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biological and environmental factors like size of fish in fall, predation pressure, competition, overwinter food availability or habitat condition (Post and Evans 1989), Although size-dependent growth of age-O Colorado pikeminnow has not been specifically studied, it is likely, because age-O fish have been noted to switch to piscivorus diets between 20 and 50 mm TL and are predominately piscivorus at 100 mm TL (Vanicek and Kramer 1969; McAda and Tyus 1984; Muth and Snyder 1995). Switching to a diet with higher fat, increasing food type availability and increasing predation proficiency are all possible mechanisms that could result in increasing growth rates as body length increases. The same factors may also conversely contribute to size-dependent mortality, because starvation tolerance, metabolic rates and stored lipids of smaller fish are lower than that of larger fish (Paloheimo and Dickie 1966; Thompson et al. 1991; Bestgen 1996), and because smaller fish are more vulnerable prey (Werner and Gilliam 1984). Size-dependent overwinter mortality has been shown to be a common life-history attribute for fish in temperate climates and can be particularly limiting to species near their northern distributional boundaries in the northern hemisphere (Toney and Coble 1979; Post and Prankevicius 1987; Shuter and Post 1990; Smith and Griffith 1994). Starvation and predation are the most likely mechanisms for overwinter mortality. However, studies have demonstrated that age-O Colorado pikeminnow are relatively starvation resistant, even at reduced temperatures (Thompson et al. 1991; Bestgen 1996). At reduced temperatures for 6 months, starved small (30 rom TL) and medium (36 mm TL) sized age-O Colorado pikeminnow experienced lower survival than large individuals (44 mm TL) (3.3% and 6.7% compared to 100% survival, respectively), but all fish fed and survived substantially better (95.1%,98.4% and 100%, respectively) when food was available (Thompson et al. 1991). While feeding, starvation was prevented; yet, during the same period, no growth occurred. An examination of stomach contents of age-O Colorado pikeminnow in ponds during December showed that only 40% of young fish had empty stomachs, suggesting that fish were feeding overwinter (Osmundson and Kaeding 1989). However, when compared to 0 to 5% empty stomachs for age-O Colorado pikeminnow during summer and fall months (Vanicek 1967; Grabowski and Hiebert 1989; Muth and Snyder 1995), it would seem that starvation could be a significant factor for at least some portion of age-O fish during winter months. Based on these former studies, it is likely that both overwinter growth and mortality occur to some degree; however their interaction is not easily discerned based solely on a length shift from fall to spring. It is more accurate to examine how length-frequency distributions change from fall to spring. To differentiate these shifts, Post and Evans (1989) plotted quantiles (numbers of fish within a categorical percentile of the total) of hypothetical distributions of fish lengths in spring verses fall and examined how distributions deviated from each other under simulated case scenarios that exemplified different selective mechanisms. In light of the logistical difficulty in determining absolute numbers of fish and standardizing catch rates among variable physical conditions in dynamic river systems like the Colorado River system, a similar analysis of age-O Colorado pikeminnow length-frequency distributions might reveal clues as to the relative importance of interaCting size-dependent mortality and growth. 2