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EXECUTIVE SUMMARY Catch rates of age-O Colorado pikeminnow (Ptychocheilus lucius) were compared between fall (September-October) and spring (March-April) sampling periods as indices of overwinter survival in two reaches of the Green River, Utah. Overwinter survival indices were determined for the year classes 1987-95 for Reach 3 (Colorado River confluence upstream to Green River, Utah; 193 km) and for the year classes 1989-94 for Reach 4 (Sand Wash upstream to Split Mountain, Utah; 169 km). Average indices were 54% (23-100%) and 51% (29-96%), respectively. Overwinter survival indices for Reach 3 were compared to total length of fish in fall, flow variability, river temperature, average backwater depth, and fall densities of non-native fishes. No clear relationships were revealed for river temperature, or densities of non-native fishes. Lower survival was demonstrated for small size in fall, higher fluctuating river flows, and significantly higher survival was related to backwater depth. Backwaters with mean depths of> 120 cm showed ,the highest overwinter survival index of 85%, compared to only 18% for backwaters with mean depth of <30 cm. These findings show that backwaters deepened by appropriate antecedent flows provide greater resilience to inuridation and desiccation as a result of fluctuating releases from Flaming Gorge Dam. We noted that sampling in spring was preceded in all years by early runoff spikes from low elevation snowmelt and river ice breakup, and hypothesize that much of the decrease in densities of age-O Colorado pikeminnow in nursery backwaters is related to a natural survival strategy of downstream dispersal. This hypothesis is supported by findings of highest densities of age-O Colorado pikeminnow in the Lake Powell inflow immediately following these March spikes in 1993 and 1994. These analyses show that interactions among variables affecting survival of age-O Colorado pikeminnoware complex. Catch rate statistics collected in fall and spring may be of insufficient sensitivity to provide accurate and reliable estimates of fish density. This paper concludes that merwinter survival of age-O Colorado pikeminnow must be considered as an i~portant aspect in (n-crall survival of fish to age of recruitment, but existing measures of fall and spring densities show high variability that is partly attributed to annual population variability as well as to sampling. We helieve that understanding the fate of age-O Colorado pikeminnow during the winter period is the key to determining cohort strength and recruitment to the adult portion of the population. It is important to understand and separate the effect of anthropogenic actions from natural life history strategies on long-term conservation of this species. IV