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growth efficiency may have been responsible for observed differences between Colorado squawfish and fathead minnow. The hypothesis that growth efficiency was responsible for the competitive advantage of fathead minnow over Colorado squawfish is consistent with the life-history strategies of the fishes. Colorado squawfish are long lived, have large body size, and delayed reproduction. Fathead minnow are short lived, have small body size, and early reproduction. Implicit in these characteristics is one other correlate of life-history theory: short-.lived species have rapid development compared to long-lived species (Pianka 1970). Faster development implies greater growth efficiency, provided that both species eat similar prey and are exposed to identical environmental conditions. Thus, natural selection may have provided fathead minnow with a competitive advantage over larval stages of Colorado squawfish. Because fathead minnow are not native to the Colorado River Basin, there has been no coevolution with Colorado squawfish to facilitate development of partitioning mechanisms that would allow coexistence. Assuming Colorado squawfish do not become extirpated in the wild, two important implications of the lack of coevolution are related to (1) effects of non-native fishes on habitat selection by young Colorado squawfish and (2) ultimate evolutionary effects of the introduction of non-native fishes into the Colorado River Basin. Mechanisms of nursery-habitat selection used by young Colorado squawfish are unknown, but it is plausible that food availability plays a role. The species that is most efficient at capturing and converting food resources to biomass will be able to persist in an environment for a longer time as resources become scarce (Tilman 1982). For larval Colorado squawfish, the alternative to living in a backwater habitat that has become unsuitable is 21