Intercrosses and the US Endangered Species Act: Should hybridized populations be included as Westslope cutthroat trout?

There are currently no policy guidelines for treating hybrids under the U.S. Endangered Species Act (ESA). We considered the scientific basis for determining whether hybridized populations should be included as part of the westslope cutthroat trout (Oncorhynchus clarki lewisi) unit considered for listing under the ESA. Westslope cutthroat trout are threatened by genomic extinction because of widespread introgressive hybridization with introduced rainbow trout (O. mykiss) and Yellowstone cutthroat trout (O. c. bouvieri). Experimental results suggest that first-generation hybrids between westslope cutthroat trout and rainbow trout have reduced fitness. However, hybridization may spread even when hybrids have severely reduced fitness because the production of hybrids is unidirectional-that is, all the progeny of a hybrid will be hybrids. In addition, heterosis resulting from the sheltering of deleterious recessive alleles in early-generation hybrids may increase the effective rate of introgression. However, such short-term increases in fitness may disrupt important long-term adaptations of native populations. The loss of these adaptations will be difficult to detect because some local adaptations might only be apparent during periodic episodes of extreme environmental conditions, such as winter storms, drought, or fire. Thus, rapid spread of hybridization could result in the loss of local adaptations in native populations of westslope cutthroat trout and decrease their probability of long-term persistence. Protection of populations with some admixture would protect sources of spreading hybridization. Treatment of hybrids in conservation planning depends primarily on the amount of evolutionary divergence between the hybridizing taxa and the geographical extent of introgression. We recommend that only nonhybridized populations be included as westslope cutthroat trout in the unit to be considered for listing. Populations of unknown status should be protected until more information about these populations becomes available.