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1944 JOHNSON ET AL. Tnet.a 1.-List and status of fish species occurring in the Yampa River, Colorado (E =federally endangered; SE =state endangered; ST =state threatened; SC =state species of special concern; CS =conservation species as designated by the 2004 Rangewide Conservation Agreement (UDWR 2004) among Arizona, Colorado, Nevada, New Mexico, Utah, and Wyoming; * =intermittent inhabitants [al] aze salmonids]; '[ _ extirpated species). Species Status Native Bluehead sucker Catostomus discobolus CS Bonytail Gila elegansj' E, SE Colorado River cutthroat trout Oncorhynchus clarkii pleuriticus* SC Colorado pikeminnow Ptychocheilus Lucius E, ST Flannelmouth sucker Catostomus latipinnis CS Humpback chub Gila cypha E, ST Mottled sculpin Cottus bairdii Mountain sucker Catostomus platyrhynchus Mountain whitefish Prosopium williamsoni* Razorback sucker Xyrauchen texanus E, SE Roundtail chub Gila robusta SC, CS Speckled dace Rhinichthys oscu[us Nonnative Black bullhead Ameiurus melas Black crappie Pomoxis nigromaculatus Bluegill Lepomis macrochlrus Brook stickleback Culaea inconstans Brook [rout Salvelinus fontinalis* Brown trout Salmo truna* Channel catfish /ctalutus punctatus Common carp Cyprinus carpio Creek chub Semotilus atramaculatus Fathead minnow Pimephales promelas Green sunfish Lepomis cyanellus Iowa darter Etheostoma exile Northern pike Esox Lucius Northern plains killifish Fundulus kansae Rainbow trout Oncorhynchus mykiss* Red shiner Cyprinella lutrensis Bedside shiner Richardsonius balteatus Sand shiner Notropis su-amineus smallmouth bass Micropterus dolomieu White sucker Catostomus commersonii electrofishing during 10 May-3 July 2003. Each fish was measured (total length [TL], mm); most were weighed (wet weight, g). Individuals exceeding a minimum size (channel catfish: 275 mm TL; northern pike: 260 mm TL; smallmouth bass.: 150 mm TL) were marked with a numbered Floy tag that was inserted below the dorsal fin. smallmouth bass were sampled on five capture occasions (average electrofishing time = 14 h/occasion); channel catfish and northem pike were sampled on three capture occasions (55 h/ occasion). Because of their lower relative densities, channel catfish and northem pike were marked and recaptured over a much larger reach, which extended to just below the Little Snake River (-~-RKM 80). Abundance estimates were computed using CAPTURE (White et al. 1982), and uncertainty was expressed as 95% confidence intervals (CIs) based on profile likelihood intervals (Evans et al. 1996). To normalize abundance of each species, abundance was divided by the river segment length river which abundance was estimated. We computed areal fish densities from a mean channel width (62 m) measured at RKM 84, 103, and 175 at a flow of 17 m3/s (Stewart et al. 2005). For predators that were not weighed, estimated weights (W) were computed from weight-length relationships derived from field measurements (W = a x TLb, where a = 2.0 x 10-~ and b = 3.671 for channel catfish [n = 365]; a = 1.0 x 10-5 and b = 2.929 for northem pike [n = 297]; and a = 7.0 x 10-~ and b = 3.538 for smallmouth bass [n = 1,135]). Body condition was expressed as relative weight (YV ; Anderson and Neumann 1996). Channel catfish, northern pike, and smallmouth bass used for diet analysis were collected from the Yampa River between RKM 165 and 191. Channel catfish and northern pike were collected during June-November 2005, and smallmouth bass were collected during June-October 2003, June 2004, and May-September 2005. A sample of smallmouth bass was also collected for diet analysis from the Grand Valley reach (GVR; Figure 1), a 55-km section of the upper Colorado River, during April- August 2004. Although crayfish were not as abundant as in the Yampa River (P.J.M., personal observation), small-bodied fishes were abundant and well studied in the GVR (McAda et al. 1994; McAda and Ryel 1999; Bundy and Bestgen 2001); thus, it served as a useful reference system, representing fish prey resource availability in the Yampa River prior to the population crashes of small-bodied fishes. Fish were measured (TL, mm) and weighed (wet weight, g). Whole fish or their stomachs were then fixed in 10% fotmalin for diet analysis. Otoliths were collected from smallmouth bass, and we used thin sections to determine their ages (Martinez 2004). Northern pike ages were determined from scales by Martinez (1995) and Nesler (1995). We obtained information on size at age of channel catfish from Tyus and Nikirk (1990). Growth rates were determined by fitting von Bertalanffy growth functions (Isely and Grabowski 2007) to size at age. We dissected predator stomachs and removed all food items from the esophagus to the pyloric sphincter. Frey items were identified to the lowest practical taxonomic level. Partially digested fish were identified from endoskeletal remains (Eddy and Underhill 1978; Hansel et al. 1988). Pharyngeal teeth and cleithra were also compared to those from reference fish to confirm identifications. Prey items were measured under a stereomicroscope, and head capsule widths (Smock 1980) of insects, carapace lengths of crayfish (Roell and Orth 1992), and vertebral column lengths of fish (Clothier 1950) were recorded. Insect measurements