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Previous studies in the Green River nursery areas (Haines and Modde 1996; Haines et al. <br />1998) have shown that population estimates can be made in 20-mile reaches of river, but that <br />accuracy of overwinter survival estimates is influenced by environmental conditions and fish <br />movement into or out of sample reaches during winter. Obtaining accurate estimates of <br />overwinter survival of age-0 Colorado pikeminnow is critical to understanding the life history of <br />the species, modeling population dynamics, and determining the factors affecting survival to <br />protect the species and its habitat. Capture-recapture population estimation is the best means for <br />estimating abundance of age-0 Colorado pikeminnow because, unlike catch per unit of effort, it <br />can account for the influence of environmental factors that may affect capture probabilities. In <br />addition, capture-recapture can be used to estimate rates of immigration and emigration. <br />This study attempted to use capture-recapture procedures to estimate overwinter survival <br />and movement of age-0 Colorado pikeminnow and relate the observed responses to stage <br />fluctuations in the Green River induced by hydropower operations at Flaming Gorge Dam. The <br />following objectives were developed for this study. <br />1) Determine if overwinter survival of age-0 Colorado pikeminnow is affected by winter <br />operations of Flaming Gorge Dam. <br />2) Determine if backwater habitats are physically affected by fluctuating releases from <br />Flaming Gorge Dam during winter. <br />3) Determine if fish movements in winter are related to fluctuating releases from <br />Flaming Gorge Dam. <br />4) Evaluate the assumptions of overwinter survival estimates and specifically determine <br />how Colorado pikeminnow movements affect these estimates. <br />A-3