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adults to portions of habitats, and manipulation of flow regimes. This study explored the mechanical ~ control methodology for nonnative cyprinids proposed in Lentsch et al. (1996). The goal of this study was to determine the feasibility of reducing the abundance of nonnative cyprinids from the lower Colorado and Green rivers in Utah to benefit native fishes, primarily Colorado pikeminnow and razorback sucker. Enhancement of growth and survival of native fishes was the desired outcome of removal efforts. 1.1 Objectives and Hypotheses Our specific objective for this study was to remove nonnative cyprinids from habitats within a 50-mile river reach in the lower Green River in Utah, and from three, 10-mile reaches in the Colorado River in Utah. We sought to reduce cyprinid abundance from pre-runoff throughout summer months at two spatial scales, i.e., at backwater and river reach levels. The working hypotheses of this study were: ~ 1) Cyprinid removal by seining all backwaters in a river reach will deplete cyprinid abundance at a backwater level. 2) Cyprinid removal by seining all backwaters in a river reach will deplete cyprinid abundance at a reach level. 3) Cyprinid removal by seining all backwaters in a river reach will shift the species composition. 4) Fall YOY ISMP sampling will be able to detect changes in cyprinid abundance (backwater ~ and reach levels) and species composition from cyprinid removal the previous spring. 5) Exclusion of nonnative cyprinid adults from portions of flooded tributaries will increase available area for larval native fishes and reduce area available for nonnative cyprinids. 2.0 METHODS 2.1 Study Area Removal efforts were conducted in portions of the lower Green and Colorado rivers (Figure 1). On the Green River, habitats were treated within a 50-mile reach from the Dry Lake/School Bus Wash -2-