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by Seethaler (1976) revealed that of 27 fish species now found in the lower Yampa River, only ten were native fishes including four species considered threatened and endangered by the Colorado Wildlife Commission. Kidd (1977) showed similar species composition for the lower Gunnison River (six native fishes among 19 total species) and the Colorado River in Mesa County (nine native fishes among 22 total species). Due to their long isolation in the Colorado River system, the endemic fishes are somewhat comparable to geograph- ically isolated island fauna in their vulnerability to displacement by intro- duced species. There is a good correlation between the spread of the introduced species in the Green, Yampa, Gunnison, and Colorado Rivers and the decline of such species as razorback suckers, bonytail chubs, and Colorado squawfish. Backwater areas formerly utilized by native fishes are now often dominated by redside shiners and sand shiners, Notropis lutrensis. These species are no doubt competing for food and space and preying on native species, thereby dis- placing them in many areas (Prewitt and Carlson 1976). As certain native species became rare,such as the razorback sucker, natural hybridization with Catostomus suckers increased rapidly (Hubbs and Miller 1953). Several authors have collected razorback sucker x flannelmouth sucker, Catostomus latipinnis, in the Colorado River system in Colorado (Jordan 1891; Hubbs and Miller 1953; Banks 1964; Holden 1973). Holden collected almost as many hybrid suckers (40) as razorback suckers (53) between 1967 and 1973. The Curecanti Project has created an environment in the Gunnison River above Delta, Colorado in which Catostomus sucker hybrids are common (Wiltzius pers. comm. Individuals of rare species are presumably often attracted to the spawning grounds of common species, where the chance of hybridization is in- creased. When both species are common, minor differences in reactions or - 9 -