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EXECUTIVE SUMMARY ~ Controlling nonnative species of fish in the Upper Colorado River Basin has been a concern of the Upper Colorado River Endangered Fish Recovery Program (Program) for many years. Beginning in 1998, the Utah Division of Wildlife Resources (UDWR) was funded to examine the effectiveness and ~ feasibility of removing nonnative cyprinids from the lower Green and Colorado rivers within Utah for the benefit of native endangered fish. The study area included 50 consecutive miles on the Green River and three disjunct 10-mile ~ sections of the Colorado River. Four or five removal trips were made each year in April and May on the Green River and July and August on the Colorado River. On each removal trip, all backwaters in the study area. were seined multiple times to remove nonnative fishes. ~ Conclusions specific to our working hypotheses were: 1) Cyprinid removal by seining all backwaters in a river reach will deplete nonnative cyprinid abundance at a backwater level. • Green River ' i. Depletion was observed within some individual backwaters within one-day sampling occasions. ii. No consistent or significant declines in catch or catch rate were observed in individual backwaters where fish were removed on consecutive removal trips. • Colorado River ~ i. Depletion was observed within some individual backwaters within one-day sampling occasions. ii. Backwaters were seldom available to be seined more than one trip so the trip removal effect could not be evaluated. ~ 2) Cyprinid removal by seining all backwaters in a river reach will deplete cyprinid abundance at a reach level. • Green River i. Significant differences were observed between treatment and control reaches in the Green River, but no decline in catch rate in treatment areas was observed relative to ~ catch in control areas. ii. Temporary reductions in abundance of nonnative adults were observed at a reach level in the Green River. iii. In five unique backwaters sampled on each trip, declines in combined catch and catch rate were observed at a reach level in consecutive removal trips. ~ iv. The net effect of removal was not significant due to increases in catch, which were likely a result of immigration, reproduction, and growth of larval nonnative fish. -viii-