki bouvieri and a hatchery rainbow trout, Oncorhynchus mykiss, strain that have been stocked in northeastern Washington. Relatively high levels of variation (numbers of alleles and heterozygosity) were observed in all collections and allele frequencies were quite variable among collections. Evidence of limited introgression by rainbow and/or Yellowstone cutthroat was found at several locations. Both FST values and tests of genetic differentiation indicated the existence of numerous, reproductively isolated populations. The population in Slate Creek was very similar to the Kings Lake Hatchery strain, and we conclude that this similarity is the result of historical introductions of this hatchery strain into what was presumably a stream without a native cutthroat population. In one stream, differences in introgression and allele frequencies were found above and below a barrier falls. Because of the substantial level of population differentiation observed among the various collections, we recommend that management and conservation actions be focused at the level of individual streams in order to maintain the productivity and genetic character of the existing populations of cutthroat trout.