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'~ f <br />endangered Colorado River squawfish (Ptychocheilus Lucius) in the Upper Basin of the Colorado <br />River Drainage. We draw upon already published information and data on movement, growth, <br />thermal experience, and stomach contents of pike that we collected during this study. <br />Study Area -Upper Colorado River <br />The upper Colorado River Basin (above Glenn Canyon Dam) includes the major subbasins of <br />the Green, Yampa, White Dueschesne, Gunnison and Colorado Rivers. The upper basin <br />currently supports four endangered fish species: Colorado squawfish (Ptvchocheilus Lucius); <br />razorback sucker (Xyrauchen texanus); humpback chub (Gila cypha); and bonytail chub Gila <br />ele ans . Since impoundment of Flaming Gorge Reservoir in 1963, densities of all four species <br />have rapidly declined (Miller and Hubert 1990). While changes in flow regime, hydrology and <br />geomorphology have undoubtedly played a major role in these declines, the addition of at least <br />20 nonnative fish species has also concerned ecologists (Tyus et al. 1982). Of the 20 <br />introductions, northern pike (Esox Lucius), smallmouth bass (Micronterus dolomieui) and channel <br />catfish (Ictalurus punctatus) are potentially the most important piscivores and may pose the <br />greatest threat to the overall survival and recovery of native species. Of these three species, <br />northern pike are by far the largest nonnative predator in the system, and since their introduction <br />in 1977 ('I'yus and Beard 1990) have shown significant increases in distribution and density <br />(Tyus and Beard 1990; Nesler 1990; Valdez, pers. comm.). <br />Methods <br />Bicenergetics Model <br />We estimated potential predation by northern pike on Colorado squawfish by combining a <br />bicenergetics model (Hewett and Johnson 1992) with field data on food habits, growth, thermal <br />