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short-lived fractional spawners (Gale and Buynak 1982, Gale 1986) and are typically present in high numbers and at appropriate forage sizes in backwaters of the Green River throughout summer and fall (Tyus et al. 1982, Karp and Tyus 1990). Karp and Tyus (1990) suggested that although the abundance of small nonnative prey fishes in the Green River System might benefit growth of young Colorado squawfish, the benefit might be countered by the aggressive nature of some nonnative fishes which could have negative effects on growth and survival of young Colorado squawfish. In their laboratory experiments on behavioral interactions, Karp and Tyus observed that red shiner, fathead minnow, and green sunfish shared activity schedules and space with Colorado squawfish and exhibited antagonistic behaviors toward smaller Colorado squawfish. We could not effectively evaluate competition for food between yay Colorado squawfish and other fishes because study design did not provide for estimation of resource abundance and availability, intraspecific diet selectivity, and the effects of interspecific use of important resources. Direct evidence for interspecific competition should be determined only through experiments which demonstrate that shared use of a limited resource negatively affects one or more of the species (Schoener 1983, Hanson and Leggett 1986, Underwood 1986, Wiens 1992). Additionally, we assume gut contents represented food consumed in the backwaters of capture, but this might not always have been the case. Tyus (1991b) observed that although young Colorado squawfish in the Green River were found mostly from backwaters, some moved to or from other habitats during 24-h periods. We found that diet overlaps for most comparisons with Colorado squawfish were below the level generally considered biologically important (Table 2). Although not conclusive, these comparisons suggest either general resource partitioning or differences in diet preferences. 12