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I I I I I I I I I I I I I I I I I I I Nonnative fish studies in the Yampa River Hawkins, John, Cameron Walford, and Tasha Sorensen Lan'al Fish LaboratOlY, Department of Fishery and Wildl~re Biology. Colorado State University, Fort Collins, CO In the Yampa River, nonnative piscivorous, channel catfish (/ctalllrus pltnctatlls), northern pike (Esox lllcius), and small mouth bass (MicroptentS dolomieu) pose a threat to native and endangered fishes. Channel catfish have occurred in the Yampa River for 75- 100 years, northern pike have occupied the river for 25 years, and smallmouth bass have occupied the river in significant numbers only in the last 10 years. Northern pike were stocked into the tributary Elkhead Reservoir in the late 1970s and colonized the Yampa River almost immediately. Smallmouth bass were extremely rare in the Yampa River until 1992 when a rapid draw down at Elkhead Reservoir introduced large numbers into the :1ver. Catfish and smallmouth bass are now abundant downstream of Craig, Colorado and northern pike occur throughout the river. Northern pike also occur upstream in the mainstem Stagecoach Reservoir, where they were illegally stocked. Range of those three species overlaps designated critical habitat for federally listed Colorado pikeminnow (Ptychocheillls 11lCillS), razorback sucker (Xyrallchen texanus), humpback chub (Gila cypha), and bonytail (Gila elegans) in the Yampa River. Management actions to reduce abundance of nonnative piscivorous fish may be necessary to recover the endangered fishes. A key piece of information required for effective management is the effort required to remove a given proportion of the targeted nonnative population. In 2.003, we designed several studies to evaluate effectiveness of removal of northern pike, smallmouth bass, and channel catfish in the Yampa River. In our 75-mile study reach within critical habitat northern pike and channel catfish were captured, tagged, and rele:ased on three sample occasions. No channel catfish were removed and northern pike were removed only in an 8-mile treatment reach. Smallmouth bass were captured, tagged, and released on tive sample occasions in a 12-mile study reach and were removed only in a 6-mile treatment site of that reach. We describe movement, density. length- frequency, population abundance, and capture probabilities for northern pike, channel catSsh, and smallmouth bass. These results will be useful to design effective control strategies for nonnative fish predators in the Yampa River. Examining Predation Demand by Nonnative Piscivores and its Utility in Evaluating and Prescribing Nonnative Fish Control Strategies Martinez, Patrick I, Brett Johnson2, and John Hawkins & Kevin Bestgen3 I Colorado Division of ~Vild/ife, Grand Junction. CO; ! Dept. ofFish & Wi/dlzfe Bio, Colorado State L'ni\'ersity. Fort Collins. CO; .3 Larval Fish Laboratory. Colorado State University, Fort Co/lins, CO 9