Westslope Cutthroat Trout, Hybridization, and the U.S. Endangered Species Act

Allendorf et al. (2004) describe introgressive hybridization and the potential “listing” of westslope cutthroat trout (WCT, Oncorhynchus clarki lewisi) under the U.S. Endangered Species Act (ESA). This issue is complicated because many natural populations have experienced small amounts of introgression detectable only by molecular genetic methods. The issue is further complicated because portions of the native geographic ranges of WCT and rainbow trout (RT, O. mykiss), the principal hybridizing species, naturally overlap. Allendorf et al. recommend only “nonhybridized” populations be considered WCT under the ESA. If this recommendation is followed, then populations with any detectable introgression would be excluded from the subspecies and would be eligible for eradication under an ESA listing. Thus, for situations like WCT, a biological dichotomy exists between (1) the need to conserve the genetic resources of an imperiled species in which introgression has occurred and (2) the need to eliminate hybridization threats posed by introduced taxa. The U.S. Fish and Wildlife Service (USFWS) has thoroughly examined this issue in response to a court order (USFWS 2003) and has reached conclusions that differ from those of Allendorf et al. We comment here on their perspectives and provide alternative viewpoints. Details of the USFWS’conclusions are described elsewhere (USFWS 2003). Allendorf et al. note (abstract), “There are currently no policy guidelines for treating hybrids under the U.S. Endangered Species Act.” This is partially true. A proposed “intercross policy” (USFWS 1996) was developed but never finalized because the range of possible hybridization situations precluded a single policy. Instead, policy