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Introduction The Colorado squawfish (Ptychocheilus lucius) was widely distributed throughout the Colorado River drainage until the early 1900s (Girard 1856, Jordan 1891, Gilbert and Scofield 1898, Miller 1961, Minckley 1973). At present, populations persist in the upper basin in the Green and Colorado rivers and their larger tributaries (Tyus et al. 1982). In the lower basin, Colorado squawfish have not been collected since the early 1960s and have presumably been eliminated from all waters by intensive development and modification of mainstream and tributary habitats (Miller 1961, Minckley and Deacon 1968, Minckley 1973). The Colorado squawfish was listed as an endangered species under the Endangered Species Act (ESA of 1973; Federal Register Vol. 39 (3):1175) and a recovery plan was prepared and subsequently approved by the U.S. Fish and Wildlife Service (USF41S 1978). Despite its listing as "endangered" and the protection afforded a listed species, upper basin habitats of the Colorado squawfish have continued to be subjected to intensive development and fragmentation by the construction of large mainstream and tributary dams and diversion structures (Holden and Stalnaker 1975, Seethaler 1978, Holden and Wick 1982, Miller et al. 1982, Miller et al. 1984, Tyus et al. 1982). Prior to 1985, recovery efforts for Colorado squawfish were negligible, although an intensive effort (Colorado River Fishery Project) was conducted to determine the status, life history and recovery prospects for endemic Colorado River fishes in the upper basin (Miller et al. 1982). It was presumed that suitable riverine conditions no