Introgressive Hybridization between Westslope Cutthroat Trout and Native Rainbow Trout in Big Creek, Idaho
Michael P. Peterson, Matthew R. Campbell, Christine C. Cegelski, and Madison S. Powell
Abstract.—Westslope cutthroat trout Oncorhynchus clarkii lewisi are currently under a second review for listing as a threatened species under the Endangered Species Act. Both natural and anthropogenically induced hybridization has been previously documented between this subspecies and rainbow trout O. mykiss and between steelhead (anadromous rainbow trout) and coastal cutthroat trout O. clarkii clarkii. However, levels of reported introgression have varied greatly. To assess natural hybridization and the extent to which it may affect the frequency and persistence of O. mykiss alleles among sympatric populations of westslope trout, we used three nuclear loci to detect hybrids, and mitochondrial DNA to assess the direction of hybridization and introgression in Big Creek, Idaho and its tributaries. Natural hybridization between westslope cutthroat and sympatric rainbow trout/steelhead appears to occur at a relatively low frequency with numerous parental types still present in varying numbers within the drainage.
Subsequent genetic analyses revealed no hybridization in samples from 2001 and percentages of hybrid genotypes within sample locations ranging from 1.6% to 13.3% in 2002. Differences between years may be attributable to sampling, time of year, and seasonal movements of westslope cutthroat trout and their hybrids. Furthermore, hybrids were more frequently observed (p < 0.01) with mitochondrial haplotypes of westslope cutthroat trout indicating a directional preference of westslope cutthroat females spawning with O. mykiss males.