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NONNATIVE FISH PREDATION THREAT estimates of small-bodied fish density bracketed those of Wick et al. (1985); Bundy and Bestgen (2001) reported a density of 30,000 fish/ha in 46 backwaters of the GVR, and Osmundson et al. (1998) reported an average density of 45,000 fish/ha. ff such densities of age-1, small-bodied fish were to occur in the Yampa River again and if potential smallmouth bass predation were to become focused on a single species, such predation could eliminate all yearling mottled sculpin (69,000 fish• ha 1 • year '), half of the age-1 speckled dace or roundtail chub (18,000 fish • ha 1 • year 1), or about a quarter of age-1 flannelmouth suckers (9,600 fish• ha 1 • year 1) across large sections of river. Obviously, if smallmouth bass prey on younger native fish with low accumulated body mass, a given level of consumptive demand will deplete a much greater number of prey individuals. Northern pike predation may affect prey populations differently because they feed on lazger, older individuals. Some of the native fishes found in the Yampa River have a life span exceeding 40 years and reproduce intermittently (Minckley and Deacon 1991). Populations with this life history type become vulnerable to collapse as mortality of older life stages increases due to such factors as predation by introduced species (Winemiller and Rose 1992; Musick 1999). Clearly, nonnative fish consumptive demand was sufficient to cause notable mortality in native fish populations, and our analysis provides managers with the information to prioritize predatory fish control programs. The next obvious question is one posed by Mueller (2005): "Is mechanical predator control feasible?" Political resistance to sport fish removal is a significant impediment in spite of continued imperilment of species protected by listing under the Endangered Species Act (see Clarkson et a1. 2005). In principle, the Yampa River system is unique enough to warrant freshwater protected area status (Suski and Cooke 2007), a designation that could build societal support for more-aggressive predator removals (Marti- nez 2005). From a biological standpoint, we believe that the likelihood of achieving a predator population suppression target is directly linked to the population's recruitment patterns. The potential number of nonna- tive fish that could be produced in such a large system could easily overwhelm removal crews. Fortunately, northern pike recruitment may be constrained some- what, because their abundance appears to depend on immigration from upstream impoundments and off- channel habitats. Concerted efforts to (1) reduce immigration to the Yampa River from these sources and (2) remove adults from the river offer a practical strategy to reduce piscivory there. smallmouth bass recmitment in the Yampa River 1949 appears to be dependent upon below-normal flows; their recruitment in streams is sensitive to high spring and summer flows, which dismpt nesting and reduce survival of young (Simonson and Swenson 1990; Peterson and Kwak 1999; Smith et al. 2005). Several successive normal to wet years could allow managers to deplete the adult stock of smallmouth bass before new recruits can replenish it. Intense removal efforts would be required to avoid the compensatory increases in reproduction (bong and DeAngelis 1998; Peterson and Kwak 1999; Weidel et al. 2007) that can occur when physical conditions became favorable. ff small- mouth bass cannot be suppressed in the Yampa River, then managers should at least make all possible attempts to contain the spread of the species. The present study and many others suggest that the recent expansion of smallmouth bass in the Yampa River and elsewhere in the upper Colorado River basin (Anderson 2005; Martinez 2005) poses a significant threat to small-bodied fishes throughout the system. The literature is replete with cases where smallmouth bass entering new systems have reduced or eliminated small-bodied fishes. Deleterious effects of smallmouth bass on .native species have occurred in locations adjacent to the native range of smallmouth bass (MacRae and Jackson 2001; Vander Zanden et al. 2004), in the northeastern United States (Whittier and Kincaid 1999), the Adirondacks (Findlay et al. 2000; Weidel et al. 2007), the western United States (Gard 2004; Fritts and Pearson 2006), and on other continents (Gore et al. 1991; Iguchi et al. 2004). These cases and the present study draw attention to the potential detrimental effects of smallmouth bass, highlight the need to prevent this species' transfer to other waters, and should assist managers in striving to protect and recover native fish assemblages. Climate forecasts and human population trends suggest that physical conditions in the Yampa River will become more favorable for nonnative fishes in the future and add urgency to predator fish control plans. Present-day water temperatures are cooler than opti- mum for all three nonnative predator species; con- sumptive demand by the three species will increase if the river becomes warmer, thus exacerbating deleteri- ous trophic interactions among native and nonnative fishes. Warming of the Yampa River should be expected in response to (1) regional climate change (Balling and Goodrich 2007) and (2) reductions in streamflow from withdrawals to satisfy growing human demand for water (Stewart et al. 2005; USFWS 2005). Warmer water may make reproductive conditions more favorable for smallmouth bass (Shuter et al. 1980; Sems 1982; Casselman 2002) and northern pike (Casselman and Lewis 1996). Simultaneously, native