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preceding fall. Larger and more age-O fish in fall resulted in larger and more age-l fish in spring, which suggests that overwinter size-selective pressures were relatively consistent from year to year. The fact that reduction in CPE from fall to spring varied from year to year indicates the magnitude of variation in year class strength overrode any effect of overwinter mortality. Two important conclusions can be drawn from this finding: I) year class size and length in fall may override effects of overwinter size-selective mechanisms in dictating year class size and length in spring and 2) current sampling in fall and spring are representative of actual conditions. Our study further suggests that Flaming Gorge Dam has not altered the length of the growing season as measured by DD based on growth potential indices. A similar finding was reported by Kaeding and Osmundson (1988). They found that impoundment had not altered the temperature regime of the middle reaches of the Colorado River and suggest that dams in the lower Colorado River basin prevent Colorado pikeminnow from using warmer downstream reaches (below Glen Canyon Dam). Yet, if this theory were true, one would expect age-O Colorado pikeminnow to be larger in lower reaches of the Green River than in middle reaches rather than being the same as noted in this and other studies (Tyus and Haines 1991). It was unexpected that year class size and mean length in fall was not related to DD, summer flows or timing and magnitude of peak discharge in the preceding spring. Possible explanations for this may be that factors other than temperature and flow dictate number and size of fish in fall and their respective condition overwinter. Survival and growth may be limited by biotic factors such as predation by and competition with native and nonnative fish species before winter. This hypothesis concurs with Bestgen (1997) who found biotic factors to be more important than abiotic factors in determining fall recruitment of age-O Colorado pikeminnow, which seems more likely than lower growth rates in the lower Green River. The lack of a size difference between lower and middle age-O and age-l Colorado pikeminnow (despite a substantial difference in temperature) could be the result of differences in habitat quantity and quality between the two areas or may simply be an artifact of better survival of fish spawned later in the season (when red shiner numbers decline) in the lower Green River (Bestgen 1997). Why a similar relationship would not occur in the middle Green River is unclear. A loose association of year class length and DD in the lower Green River and not in the middle Green River suggests that limiting factors in the two reaches may be different. More information on ecology of and predation by red shiners and other nonnatives is necessary to elicit this puzzling relationship. CONCLUSIONS Overall, scale analysis proved useful in determining the relationship between length and overwinter mortality and growth; however, our age-O scale data was limited to several sample sites within one year. Such data limitations precluded extensive statistical analysis and cautioned that any conclusions be pending further investigations and validation of our findings. To validate this relationship, we recommend that scale collection become a standard procedure for Inter- agency Standardized Monitoring and that data for future analysis include a greater temporal and spatial scale. In particular, this analysis should be evaluated for data representing more than one year-class of age-O fish because of the inherent variability of year-class strength and size 13