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INTRODUCTION The fish community of the Colorado River was historically dominated by endemic species -- species found nowhere else. Because of there limited d istribution, survival of these species is more tenuous than it is for species with wider distributions. Alteration of the Colorado River has greatly reduced the populations of some of these endemic species, and " continued alterations of the river could reduce a few species to extinc- tion. The recent decline in abundance of three endemic fishes, Colorado squawfish (Ptychocheilus lucius), humpback chub (Gila cypha), and bonytail chub (G. elegans), has been significant enough to justify classifying them as endangered by the United States Fish and Wildlife Service. TJnder the provisions of the Endangered Species Act of 1973, endangered species designation dictates that Federal agencies involved in resource development projects must not jeopardize the continued existance of endangered species. Proposed developments that would further reduce populations of endangered species might be modified or canceled. Thus, the presence of an endangered species can greatly influence the develop- ment of natural resources. This has been a particularly controversial issue affecting proposed Colorado River water projects. Information on the distribution, abundance, and habitat associations of endangered Colorado River fishes was required to determine the effects that proposed water development projects might have on these fishes. In 1979 the Fish and Wildlife Service (FWS) and the Bureau of Reclamation (BR) cooperated to develop the Colorado River Fishery Project (CRFP), a research group whose purpose was to collect and .interpret information on endangered Colorado River fishes. CRFP findings suggest that the endangered Colorado squawfish and humpback chub may experience an unusually high rate of mortality during the first few years of life (Miller et al. 1983). If these causes of early mortality could be determined, it might be possible to improve the survival of Colorado squawfish and humpback chub, Initial CRFP studies collected chubs (Gila) with morphomeristic charac- teristics that appeared to intergrade between those of the humpback and the roundtail chub (Gila robusta), suggesting that some gene exchange between these species may be occurring. It has been hypothesized that reproductive barriers between roundtail chub- and humpback chub are being ,, lost as the river habitat has changed with man's influence. Primary concern of researchers is that depletion of river discharge might eliminate or degrade spawning habitat and reduce reproductive barriers for the humpback chub, thus leading to its extinction. These early CRFP studies also documented that the bonytail chub, a close relative of the humpback chub, was virtually extirpated from the L'pper Colorado River Basin and ' was on the edge of extinction throughout the entire Basin.