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I I I I I I I I I I I I I I I I I I I environment in the summer backwaters of the Colorado River for gonad maturation or spawning purposes. McMahon and Terrell (1982) indicated all life stages of channel catfish tend to concentrate in the warmest sections of rivers. The temperature regime in the Colorado River (mainchannel and backwaters) within the study area may imPede catfish reproduction, and SUlVival and growth of young in some years. These results indicate that adult channel catfish congregate in backwater habitat available in the Colorado River. Despite the lack of optimal temperature conditions, annual reproduction and recruitment of channel catfish appears to be sufficient to maintain a consistent size frequency distribution in the riverine population. Objective 2: to detennine the incidence of predation by non-native, non-sa1monid gamef'M species upon the endangered f'lShes. Fish predation by largemouth bass appears significant based on I to 3 fish prey consumed per bass by 57 % of the bass collected, but low relative abundance of bass present in backwater habitats ( < 2 % relative abundance) and the very high relative abundance of alternative fIsh prey species suggests the impact of largemouth bass on native and endangered fish species is low under the present conditions in the Colorado River fish community. In contrast, Osmundson (1987) suggested that low densities of largemouth bass in backwaters could have a serious impact on Colorado pikeminnow recruitment. Comparison of identifIable fish species consumed by largemouth bass with fish species collected in standardized seine hauls suggests bass are probably capturing fIsh prey species in a frequency dictated by the relative abundance of each species. As such, fathead minnows and red shiners likely comprise the bulk: of the bass diet, and Colorado pikeminnow young would be eaten infrequently. If bass were selecting for Colorado pikeminnow YOY as prey (and presumably Gila sp. as well due to their morphological similarities), these species should be occurring in bass stomachs in greater proportion than their relative abundance. It is possible that one or more of the unidentifiable fish species ingested were Colorado pikeminnow. It is possible that one or more of the unidentifiable fIsh species ingested were roundtail chub, but the data from this investigation is not adequate to confirm selectivity of bass for Gila spp. as prey. Colorado pikeminnow young would be relatively more abundant in backwaters without largemouth bass if predation was a significant factor, but there were not enough pikeminnow young present throughout the study reach to be able to adequately investigate this potential relationship. Other factors such as lack of nursery habitat availability in the spring-summer to retain larvae or lack of survival at the larval stage could be contributing to the infrequent occurrence of Colorado pikeminnow in falllSMP collections. Considering the piscivorous nature of largemouth bass found in Colorado River backwaters, it is possible that a seasonal influx of abundant, drifting Colorado pikeminnow larvae or young-of-year into any backwaters containing largemouth bass would be vulnerable to predation. Results from Ruppert et al. (1993), Muth and Beyers (unpublished data cited in Lentsch et al. 1996), and Bestgen et al. (1997) suggest Colorado pikeminnow would be vulnerable to predation by the more abundant red shiner as well. Muth and Beyers suggest red shiner, green 35