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EXECUTIVE SUMMARY This report encompasses two important aspects of age-O Colorado pikeminnow life- history. First, we investigated usefulness of scale analysis and length-frequency distributions for evaluating oveIWinter mortality and growth of age-O Colorado pikeminnow. Second, we examined associations of age-O fish length and relative abundance in fall and spring with spatial and temporal degree-day accumulation and flow regimes in the Green River system. Using the relationship between total number of circuli and total length of young Colorado pikeminnow, we estimated maximum number of circuli formed in the first year for 1) fish that formed a first year growth check and 2) for those that did not. We found that most age-O Colorado pikeminnow (49%) collected in fall of 1991 (only year scales were collected from age- o fish) were too small (<40 rnrn TL) to form a first year growth check but that a first year growth check was present on scales of most adults (75%) collected between 1978-1989. We then examined length frequency information collected over eight winter seasons (1987 to 1994). In the middle Green River, 45 to 75% of age-O fish were less than 40 rnm in fall for years examined (1989 to 1993); in the lower Green River, 50 to 90% offish were less than 40 mm TL in fall for years examined (1987 to 1993). This information suggests that some size- dependent mechanism, such as mortality or growth, favored larger Colorado pikeminnow. To assess size-selective mechanisms (growth or mortality) affecting survival of age-O Colorado pikeminnow, we examined shifts in length-frequency data, plotted as quantiles, from fall to spring over the 8 year period. In the middle Green River, all years showing a significant effect of a size-selective overwinter mechanism indicated mortality was dominant. In the lower Green River, mortality was dominant in five years, but two years showed dominant size-selective growth. We were not able to discern the interaction between growth and mortality. The relative importance of these size-dependent mechanisms most likely varies from year to year depending on biological and environmental factors like nonnative predation, competition, and degree day accumulation in the first growing season. These results lead us to explore relationships between physical variables of temperature and flow with size of age-O Colorado pikeminnow in fall. If degree-day accumulation influenced size of fish in fall, we would have expected to see differences among middle and lower river reaches for both size of age-O fish entering winter and consequently, their overwinter survival assuming they were hatched during the same period. We calculated degree-day accumulation for pre- and post-dam periods to determine large-scale changes due to Flaming Gorge Dam and then for individual winter and summer seasons that corresponded with fall and spring age-O fish sampling (1987 to 1994). We found that mean daily water temperatures of the Green River near Jensen, Utah, and near Green River, Utah, were higher on average at the downstream Green River station than the upstream Jensen station, and total degree day accumulation was 37% greater at the downstream station during the period of record. However, no change was found to be associated with dam operations. We also found that degree day accumulation for summer and winter periods was consistently and substantially greater in the lower Green River during the study period. Despite the differences in degree-day accumulation between sites, age-O Colorado pikerninnow were not larger in the lower Green River in fall. Furthermore, analysis showed that overwinter degree-day accumulation did not appear to Vlll