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I I I I I I I I I I I I I I I I I I I compared to the existing presence of largemouth bass and green sunfish and native fish species so the probabilities of prey selection by centrarchids for native fish prey are likely to be small. The fish diet of largemouth bass and green sunfish collected in Colorado backwaters suggested more opportunism versus increased vulnerability of one fIsh species over another. Nyberg (1971, cited in Osmundson 1987) indicated largemouth bass rely almost exclusively on sight for detecting and pursuing prey. Turbidity in Colorado River backwaters could inhibit sight-feeding selectivity by largemouth bass and promote a more opportunistic feeding approach. Green sunfish do not seem to pose a significant predation problem in Colorado River backwaters to other fish species. This is attnbuted to the relatively small numbers and small size of green sunfISh present throughout the spring-to-fall seasons relative to other fish species. Other studies have established green sunfish as a significant piscivore in small stream habitats, and demonstrated the influence of green sunfISh on native fish species and communities (Lem1y 1985, Lohr and Fausch 1996). Other studies, such as Osmundson (1987), Marsh and Langhorst (1988) and Muth and Beyers (Lentsch 1996) demonstrate green sunfish will prey on Colorado pikeminnow and razorback sucker young in laboratory and non-riverine situations. Evidence for similar effects in habitats in the Colorado River was not supported by these results. The ,relatively low frequency of occurrence of fIsh predation by green sunfish in Colorado River backwaters may be in part due to traits described in Lohr and Fausch (1996) concerning crepuscular feeding habits and high gastric evacuation rates associated with this species. The authors suggested these traits contnbuted to a lack of fish in green sunfish stomachs from samples collected during the day. Crepuscular activity by green sunfish is supported by the 1991 nighttime sampling of backwaters conducted in this investigation. It is possible that the frequency of occurrence of fish prey in green sunfish in the Colorado River is higher than indicated by these results since all stomach samples were collected during the daytime. Another potential effect might be that daytime sampling contributed to the higher percentage occurrence of fish remains and unknown species as categories in the diet assessment since digestion of fISh prey ingested the previous night had progressed too far to enable identification of species. However, the relative abundance of nonnative cyprinid prey and prey species occurrence in green sunfish stomachs suggest predation on native fish species by green sunfish is not disproportionate. Encounters between centrarchid predators and Colorado pikeminnow YOY prey would be infrequent compared to interactions with red shiner and fathead minnow. Diet overlap assessments (McAda and Tyus (1984), McAda and Kaeding (1989), Muth and Snyder (1995)) and the competition study results of Beyers et aI. (1994) suggest red shiner and fathead minnow may pose a significant competitive threat to the growth and survival of Colorado pikeminnow and razorback sucker young. Based on the numerical dominance of these species in Colorado River backwaters found in this and other studies, the potential impacts from nonnative cyprinids appears more significant compared to threats posed by mostly juvenile nonnative centrarchid gamefish species. An alternative hypothesis would be that these centrarchid predators, distributed in small numbers over the available backwater habitat, are selectively preying upon Colorado pikeminnow larvae ad YOY throughout the summer season, and reducing them to the levels of scarcity observed annually in standardized monitoring in September. The prey species frequency of 37