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19951 DIETS OF FISHES IN BACKWATERS 101 varies, but generally fish become an important food item after Colorado squawfish attain a length of >40 mm. Osmundson and Kaeding (1989) suggested that slower growth and poor- er condition of YOY and especially yearling Colorado squawfish in grow-out ponds with lower densities of appropriate-size forage fish might have been caused by higher reliance on insect forage. Identifiable fish reported in digestive tracts of YOY Colorado squawfish here and by McAda and Tyus (1984) and Grabowski and Hiebert (1989) were either red shiner or fathead minnow larvae. These non- native species are short-lived fractional spawn- ers (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 autumn (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 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 exper- iments on behavioral interactions, Karp and Tyus observed that red shiner, fathead min- now, and green sunfish shared activity sched- ules and space with Colorado squawfish and exhibited antagonistic behaviors toward small- er Colorado squawfish. We could not effectively evaluate competi- tion for food between YOY Colorado squaw- fish and other fishes because study design did not provide for estimation of resource abun- dance and availability, intraspecific diet selec- tivity, and effects of interspecific use of impor- tant resources. Direct evidence for interspecific competition should be determined through experiments demonstrating that shared use of a limited resource negatively affects one or more of the species (Schoener 1983, Under- wood 1986, Wiens 1992). Additionally, we assume gut contents represented food con- sumed in the backwaters of capture, but this might not always have been the case. Tyus (1991b) observed that although young Colo- rado squawfish in the Green River were found mostly in backwaters, some moved to or from other habitats during 24-h periods. We found that diet overlap for most comparisons with Colorado squawfish was below the level gen- erally considered biologically important (Table 2). Although not conclusive, these compar- isons suggest either general resource parti- tioning or differences in diet preferences. Diet overlap values were considered biologically important only for comparisons with certain size-interval, river-reach groups of five fishes. Because interspecific demand for resources might not exceed supply, Bowen (1983) noted that even extensive diet overlap is not conclu- sive evidence for competition. Accordingly, McAda and Tyus (1984), who also used Schoener's index to examine diet overlap between YOY Colorado squawfish and nonna- tive fishes in the Green River, suggested that high diet overlap they observed between Colorado squawfish 22-40 mm TL and chan- nel catfish 19-55 mm TL (overlap value = 0.60) and especially red shiner 15-69 mm TL (overlap values 0.70-0.80) might reflect shared use of abundant resources, primarily imma- ture dipterans, rather than competition. The same may be true for higher diet overlaps we observed. Ward et al. (1986) reported that chi- ronomids, the principal food category result- ing in high diet overlap, were among the more common benthic invertebrates in the Colo- rado River basin. We observed that overlap values were gen- erally higher and, for most fishes, diet variety was greater in the lower than upper reach, perhaps because food resources were more abundant and diverse in backwaters of the lower reach. Based on observations during summer and autumn 1979-1988, Haines and Tyus (1990) found that backwaters in the upper and lower reaches were similar in mean surface area, but that those in the lower reach were shallower and warmer, conditions that may favor higher productivity. Also, within the upper reach, Grabowski and Hiebert (1989) noted that during summer and autumn 1987-88 concentrations of backwater nutri- ents, particulate organic matter, phytoplank- ton, zooplankton, and benthic macroinverte- brates (particularly chironomid larvae) in- creased progressively downstream. They sug- gested this trend was due to attenuation of flow releases from Flaming Gorge Reservoir (located near the Wyoming-Utah border) at downstream sites that reduced the degree of water exchange between the main channel and backwaters and allowed for greater backwater warming and stability.