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Bioenergetics Model Component • Bioenergetics modeling predictions indicated that overwinter survival of age-0 Colorado pikeminnow may be negatively affected if fish activity increases in response to physical or natural changes in the environment. When activity rates were spontaneous or 0.5 bl/s, 35-, 45-, and 60-mm TL fish survived to the end of the winter period. When activity was 1.0 bl/s fish did not survive to the end of the winter period regardless of size. • Simulations showed that fish mass and condition consistently decline when water temperatures are below PC. • Mass and condition of fish decline faster when activity is increased because energetic reserves must be used to offset the cost of higher metabolic rate. • Data describing fish growth and food consumption rates during winter are potentially useful for additional confirmation of the accuracy of bioenergetics model predictions. Required data are repeated measurements of mass (to nearest 0.01g) and total length (to nearest 1 mm) of individual fish before and after the winter period. RECOMMENDATIONS Effects of stage fluctuations on overwinter survival could not be evaluated in this study, but the bioenergetics modeling predictions indicates that overwinter survival of age-0 Colorado pikeminnow may be negatively affected if fish activity increases in response to natural or physical changes in the environment. 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 evaluation should be conducted to determine the likelihood that natural mortality rates are affected. xx