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to leave the habitat, venture into the river proper, and search for a new nursery area. This process is inherently risky and an alternate nursery area may not be found. Colorado squawfish probably inhabit nursery habitats where food is abundant so that growth is maximized. However, when resources are limited, and competitors are present, food availability for Colorado squawfish is diminished. This resource reduction may cause a corresponding increase in (1) time to size of reproductive maturity, and (2) mortality because of longer time spent in smaller size classes in which fish are more vulnerable to predators and other mortality factors (Werner 1984; Thompson et al. 1991). The species with greatest growth efficiency has an advantage because it can maintain a higher growth rate as resources become scarce, and can persist in a resource-poor environment for a longer period of time. Less efficient species like Colorado squawfish may be forced to abandon nursery habitats more frequently when superior competitors are present, a consequence which may reduce survival. Ultimately, the impact of non-native fishes may influence the life-history strategy of Colorado squawfish. Historically, adult Colorado squawfish attained larger size compared to fish captured recently. The cause of this change is unknown but has been attributed to a variety of factors including: elimination of historical prey (Behnke and Benson 1983), and modification of historical temperature regime (Vanicek and Kramer 1969; Kaeding and Osmundson 1988). An alternative explanation is that, because of competition with non-native fishes, natural selection has favored Colorado squawfish offspring with a life-history strategy that results in smaller adults, and smaller size at first reproduction. This response is consistent with predictions of life-history theory for organisms that simultaneously 22