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LIFE HISTORIES OF COLORADO RIVER ENDANGERED FISHES The Service is mandated to determine critical habitat, and to use the best scientific and commercial data available (50 CFR 424.12). Current knowledge of the life history needs of the razorback sucker was presented in the Service's final rule (56 FR 54957). The bonytail, humpback chub, and Colorado squawfish life history needs were presented in the recovery plans for each of those species (USFWS 1990a, 1990b, 1991). Biological information developed since listing the razorback sucker and preparation of recovery plans has also been used to determine habitat needs of the fishes. Also, the Service incorporated unpublished information from current research on the Colorado River endangered fishes to ensure the best and most recent scientific and commercial data were used for this determination. These fishes evolved in the Colorado River and were adapted to the natural environment that existed prior to the beginning of large-scale water development. Thus, they were adapted to a system of fluctuating seasonal and annual flows influenced by wet, average, and dry climatic periods. Recent population declines and disappearances of endemic fish species in much of their former range have been associated with relatively rapid and widespread anthropogenic changes. These changes have altered the physical and biological characteristics of many mainstream rivers in the Basin and occurred so rapidly that the fishes have not had time to adapt to them (Carlson and Muth 1989). Dams and diversions have fragmented former fish habitat by restricting fish movement. As a result, genetic interchange (emigration and immigration of breeding individuals) between some fish populations is nonexistent. Large floods were once normal in the Basin, and provided food and nutrient exchange between river channels and shallow-water floodplain habitats. These floods are now controlled by numerous dams. As a result of these dams, major changes also have occurred in water quality, quantity, temperature, sediment and nutrient transport, and other characteristics of the aquatic environment (Carlson and Muth 1989). The altered habitats that have resulted are now more suitable for introduced, nonnative fishes, some of which have flourished (Tyus et al. 1982a; Carlson and Muth 1989; Minckley and Douglas 1991). These changes have greatly altered the river environment and little or no unaltered habitat remains in the Basin for the four Colorado River endangered fish species. RAZORBACK SUCKER (Xyrauchen texanus, Abbott 1861) The razorback sucker is part of a unique fish fauna endemic to the Colorado River Basin (Miller 1946, 1959). This species was once one of the most abundant and widely distributed fish in the mainstream rivers of the Basin (Jordan and Evermann 1896; Minckley 1973). Historic riverine systems provided a wide variety of habitats occupied by razorback suckers including backwaters, sloughs, and oxbow lakes (Holden and Stalnaker 1975a; Minckley 1983; Tyus 1987; Tyus and Karp 1990). 8