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appears to be an important factor in maintenance of nursery backwaters in winter. Kaeding et al. ( 1985) hypothesized that high early-life mortality might be an important factor limiting Colorado pikeminnow populations in the upper basin. Hypothesis 4: Backwater Depth The significant relationship (r=0.74, P=0.0417) between greater average backwater depth and lower overwinter decrease in density of age-O Colorado pikeminnow strongly suggests higher survival by fish in the deeper, more persistent backwaters. Deeper backwaters are less likely to become flooded or desiccated with changes in river stage, and hence, tend to be more persistent in winter. This persistent habitat enables the young fish to limit unnecessary movements during the \vi nter low metabolic period, and to remain in a sheltered, food-rich environment. Antecedent flows that shape deep backwater channels are desirable for enhancing this geomorphic feature in nursery backwaters. H,Jpothesis 5: Densities of Non-Native Fishes The hypothesis that overwinter survival is significantly higher at low fall densities of non- native fish in nursery backwaters was not accepted. The data fit the linear model poorly, and analysis showed that the slope of the relationship was approximately equal to zero. Perhaps predation and competition forces are not as intense during winter as in other times of the year, or these interactions are occurring outside of backwaters as young pikeminnow are forced to seek alternative habitats. Moreover, other factors may be having a more important impact on age-O Colorado pikeminnow in winter than non-native fishes. Reliability of Catch Rate Data Regression analyses showed poor relationships between overwinter survival of age-O Colorado pikeminnow and fall total length, flow variability, river temperature, and non-native fish densities. Possibly interactions among these and other variables would explain measured decreases in age-O fish densities from fall to spring. Alternatively, catch rates of age-O Colorado pikeminnow may be insufficiently sensitivity to accurately assess real changes in fish density. Haines and Modde ( 1996) suggested that survival estimates of age-O Colorado pikeminnow from mark-recapture data were more accurate and precise than those from catch-per-unit-effort data. For the 1992 year class, they showed overwinter survival of 45% from mark-recapture estimates in Reach 4, compared to 38% decreases in catch rate for this study. For the study period (1987-95) for Reach 3, we found a mean survival index of 54% (range, 23-100%), while Haines et al. (1998) reported a mean overwinter survival of 45% (6-62%), using mark-recapture methods. Some individual comparisons and mean comparisons are strikingly similar, indicating that both catch rate and mark-recpature techniques may provide reasonable indices of overwinter survival. Additional comparisons of mark- recapture and catch rate data are needed to determine the best and most cost-effective technique for esti mating overwinter survival. 19