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species) is required. In this paper, we employ a bioenergetics approach to estimate the potential <br />' impact introduced northern pike might have an the endangered Colorado squawfish in the Upper <br />Basin of the Colorado River Drainage. We draw upon already published information as well as <br />' movement rowth thermal ex erience dat and stomach contents of northern ike collected <br />,g P ~ P <br />t during this study. <br />Study Area <br />1 <br />' The Upper Colorado River Basin (above Glen Canyon Dam) incudes the Green, Yampa, White <br />Duchesne, Gunnison, and Colorado Rivers as the major subbasins. The upper basin currently <br />supports four endangered fish species: Colorado squawfish, razorback sucker, humpback chub, <br />' and bonytail. Since the impoundment of Flaming Gorge Reservoir in 1963, densities of all four of <br />these species has rapidly declined (Miller et al. 1989). While changes in flow regime, hydrology <br />and geomorphology have undoubtedly played a major role in these declines, the addition of at <br />' least 20 nonnative fish species has also been of concern to ecologists (Tyus et al. 1982). Of the <br />20 introductions, northern pike, smallmouth bass (Micropterus dolomieui), and channel catfish <br />' c se the reatest threat to the <br />(1 talurus punctatus) appear to be the largest p~sc~vores and may po g <br />overall survival and recovery of the native species. Of these three species, northern pike are by <br />' far the largest bodied nonnative predator in the system and, since their introduction in 1977 (Tyus <br />and Beard 1990), have shown significant increases in distribution and density (Tyus and Beard <br />' 1990, T. Nesler pers. comm., R. Valdez pers. comm.). <br />3 <br />