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<br /> r <br /> r <br />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 cypha, and bony~tail G. elegans. A fourth fish, razorback sucker <br />Xyrauchen texanus, is extremely rare and current management efforts are <br />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 fish <br />species in the Upper Colorado River Basin (USFWS 1987). The recovery program <br />identifies options for the management and recovery of these species. Among <br />these options is management of the physical habitat for the endangered fishes <br />in the upper-basin rivers. Such management would be facilitated by <br />appropriate operation of the numerous dams and reservoirs in the upper basin. <br />Considerable effort has been made to develop models describing the <br />relations between-river discharge and-the quantities of the various habitats <br />used by endangered fishes in the Upper Colorado .River Basin. Among other <br />things, researchers hope to determine whether it might be possible, through <br />flow management, to maximize the quantities of the important habitats for the <br />various life stages of the endangered fishes, while minimizing such habitats <br />for undesirable sympatric fish species. The objective of this study was to <br />determine the feasibility of such flow-related habitat management by comparing <br />the habitat use of age-0 Colorado squawfish with those of other sympatric fish <br />species. <br />Methods <br />Data were collected from the Green and Colorado rivers during the <br />