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03/13/96 15:04 0303 245 8933 FISH & WILDLIFE 444 R 0 4 Denver [moos/011 in the Upper Colorado River Basin, only occurring now in small numbers in Lake Monave. 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, reproductive problems from both human. produced contaminants and harmful natural trace elements, reduced or complete recruitment failure due to a loss of available spawning or nursery habitat, reduced food base. and angler-based mortality of adult fish. The warmwater reaches of the Gunnison River are historical habitat for four of the endangered fish of the Colorado River Basin previously mentioned. Historically, humpback chub were not collected from the Gunnison River until one specimen was captured in 1993 (Burdick 1995); the nearest known population of humpback chub is located at Black Rocks in Ruby Canyon on the Colorado River 38 miles downstream of the Redlands Diversion Dam. Fishery survey studies conducted over the past 15 years indicate that Colorado squawfish occupy both up- and downstream reaches from the Redlands Diversion Dam (Valdez et al. 1982; Wick et al. 1985: Burdick 1995). The population of Colorado squawfish in the Lowe- Gunnison River downstream of the Redlands Diversion Dam has been considered parr 1 Scientific napes and two letter codas of all fishes mentioned in this report are given in Appendix A: Table A.1. Only common naves for these fishes are used in the text. of the Upper Colorado River population, The Redlands Diversion Dam, located approximately 2.3 miles upstream of the confl uence with the Colorado River, has prevented the upstream movement of all fishes since it was constructed in 1918. Colorado squawfish found upstream o° Redlands constitute a remnant population. Except for hatchery-reared juvenile