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Endangered Fishes Status At present, four of the seven large fishes of the Big River fish community are federally listed as endangered species. These fishes, the Colorado pikeminnow (Ptychocheilus lucius), razorback sucker (Xyrauchen texanus), humpback chub (Gila cypha), and ' bonytail (Gila elegans) once populated warm water reaches of the mainstream rivers of the Colorado River basin from Wyoming to Mexico. The numbers and ranges of these formerly abundant fishes have been drastically reduced, and these species are now threatened with extinction. Little was known about the Big River fishes in the UCR basin until the late 1960's and 1970's when research studies at Utah State and Colorado State Universities were done, mostly by graduate students. Beginning in 1979, government agencies have invested heavily in surveys to establish the distribution and relative status of each species as part of the effort to recover them, and an excellent baseline record exists. Subsequent monitoring efforts have been less comprehensive in terms of spatial coverage and sampling gear, but can detect changes in distribution patterns and 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 due to local changes in the riverine environment. In addition, high mobility and migratory habits of species like the Colorado pikeminnow and difficulties of identifying early life history stages of the endangered species (especially when those stages may be segregated spatially from the adults) has made data difficult tCi`interpret. Populations of three of the four endangered fishes presently exist in the UCR basin above Lake Powell, while historical populations of bonytail have been extirpated. Populations of the remaining three fishes are now restricted to sections of the Green and Colorado Rivers, where populations appear relatively stable. However, anecdotal information suggests that populations of the fishes have declined in tributary streams. Of these tributaries, the Yampa and Gunnison Rivers continue to ,support adult Colorado pikeminnow and spawning areas for them. A large standing stock of the fish also occurs in the White River. A population of humpback chub exists in the Yampa River and individuals have been reported in the Little Snake River (Wick et al. 1991). The tributary UCR above Palisade, Colorado, probably supported individuals recently (Valdez et at. 1982). Adult razorback suckers have recently been reported in the lower Yampa River, tributary Green River, Gunnison River, and in a pond once connected to the upper Colorado River near Palisades, Colorado. In addition, razorback suckers stocked in the Gunnison River have been repeatedly recaptured, indicating that habitat conditions there remain suitable for them. Life History Requirements The role of tributary streams in maintaining populations of the endangered Big River fishes can be best determined by understanding the direct ways (such as providing physical habitat) and indirect ways (such as providing inputs of water, sediment, and 10