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1 The native fish fauna of the Colorado River is characterized by a high level of endemism. Of the 46 native fishes (species and subspecies) present in recent times, 38 are endemic (Miller 1958, Stanford and Ward 1986b). The high level of endemism was heavily influenced by the Quaternary history of the intermountain area of western ~ North America. Populations were isolated by desertification, and faunal composition was changed by local extinctions during the Pleistocene (Smith 1978, Stanford and Ward 1986b). At one point, native Colorado River fishes consisted of only 32 to 36 species, depending on taxonomic interpretation (Stanford and Ward 1986b, Carlson and Muth 1989). River systems of similar size elsewhere (e.g., Missouri River) typically ~ have an order of magnitude more fish species. Present Distribution and Abundance ~ Four of the big river fishes, the Colorado pikeminnow (Plychocheilus lucius), razorback sucker (Xyrauchen texanus), humpback chub (Gila cypha), and bonytail (Gila e/egans)(Frontispiece), once populated warmwater reaches of the mainstream rivers of the Colorado River basin from Wyoming to Mexico. The abundance and distribution of these fishes have been drastically reduced, and the species are now threatened with ~ extinction. As part of the effort to recover these species, government agencies have invested heavily in surveys establishing the present distribution and abundance of each species. The results of these surveys provide the basis for assessing essential aspects of Iffe history needs (e.g., habitat preference, migrations, etc.), and, in time, progress toward recovery goals. ~ Beginning in-1979, the USFWS initiated an ambitious program for monitoring the abundance of endangered fish species throughout the UCRB. Not only was spatial coverage thorough in a statistical sense, but several types of sampling gear were used to optimize collection of all species and life history stages. Sampling efficiency was optimized by selecting river reaches (strata) representing different habitat ~ characteristics. The results of this comprehensive program, summarized in W.H. Miller et al. (1982d), provide an excellent baseline record of fish abundance in the UCRB. Subsequent monitoring efforts have been less comprehensive in terms of spatial coverage and sampling gear, but can detect major trends in abundance (McAda et al. ~ 1994). However, identifying trends in fish abundance has been hampered by high variability in capture data among years, because local changes in the riverine environment and stochastic events can affect population size. In addition, elucidation of trends in the UCRB is complicated by the migratory habits of species like the Colorado pikeminnow, and by the difFculties of identifying early life history stages of the ~ endangered species, especially when those stages may be segregated spatially from the adults. A number of investigators have used standardized methods to sample fish populations in the various rivers of the UCRB and have described the physical habitat in which the 4