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growth check. Although some element of error would be associated with this criterion, it would account for the natural spatial and temporal variation in growth of age-O Colorado pikeminnow. Association between fish length and survival in their first year Our results support the hypothesis presented by others that the length of age-O fish in fall is linked to survival in their first year. As Tyus (1986) noted, Colorado pikeminnow have survived the dynamic Colorado River system by exploiting life-history strategies such as large size and long life. These traits may represent a trade-off of low early life-stage survivorship for high adult survivorship after a certain size is attained (Pianka 1988). It seems logical that Colorado pikeminnow would put energy into many small individuals that suffer high mortality in the first year when one considers the high risk distribution mechanism of passive post-larval drift over hundreds of kilometers into low-velocity backwater refuges in alluvial reaches (Haynes et al. 1984; Nesler et al. 1988; Tyus 1990; Tyus and Haines 1991). Tyus and Haines (1991) found a size difference between fish captured in the fall and in the spring; they suggested that overwinter mortality was insignificant and attributed the difference to growth. However, Thompson et al. (1991) found that age-O Colorado pikeminnow showed minimum growth at winter temperatures and that greater lipid content, which is associated with greater body length, was important to survival during periods of reduced temperature and food availability. Particularly important to note is the large discrepancy in survival Thompson et al. (1991) reported between starved fish at 44 mrn TL versus 36 and 30 mm TL (100% v. 6.7 and 3.3% survivorship, respectively, at reduced temperatures). A relatively small difference in body length resulted in an extremely large difference in survivorship when food was not available. Although some feeding has been noted during winter (60% of age-O Colorado pikeminnow with food in stomachs during winter reported by Osmundson and Kaeding, 1989), this feeding rate is significantly lower than that reported from summer and fall months (100 to 95% with food in stomachs during August and September; Vanicek 1967; Grabowski and Hiebert 1989; Muth and Snyder 1995). Therefore, it is logical to assume that Colorado pikeminnow can starve in winter, that smaller fish starve more readily and that 40 mm may be a crucial size threshold in fall for survival into spring, all of which suggests overwinter size-selective mortality. Our results also suggest that age-O Colorado pikeminnow experience a combination of size-selective growth and size-selective mortality in their first winter. We believe that enough evidence exists herein and in other studies, both in and out of the Colorado River system, to demonstrate the likelihood of these mechanisms occurring. However, the intensity of these size- selective mechanisms and their interaction should be evaluated to quantify their limitation on age-O to age-l recruitment and to determine management options for facilitating species recovery. The main contention on this issue in the past seems to be due to a tendency to ascribe an all-or-none criterion to these mechanisms rather than evaluating the extent and interacting strength to which they occur. With the degree of variability in the Colorado River system and in annual year class strength and mean size, it is likely that dominant mechanisms vary in their occurrence and severity. Another significant finding from this study is that the size and relative abundance of age- l fish in the spring is primarily dictated by the size and relative abundance of age-O fish in the 12