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Ictalurus punctatus and northern pike Esox lucius suggested minimal consumption of crayfish, but diet analyses in 2005 revealed increased consumption of crayfish by these nonnative piscivores. The present high density of crayfish in the Yampa River, potentially attributable to a bioenergetic response to elevated summer stream temperatures during the drought, has provided an abundant alternate prey source for nonnative piscivores. Nonnative crayfish in general and verile crayfish in particular should be of further concern to native fish managers given their potential to prey upon native suckers or compete with them for food in the Colorado River Basin. A review of crayfish stocking activity, policies and regulations is recommended to slow the spread of these nonnative crustaceans and to prevent the inadvertent, illicit or intentional introduction of additional new crayfish species into Colorado's western rivers. 14. Investigations of roundtail chub populations in the presence of nonnative predators in the Yampa River, 2005. Authors: CAMERON WALFORD (presenter) and JOHN HAWKINS, CSU Larval Fish Laboratory. Abstract: Native, roundtail chub (Gila robusta) populations in the middle Yampa River, western Colorado, are threatened by competition and predation from abundant nonnative fish. Roundtail chub might serve as an ideal indicator species to monitor the response to nonnative fish control. We used data collected during northern pike (Esox lucius) and smallmouth bass (Micropterus dolomieu) removal studies to begin a status assessment of roundtail chubs in the Yampa River. Sampling occurred in the spring of2005, in smallmouth bass study reaches, which included a 5-mile long site in Lily Park (RM 50.5-55.5) and two, 12-mile long sites in Little Yampa Canyon (RM 100-124). Sma1lmouth bass were captured by boat electro fishing and removed from the Lily Park site (LP-Treatment) and from one of the Little Yampa Canyon sites (LYC-Treatment). The other Little Yampa Canyon site served as a control (L YC-Control) and all bass were returned alive. Northern pike were removed river-wide, including all three study sites. Roundtail chub were also captured and measured for total length (TL, mm), weighed (g), scanned for a PIT tag, tagged if not previously marked, examined for pike bites and reproductive condition, and returned to the water. A total of 195 roundtail chub was tagged including 46 in L YC-Control, 79 in L YC- Treatment, and 70 in LP- Treatment. Using a capture-recapture abundance estimator, we estimated that 119 roundtail chub resided in L YC-Control (76-228 95% CI; SE=36.1; CV=0.303) and 226 resided in L YC- Treatment (137-42595% CI; SE=69.1; CV=0.306). A population estimate for round tail chub in LP- Treatment was not possible because no fish were recaptured. Catch per unit effort of round tail chub in LP-Treatment (2 fish/hr) was about twice the catch per unit effort in L YC-Control (1 fish/hr) and L YC- Treatment (1 fish/hr). There was a distinct difference in the size of roundtail chub in LP- Treatment compared to those in the two upstream sites in L YC. Roundtail chub in both L YC- Treatment and L YC-Control were larger (>350 mm TL) and those in LP-Treatment were smaller (<350 mm TL). Movements of recaptured roundtail chub indicated a small home range from April through mid June and greater movements in June and July_during apparent spawning migrations. Seventy-one percent of recaptured roundtail chub moved short distances of 14