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I I I I I I I I I I I I I I I I I I I five species can be conducted on a scale large enough to be effective in reducing their numbers without also impacting the native fish community negatively. Mechanical removal also impacts native fish species through disturbance and sampling and handling stress, so these actions must carefully target river reaches and vulnerable life stages of nonnative fish species in order to minimize these impacts. Lentsch et al. (1996) concluded that control of these nonnative fish species would be difficult at best due to their ubiquitous distribution, abundance, and ability to rapidly replenish and restore populations following reductions. Duffy (1998) determined that fathead minnow populations in prairie wetlands were unaffected by a simulated commercial harvest. Densities of fathead minnow ranging from 52,000 - 356,OOO/ha demonstrated no change in density, mortality rates or size structure that could be attributed to harvest rather than natural mortality factors. The author concluded that the productivity base in the wetlands accounted for high densities and production in fathead minnow and harvest mortality rates were inadequate to impact the populations. In contrast, studies in the Missouri River (Mest11999, Stanovich 1999) and the Mississippi River (Pitlo 1997) have demonstrated the size structure and abundance of channel catfish populations can be suppressed by commercial harvest. Studies in the Upper Colorado River basin suggest channel catfish size structure and abundance in the Yampa River (Modde and Fuller 2001, draft) and the San Juan River (Brooks et al. 2000) may be reduced by mechanical removal. Backwater habitats appear to concentrate channel catfish and common carp in July and adult Colorado pikeminnow are not present at that time. Commercial-type harvest of these two species may be possible unless recent stockings of razorback sucker and bonytail result in the occUPancy of summer backwaters by these two endangered species. By-catch mortality of native species in any commercial-level harvest of nonnative fish species remains a concern that must also be examined. Evaluation of these control options must also take into account that variability in flow and water teDlPerature regimes likely affect use of backwaters by both native and nonnative fish species. The variability in fish use of backwaters due to the influence of these environmental factors will complicate the evaluation of the effectiveness of the nonnative fish removal treatments. CONCLUSIONS Objective 1: to determine trends in abundance, distribution, reproductive success, and size strocture of non-native, non-salmonid gamef'lSh populations in the Colorado River. 1. Nonnative gamefish species other than adult channel catfish are consistently a small percentage of the numbers of fish present in backwater habitats in the Colorado River study area. 2. Nonnative cyprinid fish species, primarily fathead minnow, red shiner, and sand shiner, are consistently the most abundant fish species in Colorado River backwaters. 3. Adult channel catfish and common carp are the most abundant, large-bodied fish species in Colorado River backwaters during April and July. 41