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North American Journal of Fisheries Management 28:1941-1953, 2008 ~Art1C~e~ © Copyright by the American Fisheries Society 2008 DOI: 10.1577/M07-199.1 Ranking Predatory Threats by Nonnative Fishes in the Yampa River, Colorado, via Bioenergetics Modeling BRETT M. JOHNSON Department of Fish, Wildlife, and Conservation Biology, Colorado State University, Fort Collins, Colorado 80523, USA PATRICK J. MARTINEZS` Colorado Division of Wildlife, 711 Independent Avenue, Grand Junction, Colorado 81505, USA JOHN A. HAWKINS AND KEVIN R. BESTGEN Larval Fish Laboratory, Department of Fish, Wildlife, and Conservation Biology, Colorado State University, Fort Collins, Colorado 80523, USA Abstract.-Because of its relatively natural hydrograph, the Yampa River, Colorado, is considered the crown jewel of native fish habitat in the upper basin of the Colorado River and has supported a relatively intact native fish assemblage. Nonnative fishes aze thought to pose the greatest threat to native fishes in this system. Removal programs for nonnative northern pike Esox Lucius and channel catfish Ictalurus punctatus have highlighted managers' perception of the threat posed by each species. Recent expansion of nonnative smallmouth bass Micropterus dolomieu in the Yampa River attracted an avid angling clientele but also coincided with a precipitous decline in native fishes, necessitating a rigorous assessment of the relative impact of all three nonnative predators on the native fishes. We used abundance, growth, and diet estimates for each predator species to quantify consumptive demand using bioenergetics models. Despite a low abundance of small-bodied fishes and thus a low number of fish in the smallmouth bass diet, total fish consumption by smallmouth bass (mean =15.2 kg• km`t • year t; 95% confidence interval [CI] =13.3-17.1 kg • km-1 • year 1) was similaz to that estimated for northern pike (mean = 13.7 kg•km r•year 1, 95% CI = 11.4-16.0 kg•kta r•year l) and was about 65 times higher than the estimate for channel catfish (mean = 0.22 kg• km t • yearn; 95% CI = 0.05-0.40 kg• km ~ • year ~). Diet data from the upper Colorado River, where small-bodied fish were plentiful, suggested that piscivory by smallmouth bass in the Yampa River could be 10 times the piscivory by northern pike and. channel catfish, or about 168.5 kg • km 1 • years (95% CI = 147.0- 189.9 kg• km-~ • year t), if prey fish were more available. This level of piscivory suggested that smallmouth bass presented the greatest predatory threat to native fishes of the Yampa River, As envirotunental conditions change, use of field monitoring together with bioenergetics modeling will be an effective framework to assist managers in adapting their nonnative fish control efforts to maximize the likelihood of native fish recovery. Native fishes are declining at alarming rates (Leidy and Moyle 1998), and imperilment is most severe in the arid southwestern United States (Warren and Burr 1994), where persistence of more than 60% of the native fishes of Nevada and Arizona is at risk (Master et al. 1998). The upper Colorado River basin (above Glen Canyon Dam) is inhabited by only 14 native fish species, including several endemics and four species that are federally listed as endangered (bonytail Gila elegans, Colorado pikeminnow Ptychocheilus Lucius, humpback chub G. cypha, and razorback sucker Xyrauchen texanus). The roundtail chub G. robusta is listed as endangered, threatened, or aspecies of special concern by five of the seven states within the Colorado * Corresponding author: pat.martinez@state.co.us Received November 8, 2007; accepted April 8, 2008 Published online January 5, 2009 River basin, and the species is under review for listing at the federal level (Bezzerides and Bestgen 2002; Brouder 2005). Habitat degradation, primarily hydro- graphic alteration (Richter et al. 1997), is an important factor in the declines of native fish in the Colorado River (USFWS 2004). Water depletions for human use, streamflow regulation, and associated instream barriers to migration continue to disrupt natural riverine processes (Poff et al. 1997) to the detriment of the fishes that have evolved over millions of years in the warm, turbid water and extreme flow variations. The introduction or invasion of nonnative fishes is also a major contributing factor in the decline of native fish faunas (Courtenay 1995; Rahel 2002; Eby et al. 2006), including that of the Colorado River (Tyus and Saunders 2000; Olden et al. 2006). Over the past century, more than 60 nonnative fish species have become established in the Colorado River basin (Rinne 1941 ~~ ~ ~ ~~~_~