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DISCUSSION We found that abundance of adult nonnative smallmouth bass declined after mechanical removal at two study sites in the Yampa River. We propose that our annual removal rates of 40-48% at Little Yampa Canyon and 44-65% at Lily Park from 2004 through 2006 assisted with the decline in abundance. At Little Yampa Canyon we removed 3,879 adult smallmouth bass from 2004 through 2006 and abundance declined 17% from 2,888 fish in 2004 to 2,394 fish in 2007. At Lily Park we removed 2,588 adult smallmouth bass from 2004 through 2006 and abundance declined 19% from 1,519 fish in 2004 to 1,233 fish in 2007. Additional evidence of a decline of smallmouth bass numbers from 2004 to 2007 included declining CPUE at both sites during the study period. CPUE was well correlated with density and CPUE may serve as an occasional substitute for annual abundance estimates; however, abundance estimation using mark-recapture methods is superior for measuring changes in smallmouth bass populations in the Yampa River and we encourage its continued use. Removal rates improved with increased effort. In Little Yampa Canyon in 2004 and 2005, removal rates were not very high because our treatment (removal) area at the time was only 12 miles long or 50% of the 24-mile study site for which abundance was estimated. In 2006 even though smallmouth bass were removed from the entire 24- mile reach, removal rate was low because we only completed five removal occasions that year due to a short-duration water year. We achieved the highest removal rates at Little Yampa Canyon (64%) and Lily Park (83%) in 2007; however, those removals did not affect the observed decline in abundance because removals in 2007 were done after abundance was estimated. . Increasing the rate of removal will require applying removal at a geographic scale that matches the distribution and home-range of the targeted species, increasing effort, or increasing capture efficiency. Removal rate could be improved with more effort such as increasing the number of electrofishing boats operating concurrently. However, we suggest increasing effort by adding one or two more removal occasions earlier in the year. Capture efficiency also affects removal rate 22