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INTRODUCTION Background The Colorado River basin was originally occupied by only thirteen native fishes (Behnke and Benson 1983). However, today native fishes of the Colorado River basin have been adversely affected by major environmental changes from human alterations to the ecosystem. As a result of these changes, four of these endemic fishes to the Colorado River system, the Colorado squawfishl, razorback sucker, bonytail, and humpback chub, are listed as endangered under the Endangered Species Act (ESA; U. S. Fish and Wildlife Service [FWS]). The Colorado squawfish has been extirpated from the lower basin, and it now occupies approximately 20% of its historic range (Tyus 1990). In the Lower Colorado River, below Glen Canyon Dam, a substantial population of razorback sucker persists in Lake Mohave but occurs only sporadically in riverine reaches (Marsh and Minckley 1989). In the Upper Colorado River Basin, the largest populations of both Colorado squawfish and adult razorback sucker are found in the Green and Yampa rivers (Holden and Wick 1982; Lanigan and Tyus 1989). A small remnant population of Colorado squawfish occurs in the San Juan River; no wild razorback sucker have been found there in recent times (Ryden and Pfeifer 1995). In the Upper Colorado River, only 25 adult razorback sucker were captured between 1980 and 1990 from ri veri ne habitats (Valdez et al . 1982a) ; only seven adult fish have been captured since 1990 (Osmundson and Kaeding 1991; Burdick 1992; unpublished FWS data). Colorado squawfish continue to persist in the Upper Colorado River but it is uncertain whether recruitment is adequate to maintain a self-sustaining population. Humpback chub now exist only in five widely-separated canyon habitats. Bonytail, the rarest native fish in the Colorado River, is considered "virtually" extinct in the Upper Colorado River Basin. Bonytail now only occur in small numbers in Lakes Mohave and Havasu. Some of the major factors that are suspected in reducing populations of these four "big river" endangered fishes include alteration of the hydrologic regime and reduced water quality, competition and predation from the introduction and proliferation of nonnative fishes, possible reproductive impairment from both human-produced contaminants and harmful natural trace elements (Hamilton and Waddell 1994; Waddell and May 1995), reduced or complete recruitment failure due to a loss of available spawning or nursery habitats, reduced food base, and angler-related mortality of adult fish. Both the mainstem and the North Fork of the Gunnison River are regulated by Federal dams and reservoirs upstream of the warmwater reaches. The largest of these reservoirs is the Aspi nal 1 Unit, which is a series of three federal (U . S Bureau of Reclamation [BR]) reservoirs and dams on the upper South Fork or main Gunnison River. Taylor Park Reservoir is located upstream on the Taylor River. Other water development projects constructed by BR include Paonia Dam on the North Fork, Crawford Dam (Smith Fork Project) on the Smith Fork, Ridgeway Reservoir (Dallas Creek Project) on the Uncompahgre River, and Fruitgrower's Reservoir. Although the three major reservoirs of the Aspinall Unit and other 1 Scientific names and two letter codes of all fishes mentioned in this report are given in Appendix A: Table A.1. 0n1v rnmmnn namee fnr thccc fichcc arc uccrl in the +cvt