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s,,~ <br />Abstract: The upper basin of the Colorado River currently supports four species of endangered <br />fish. These include the Colorado squawfish, razorback sucker, bonytail chub and the humpback <br />chub. The introduction of nonnative fish predators is currently one of the main hypotheses for <br />why these fish continue to decline in distribution and abundance. We conducted a bioenergetics <br />study to determine the predation potential of northern pike on the endangered Colorado <br />squawfish in the Green River within the Upper Colorado River basin. Patterns of movement <br />and thermal experience were measured by tracking pike implanted with radio transmitters. <br />Growth and diet composition were measured from these and other pike captured by <br />electrofishing and with trammel nets. These measurements were used in a bioenergetics model <br />to predict the sizes and numbers of juvenile squawfish that might be consumed by 1,000 northern <br />pike during a year. Model simulations indicated that if pike specialized on squawfish (i.e., <br />representing 25 % of the diet), 760 kg of squawfish could be consumed during summer and fall <br />for every 1,000 pike in the basin. A further 350 kg could be consumed during winter-and <br />spring. A more conservative estimate, based on squawfish composing an observed 4 % of the <br />pike diet, resulted in consumption of slightly over 100 kg of squawfish during summer-fall and <br />75 kg during winter-spring. We suggest that this methodology may become very important for <br />determining which species of predators represent the greatest threat to rare fishes because it does <br />not depend on handling large numbers of the sensitive species. <br />