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i~ Introduction <br />Three fishes native to the Upper Colorado River Basin are currently <br />listed as endangered: Colorado squawfish Ptychocheilus lucius, humpback chub <br />Gila cy~a, and bonytail G. elegans. A fourth fish, the razorback sucker <br />Xvrauchen texanus, is extremely rare and current management efforts are <br />r directed toward preventing its further decline. The Fish and Wildlife <br />Service, in cooperation with other federal, state and private organizations, <br />recently developed a Recovery Implementation Program for endangered fishes in <br />the upper basin (USFWS 1987). The recovery program identifies options for the <br />management and recovery of these species. Among these options is management <br />of the physical habitat for the endangered fishes in upper-basin rivers. Such <br />management would be facilitated by appropriate operation of the numerous dams <br />and reservoirs in the upper basin. <br />Considerable effort has been made to develop physical-habitat simulation <br />models describing the relations between river discharge and the quantities of <br />the various habitats used by endangered fishes in the Upper Colorado River <br />Basin. Although the outputs of these habitat models have yet to be correlated <br />with numerical changes in the populations of endangered fishes, researchers <br />hope to determine whether flow management might be used to maximize the <br />quantities of the important habitats for the various life stages of the <br />endangered fishes while minimizing such habitats for undesirable species of <br /> sympatric fishes. The objective of this study was to examine the data <br /> <br /> collected by the Fish and Wildlife Service during the Standardized Sampling <br /> Program of 1979-1981 (Tyus et al. 1982; Valdez et al. 1982) and to compare the <br /> habitat us e of age-0 Colorado squawfish with those of other sympatric fish <br /> species. Differences in habitat use among the fish species will help <br /> <br />1 <br />