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operations would likely jeopardize the continued existence of some listed fishes. An interagency program (i.e., Recovery Implementation Program) has been established in the upper Colorado River basin in an effort to recover listed fishes without violating eXisting state and federal water agreements (USFWS 1987; Rose and Hamill 1988). This program oversees recovery activities in the upper Colorado River, provides funds for evaluating habitat requirements of the fishes, and seeks ways to obtain water needed by the fish. In this paper, I presen,t information about endangered Colorado River fishes, and research and management activities for them. COLORADO RIVER FISHES Decline Geographic isolation and extreme climatic and hydrologic conditions have resulted in a unique Colorado River fish fauna (Miller 1959, 1961; Molles 1980). This fauna can be separated into three categories: (1) fishes that inhabit high or intermediate elevations that either share, or have closely allied forms in adjacent drainages, (2) endemic species of small streams at low to intermediate elevations, and (3) big river fishes, commonly called the Colorado River fishes, which are mostly endemic species of mainstream rivers (Minckley et al. 1986). Native big river fishes, consisted only of cyprinids (minnows) and catostomids (suckers), that were widely distributed in mainstream habitats of the historic Colorado River basin (Jordan and Evermann 1896). Four of the big river fishes, the Colorado squawfish Ptvchocheiluslucius, razorback sucker Xvrauchen texanus, humpback chub Gila ~, and bony tail Gila eleqans, once ranged throughout warmwater reaches of the mainstream rivers of the Colorado River basin from Wyoming to Mexico. The range of these fishes has been drastically reduced and they are now threatened with extinction. The endangerment of the big-river fishes of the Colorado River has been attributed to conversion of riverine habitat to artificial impoundments, 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 in the mainstream. 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). About 80% of the native fishes there are endangered (W.L. Minckley, personal communication). Colorado squawfish has been extirpated from the lower Colorado River; relict populations of bony tail 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. latiDinnis). 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 there 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, bony tail chub, roundtail chub ~ robusta , speckled dace Rhinichthvs osculus, and Kendall Warm Springs dace ~ osculus thermal is), catostomids (razorback sucker, flannelmouth sucker, bluehead sucker C; discobolus, mountain sucker C. Dlatvrvnchus), salmonids (Colorado River cutthroat trout O. clarkii pleuriticus, Rocky Mountain whitefish P. williamsoni) and. sculpins (Cottus species; Tyus et al. 1982; Behnke and Benson 1983; Woodling 1985). 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 bony tail, 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 1t6 .I......!.....' , "