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replacement of warmwater habitats to cold tailwaters, and migration barriers (Holden and Wick 1982; Seethaler 1978; Vanicek 1967). More insidious effects, including introduction of non-native species; small, but cumulative water depletions; and downstream effects of water projects have also have an effect. As a result, the combined effects of habitat loss (including regulation of natural flow, temperature, and sediment regimes), proliferation of introduced fishes, and other man-induced disturbances (Miller 1961; Minckley 1973; USFWS 1987) have had profound effects on native fish habitats. In the Lower Colorado River, change in natural flow regimes, stream blockage, and conversion of many miles of warmwater stream habitat to reservoirs and cold tailwaters have largely extirpated native fishes. They have been replaced by a new fauna of about 44 forms (Minckley 1982), many of which were introduced from more mesic environments. Of these, 20 species are abundant either locally or regionally (Minckley 1982). ~~ M°~~~~+ Colorado squawfish has been extirpated from the lower Colorado River; relict populations of bonytail and razorback sucker remain in some impoundments but neither species are presumed self-sustaining; and humpback chub reproduction in the Grand Canyon is restricted to the Little Colorado River (Minckley 1973, 1983; Kaeding and Zimmerman 1983). The range of other native fishes have also been reduced, including the flannelmouth sucker (C. latiuinnis). Colorado River fishes .are more widespread in the upper Colorado River basin, where about 2000 km of occupied habitat remains in mainstream rivers (Tyus et al. 1982). The native fish fauna includes six species that are endemic large river cyprinids and catostomids, and six headwater forms that also occur elsewhere. Although 42 introduced fishes are presently reported, less than 10 are considered abundant (Tyus et al. 1982). The native fish fauna includes cyprinids (Colorado squawfish, humpback chub, bonytail chub e roundtail chub G. robusta speckled dace Rhinichthys osculus, and endall Warm Springs dace R. osculus thermalis), catostomids (razorback sucker, flannelmouth sucker, bluehead sucker C. discobolus, mountain sucker C. platvrvnchus), salmon ids (Colorado River cutthroat trout 0. clarkii pleuriticus, Rocky Mountain whitefish P. wiiliamsoni) and sculpins (Cottus species; Tyus et al. 1982; Behnke and Benson 1983; Woodlin~~,{g 19$5). "IV~G Win.\~~ ~,~v ~~ In the upper basin, Colorado squawfish persists in the Yampa River, the Green River below its confluence with the Yampa River, the upper Colorado River mainstream, and the lower San Juan River (Archer et al. 1985; Meyer and Moretti 1988; Tyus, 1990). The humpback chub is reproducing successfully in the Yampa and upper Colorado rivers (Archer et al. 1985; Karp and Tyus 1990). The razorback sucker persists in the lower Yampa and Green rivers, the mainstream Colorado River, and the lower San Juan River, but there is no indication of recent recruitment in these remnant populations (McAda and Wydoski 1980; Meyer and Moretti 1988; Lanigan and Tyus 1989). The remaining endangered large river fish, the bonytail, is extremely rare in the upper Colorado River basin (Valdez and Clemmer 1982; Kaeding et al. 1986). To date, almost all of the interest in determining and protecting Colorado River fishes has been associated with the need to protect federally- listed endangered species. However, the "fundamentally insular" (Molles 1980) nature of the fauna suggests that other species may also become endangered or 3