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chub Gila cypha, bonytail chub G. ele4ans, roundtail chub G. robusta, speckled
<br />dace Rhinichthys osculus, and Kendall Warm Springs dace Rhinichthys o.
<br />thermalis), catostomids (razorback sucker Xyrauchen texanus, flannelmouth
<br />sucker Catostomus lati~innis, bluehead sucker C. discobolus, mountain sucker
<br />C. platyrhvnchus), salmonids (Colorado River cutthroat trout Oncorhynchus
<br />clarki pleuriticus; Rocky Mountain whitefish Prosoaium williamsoni), and
<br />sculpins (Cottus species; Tyus et al. 1982a; Behnke and Benson 1983; Woodling
<br />1985). All mainstream fishes persist today despite the introduction of
<br />numerous non-native fishes of which about 10 species are common in the Green
<br />River basin (Tyus et al. 1982a; Wick et al. 1985; USFWS, unpublished data).
<br />Rare fishes usually occur in greatest numbers in unaltered or natural river
<br />systems (e.g., Colorado squawfish in Yampa River, humpback chub in Little
<br />Colorado River) and their presence is presumably associated with maintenance
<br />of usable fish habitat due to a natural regimen of fluctuating seasonal and
<br />annual flows.
<br />Historically, native cyprinids and catostomids were widely distributed
<br />and abundant in mainstream habitats of the Colorado River basin (Jordan and
<br />Evermann 1896). However, four of these fishes, the Colorado squawfish,
<br />razorback sucker, humpback chub, and bonytail chub, are now threatened with
<br />extinction presumably due to the combined effects of habitat loss (including
<br />regulation of natural flow, temperature, and sediment regimes); proliferation
<br />of introduced fishes; and other man-induced disturbances (Miller 1961;
<br />Hinckley 1973; U.S. Fish and Wildlife Service 1987a). The Colorado squawfish,
<br />humpback chub, and bonytail chub are federally protected as endangered species
<br />under provisions of the Endangered Species Act of 1973 (U.S. Fish and
<br />Wildlife Service 1986). The razorback sucker, a species proposed for federal
<br />listing as endangered, is protected by state statutes in Arizona, California,
<br />Colorado, Nevada, and Utah (U.S. Department of Interior 1990).
<br />In the lower Colorado River basin (below Lee Ferry, AZ), the Colorado
<br />squawfish has been extirpated, relict populations of bonytail chub and
<br />razorback sucker remain in some impoundments but neither species are presumed
<br />self-sustaining, and humpback chub reproduce only in the Little Colorado River
<br />and presumably in nearby mainstream Colorado River (Hinckley 1973, 1983;
<br />Kaeding and Zimmerman 19$3; C.O. Hinckley, pers. comm.). In the upper Colorado
<br />River basin, Colorado squawfish persists in the Yampa and lower Green (below
<br />the confluence of the Yampa River) rivers, the upper Colorado River, and the
<br />lower San Juan River (Archer et al. 1985; Meyer and Moretti 1988; Tyus, in
<br />press). The humpback chub is reproducing in the Yampa, Green, and upper
<br />Colorado rivers (Archer et al. 1985; Kaeding et al. 1990; Karp and Tyus
<br />1990a). The razorback sucker persists in the lower Yampa and Green rivers, the
<br />mainstream Colorado River, and the lower San Juan River, but there is little
<br />indication of recruitment in these remnant populations (McAda and Wydowski
<br />1980; Meyer and Moretti 1988; Lanigan and Tyus 1989; Tyus and Karp 1990).
<br />Bonytail chub are extremely rare in the upper Colorado River basin (Valdez and
<br />Clemmer 1982; U. S. Fish and Wildlife Service 1987a). All four fishes have
<br />been extirpated in the Green River between Flaming Gorge Dam and the Yampa
<br />River confluence, presumably due, in part, to the loss of usable habitat
<br />following closure of the dam (Vanicek et al. 1970). The Yampa River supports
<br />all of its native fish fauna including self-sustaining populations of some of
<br />the rare species and is considered important to the maintenance and
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