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1 1 1 1 Introduction In 1980, the bonytail (Gila elegans) was listed as federally endangered by the U.S. Fish and Wildlife Service (USFWS) due to its virtual disappearance. It is one of four native and endemic fish in the Colorado River system that are listed as federally endangered. Their decline is attributed to loss of habitat from dams that have altered or destroyed the natural, physical environment of the river, and to interactions with nonnative species (Vanicek and Kramer 1969; USFWS 1990; Chart and Cranney 1993). Catastrophic loss has been attributed to active elimination of native fish with replacement by sport fish in dam tailwaters (Holden 1991) and to extensive water pollution from industry and railroads as early as the 1930's (Quartarone 1993). The USFWS has ranked the recovery priority of the bonytail as 5C (high n I 1 priority) because of its low numbers and undocumented recruitment in the wild and because of conflict of its recovery with economic development (USFWS 1990). Although complete removal of dams and nonnative fish is not feasible, recovery efforts may prove worthwhile for several reasons. First, legislation from the Endangered Species Act of 1972, as amended, provides priority protection to endangered species and their habitat; hence, any recovery action is given priority over other agency activities (Rohlf 1989). Consequently, future introduction and supplementation of exotic species is discouraged, and dam operations are being determined with regard to benefits for endangered fish. For example, in the case of the bonytail and other endangered Colorado River fish, seasonality has been incorporated into the annual flow regime of Flaming Gorge Dam, as well as other water development units. 6