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The cutthroat trout (Salmo c1arki)present.s a series of unusual and difficult problems in conservation biology. As many as 16 subspecies have been recognized in the recent literature. The genetic distance between subspecies based upon 46 enzyme loci ranges from that usually seen between congeneric species to virtual genetic identity. Subspecies from the western portion of the range of the cutthroat trout are genetically more similar to rainbow trout (Salmo gairdneri) than they are to the other subspecies of cutthroat trout. I n &ition, much of the genetic variation within the westslope cutthroat trout ( S . c. lewisi) results from alleles found in only one or two local populatiotq but they often occur at high frequencies in thosepopulations. Thus, preserving the genetic variation in westslope cutthroat trout entails preserving as many local populations as possible.Captive populations of cutthroat troutpresent a series of opportunities and genetic problems. A number of management agencies are using captive populations to supplement and reestablish natural populations. Basic genetic principles must be understood and followed in establishing and maintaining captivepopulations. We describe examples of unsuccessful and successful efforts by management agencies to develop captive populations.The greatest danger to the conservation of the cutthroat trout is introgressive hybridization among subspecies and with rainbow trout. Several factors make salmonid fishes especially susceptib le to problems associated with introgressive hybridization. We conclude that biochemical analysis provides a more reliable and informutive means of detecting interbreeding than morphological characters. Interbreeding between westslope and Yellowstone cutthroat trout and non-170 Conservation Biology Volume 2, No. 2, June 1988 AUendorf and Leaty The Cuftbroaf Trout 171 native S a h O appears to be common and widespread throughout the natural range of these subspecies.provee un medio m k infomzativo y confiablepara detectar entrecnuamiento que 10s caracteres morfologicos. El entrecruzamiento entre S. clarki de la vertiente occidental y de Yellowstone con especies de Salmo no-nutivo parece ser comzin y extensa a lo latgo de toda la extmi6n natural de estas subespecies.

Human-mediated hybridization is a leading cause of biodiversity loss worldwide. How hybridization affects fitness and what level of hybridization is permissible pose difficult conservation questions with little empirical information to guide policy and management decisions. This is particularly true for salmonids, where widespread introgression among non-native and native taxa has often created hybrid swarms over extensive geographical areas resulting in genomic extinction. Here, we used parentage analysis with multilocus microsatellite markers to measure how varying levels of genetic introgression with non-native rainbow trout (Oncorhynchus mykiss) affect reproductive success (number of offspring per adult) of native westslope cutthroat trout (Oncorhynchus clarkii lewisi) in the wild. Small amounts of hybridization markedly reduced fitness of male and female trout, with reproductive success sharply declining by approximately 50 per cent, with only 20 per cent admixture. Despite apparent fitness costs, our data suggest that hybridization may spread due to relatively high reproductive success of first-generation hybrids and high reproductive success of a few males with high levels of admixture. This outbreeding depression suggests that even low levels of admixture may have negative effects on fitness in the wild and that policies protecting hybridized populations may need reconsideration.