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<br />Wild populations of the federally endangered Colorado squawfish Ptvchocheilus <br />lucius now exist only in the Upper Colorado River Basin. They are most abundant in the <br />Green and Yampa rivers of eastern Utah and northwestern Colorado (Tyus 1991a). The <br />decline of this species and other native fishes in the Colorado River Basin has been <br />attributed to habitat alterations caused by water-resource development and the introduction <br />and proliferation of nonnative fishes (Carlson and Muth 1989, Minckley 1991). <br />Backwaters of the Green River below its confluence with the Yampa River are <br />important nursery areas for young-of-year (YOY) Colorado squawfish in summer and fall <br />(Nesler et al. 1988, Haines and Tyus 1990, Tyus and Haines 1991). The natural integrity of <br />these backwaters is compromised by flow regulation via Flaming Gorge Dam, located near <br />the Wyoming-Utah border, which affects their stability and productivity and by the <br />dominance of nonnative fishes (Tyus et al. 1982, Stanford 1993). Several of the <br />nonnatives, particularly cyprinids and especially red shiner Cvprinella lutrensis, are <br />perennially represented in backwaters by all life stages (Haines and Tyus 1990). This <br />observation has led to the hypothesis that nonnative fishes are adversely affecting survival <br />of young Colorado squawfish through competition for available food resources or predation. <br />Although Stanford (1993) suggested that strong food-web interactions between native and <br />nonnative fishes are probably occurring, little is known about dietary relationships of these <br />fishes (Haines and Tyus 1990, Ruppert et al. 1993). Our objectives were to (1) describe <br />diets of yay Colorado squawfish and other small fish in backwaters of the Green River and <br />(2) examine diet overlap and potential for competition with Colorado squawfish. <br /> <br />3 <br />