Incidence of hybridization between naturally sympatric westslope cutthroat trout and rainbow trout in the middle fork salmon river drainage, Idaho

Introgressive hybridization has been widely reported for westslope cutthroat trout (WCT) Oncorhynchus clarkii lewisi and rainbow trout (RBT) O. mykiss and is often a result of introductions of normative RBT into previously allopatric populations of westslope cutthroat trout. The WCT evolved in sympatry with RBT in a portion of its native range. Few studies have evaluated natural hybrid zone structure in sympatric populations or the effects of normative introductions within sympatric populations. We used one mitochondrial DNA marker and three co-dominant nuclear DNA markers to examine 17 populations of WCT that were sympatric with native RBT-steelhead (anadromous RBT). As 5 of the I I sample locations were situated downstream of stocked headwater mountain lakes, we wanted to determine the effects of headwater lake introductions on naturally sympatric populations of WCT and RBT-steelhead below the lakes. Hybrids were found in streams below stocked and unstocked headwater lakes. Our results indicated that the majority of the populations displayed a bimodal hybrid structure, linkage disequilibrium, and Hardy-Weinberg disequilibrium. This suggests recent, ongoing hybridization but also strong assortative mating, which may be indicative of minimal impacts from stocking or independent events of natural hybridization. Natural hybrid zones should be addressed in current political deliberations about the inclusion of hybridized populations of WCT in Endangered Species Act considerations. A decision to discount protective actions for such stocks may lead to a loss of potentially adaptive genetic diversity and the fragmentation of populations.