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young Colorado pikeminnow in spring and is well above estimates of potential overwinter growth (Vanicek and Kramer 1969; Thompson et al. 1991; Valdez and CowdelI 1996). This assumption of 10 mm of overwinter growth was conservative considering Thompson et al. (1991) found that fed Colorado pikeminnow ranging from 30 to 44 nun TL grew less than I mm over a 260 day period of exposure to reduced water temperatures (3.50C) and considering Kaeding and Osmundson (1988) suggested that growth ceases at water temperatures below 13 oC. However, sampling occurred in late September and early October which may have allowed age-O fish growth for an additional several weeks before water temperatures dropped below 13 0 C in fall and after they rose above 130 C in spring. The designated 80 mm maximum length cutoff did not alter normality of distributions except for the 1994-5 season; therefore, this year was excluded from analysis. Four of seven winters (88-89, 89-90,92-93 and 93-94) examined from the lower Green River showed a dominant effect of overwinter size-selective mortality occurring as indicated by significant differences in the regression slope of fall quantiles verses spring quantiles; two winters showed a greater effect of size-selective overwinter growth (slope greater than one) (87- 88 and 90-91) and one winter's effects were indeterminable (no significant change in slope) (91- 92) (Figure 8a-8g). In the middle Green River, three of five winters examined showed a dominant effect of size-selective overwinter mortality (89:-90,90-91,93-94) and two winters effects were indeterminable (91-92, 92-93). Only in one season (1990-91) was the mechanism different between the two areas (Figure 9a-e). Other winters showed more size-selective growth or an unquantifiable combination of overwinter size-selective growth and mortality. For comparison, slopes and p-va1ues for these regressions are reported with other variables in Appendix la and b. It was not possible to guarantee that all fish under 80 mm were age-l fish in spring (a few may have been older); therefore, our QQ plots may have been biased towards demonstrating size-selective growth and against size-selective mortality if upper quantiles included fish beyond age-I, which further supports the contention that size-selective mortality is the dominant mechanism acting on age-O Colorado pikeminnow in their first winter. Associations among year class and physical characteristics Annual DD in the middle Green River (near Jensen, Utah) and at the lower Green River (at Green River, Utah) was not different in the pre- and post-dam periods (Table 2). However, Table 2. Comparison ofDD for pre- and post- impoundment periods at 2 gage stations on the Green River, Utah. Station Pre Post Jensen Green River 67.04 104.37 67.16 110.87 9