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8.57; sd = 2.01 (Figure 6). This low C1 count strongly suggests that at least some portion of adults did form a growth check in their first year. Adults were then separated into two groups based on the assumption that a maximum of 9 circuli could form in the first year (calculated from the regression with an age-O maximum of 70 rom TL). The C1 mean was 7.65 (sd = 1.17), with a minimum of 4, for adult scales that had C1 less than or equal to 9 (n = 100). This group of adults was assumed to have formed a first year growth check. This mean was compared to 11.36 C1; sd = 1.34, and a maximum of 16, for adult fish with greater than 9 C1 (n = 33). We assumed that these adults formed their first growth check in their second winter. The distribution of adult C1 for both groups combined offered some support of the above assumptions, as the means of two apparent modal peaks on the distribution approximately equaled the mean C 1 of adults that formed first year growth check and the mean of those that did not (Figure 6). We used adult C 1 counts, which ranged from 4 to 16, to determine the pre-winter minimum lengths of adults that formed a first-year growth check using Equation 1, because the growth check represents onset of somatic growth after winter. Based on a minimum of 4 C1, adults that formed a first year growth check were probably greater than 40 rom TL in the fall of their first year. Age-O scale data showed that age-O fish with 4 C1 ranged from 29 to 49 rom. Of 133 adults, 100 (75 %) were assumed to have formed a first year growth check and, hence, were probably greater than 40 rom TL upon entering their first winter. However, of the 205 age-O Colorado pikeminnow for which scales were examined in fall of 1991, only 110 (54%) were greater than 40 rom TL and 105 (51 %) age-O had greater than 4 C 1 upon entering their first winter. For fall seasons 1989 through 1993,47,45,63,45 and 75% ofage-O fish were less than 40 rom in fall, respectively in the middle Green River. In the lower Green River from 1987 through 1993, 90, 67, 52, 50, 75, 55, and 59% ofage-O fish were less than 40 rom TL in fall, respectively, and by spring, approximately 35% of age-l fish from both reaches were less than 40 mID. Figure 7 shows the percent of fish that exceed the mean length for the years combined in the lower and middle Green River in fall and respective spring. The discrepancy between percent of age-O fish likely to form a first year growth check and percent of adults that showed a frrst year growth check could be due to size-selective overwinter growth of age~O fish among years, size-selective survival oflarger age-O fish overwinter or some combination of these. Some additional error may be due to differential survival at later life-stages as well, which was not considered in this analysis. Overwinter size-selective growth v. mortality To assess size-selective mechanisms in the first winter, we examined fall and spring length- frequency histograms of age-O and corresponding age-l year classes. We developed QQ plots for winter seasons from 1987 to 1994 for age-O and age-l Colorado pikeminnow length- frequency distributions from the lower Green River and from 1989 to 1994 for fish from the middle Green River. To discern between age-l and juvenile fish in these distributions, we assumed late fall and overwinter growth of age-O Colorado pikeminnow of 10 mm TL beyond the maximum fall size of 70 rom; hence, the largest age-l Colorado pikeminnow in March would have been 80 mm. This maximum length was consistent with those reported in other studies of 8