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abundance of young Colorado pikeminnow during this investigation. Because few fish were captured, estimates were imprecise and reliable overwinter survival estimates could not be made. The fish bioenergetics analysis showed that in some circumstances, increased activity can reduce overwinter survival of young Colorado pikeminnow. Consequently, the bioenergetics analysis could not exclude the hypothesis that stage fluctuations caused by Flaming Gorge Dam operations influence overwinter survival of age-0 Colorado pikeminnow. The effect of Flaming Gorge Dam on fish activity remains unknown because we were not able to observe changes in fish activity in response to flow fluctuations. For age-0 Colorado pikeminnow, winter conditions produce a negative energy balance where the costs of survival are greater than the energetic gains realized by food consumption. If it can be shown that the operation of Flaming Gorge Dam increases activity of age-0 Colorado pikeminnow in the Green River, then further evaluations should be conducted to determine the likelihood that natural mortality rates are affected. CONCLUSIONS Overwinter Survival and Movement of Young-of-Year Colorado Pikeminnow • Population estimates for the 40-mile reach were imprecise; CV ranged from 0.25 to 0.59. The imprecision is the direct result of not catching, marking, and recapturing enough fish. Colorado pikeminnow young-of-year population densities, as measured by ISMP, were one-tenth the densities during the three years of this study compared to the 13 years prior to the study. • Overwinter survival estimates could not be made because population estimates were too imprecise. Size-selective overwinter mortality does occur among young-of-year Colorado pikeminnow, but does not necessarily occur every winter. • Young Colorado pikeminnow moved only short distances both within autumn and spring sampling and between autumn and spring sampling. Within autumn and spring sampling, 95% of xviii