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EXECUTIVE SUMMARY <br />1 <br />This study was initiated (proposed and funded in 1992; FY-1993) to address the effect of the <br />introduction of northern pike (a nonnative fish predator) on the decline in distribution and <br />abundance native fishes, particularly Colorado squawfish (Ptychocheilus lucius) in the Green <br />River. The objectives of this study were to: 1) determine the size distribution of northern pike in <br />the Ouray section of the Green River, defined here as the reach between the Jensen Bridge and the <br />confluence with the White River; 2) determine the composition offish in pike diets; and 3) <br />estimate the potential of pike predation on Colorado squawfish. We conducted a bioenergetics <br />study to determine the predation potential of northern pike (Esox Lucius) on the endangered <br />Colorado squawfish in the Green River subbasin, within the Upper Colorado River Basin. <br />Movement, temperature regime, growth and prey consumption were all measured using pike <br />(N=29) with implanted radio transmitters. These parameter estimates were then used in a <br />bioenergetics model to predict the sizes and numbers of juvenile squawfish that might be <br />consumed annually by 1,000 Northern pike. The bioenergetics model suggests that if pike <br />specialized on squawfish (squawfish comprised 25% of the diet), as many as 760 kg of <br />squawfish could be consumed during the summer and fall months by 1,000 pike. A further <br />350 kg could be consumed during the winter and spring period. Amore conservative estimate <br />is that squawfish actually only make up about 5% of the pike diet which would result in <br />slightly over 100 kg of squawfish being consumed during the summer/fall period and 75 kg <br />during the winter/spring time. We suggest that the application of bioenergetics modeling may <br />be important for determining which fish predators represent the greatest <br />vii <br />