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EXECUTIVE SUMMARY We implemented removal of invasive nonnative smallmouth bass in the Yampa River from 2003 to 2007 with a goal of decreasing small mouth bass abundance to increase survival and abundance of native and endangered fishes. In 2003, we used mark- recapture data to estimate smallmouth bass distribution, density, capture probability, and movement. From 2004 through 2007, we estimated abundance of adult smallmouth bass (>150 mm) at the beginning of each year prior to removal using mark- recapture methods at two study sites: a 24-mile (39 km) site at Little Yampa Canyon and a 5-mile (8-km) site at Lily Park. Abundance estimates were used to gauge subsequent removal efficiency in each year. From 2003 through 2007, we removed 15,190 smallmouth bass (2,441 kg) of all sizes with boat electrofishing and from 2005 through 2007 we removed 18,166 small, young-of-year smallmouth bass (126.5 kg) with electric seine. Of the bass removed, 5,448 (1,805 kg) were translocated to either Elkhead Reservoir or the Justice Center Pond in Craig for future fishing opportunities. Abundance of adult small mouth bass declined after mechanical removal at each study site. 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 2,545 adult smallmouth bass from 2004 through 2006 and abundance declined 19% from 1,519 fish in 2004 to 1,233 fish in 2007. We achieved annual removal rates of 40-64% of the bass in Little Yampa Canyon and 40-83% of the bass in Lily Park. Although we observed declines in abundance that we attributed to our annual removals, compensatory mechanisms (e.g., recruitment and immigration) likely off-set some of our removal effort. Recruitment appeared robust based on abundant young-of-year small mouth bass captured with electric seine and the wide distribution and high abundance of yearlings captured by boat electrofishing. Immigration of small mouth bass into our study sites was documented based on the movement of fish between our two study sites, invasion of escapees from Elkhead Reservoir, and movement of tagged fish from other reaches such as South Beach, vi