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NONNATIVE FISH PREDATION THREAT 1947 Taat.E 2.-River temperature (°C) and per-capita consumption (g) by smallmouth bass, northern pike, and channel catfish in the Yampa River, Colorado, over four seasonal time periods. Two simulations were performed: (1) realized piscivory (RP) based on diet data from 2003-2005 and (2) potential piscivory (PP) based on diet observations when small-bodied fishes were abundant (see Methods). The PP scenario for channel catfish is not presented, because diet did not differ from that measured during the late 1980s (Tyus and Nikirk 1990). smallmouth bass Northem pike Channel catfish Time Mean Crayfish, Fish Fish Crayfish, Fish Fish Crayfish, Plant matter Fish period temperature insects (RP) (RP) (PP) insects (RP) (RP) (PP) insects (RP) (RP) (RP) Jarr-Maz 0.8 18.0 1.2 12.8 63.0 163.7 205.3 23.0 13.0 0.1 Apr-Jun 11.9 259.6 16.6 183.8 288.3 748.8 939.5 354.0 199.7 1.1 Ju]-Sep 19.8 578.8 36.9 409.7 577.0 1,498.5 1,880.3 653.2 368.6 2.0 Oct-Dec 3.9 35.9 2.3 25.4 115.8 300.8 377.4 37.4 21.1 0.1 Annual 9.2 892.4 57.0 631.7 1,044.1 2,711.7 3,402.5 1,067.6 602.4 3.3 potential piscivory by the smallmouth bass and northern pike populations was about 186 kg• km-1 • year], and total consumptive demand (of all food types) by these two populations was 269 kg• km-t • years. The majority (55-67%) of each predator's annual consumption occurred during July- September (Table 2), when water temperatures were closest to the thermal optima of these species. The potential number of age-1 prey (i.e., SBFs) consumed by smallmouth bass ranged from 23,500 to 470,000 fish•km-1•year 1 (3,800-75,800 fish- ha t • year 1), depending on prey species. This amount was approximately an order of magnitude higher than SBF consumption by northern pike (Table 3) and was proportional to the difference in biomass consumed by the two species. Because of their relatively small weights at age 1, the potential number of roundtail chub, speckled dace, and mottled sculpin consumed was about 2-20 times higher than that of the remaining five species (Table 3). If smallmouth bass consumed fish of a size equal to the median prey :predator length ratio calculated from gut analyses, then the number consumed would range from 93,600 bluehead suckers • km-1 • year 1 (15,100 fish • ha 1 • year 1) to 301,000 speckled dace•km-1•year l (48,500 fish- ha 1 • year l). Alternatively, if smallmouth bass con- sumed the same biomass of fish but at sizes near the p5 of the prey :predator length ratio, then the number of fish consumed would be about sevenfold higher. Northem pike chose to consume much larger individ- uals and thus a lower number of prey (700-1,300 fish • km 1 • year t or 113-210 fish • ha 1 • year] ). Growth trajectories of prey indicated that speckled dace and mottled sculpin do not ultimately grow large enough to exceed the smallmouth bass gape limit, but the other six species outgrow that size at some point between ages 1 and 2. Only Colorado pikeminnow and flannelmouth suckers reach a body size greater than the gape limit of the average northern pike; this occurs at a point between ages 3 and 4. Discussion Bioenergetics model simulations allowed us to (1) provide quantitative evidence that nonnative fish predation was probably having a significant impact on native fish populations of the Yampa River and (2) Taet,e 3.-Potential annual consumption of small-bodied native fishes by smallmouth bass and northern pike populations in the Yampa River, Colorado, 2003-2005. Small-bodied fish equivalents (SBFs) were computed from annual biomass consumed by each predator population and (1) the mean weight of each prey species at age 1 (see Methods) or (2) the median (and 5th and 95th percentiles) of the prey :predator size ratio (total length [TL], mm) estimated from predator stomach contents. Blank cells indicate that the size computed from the prey :predator ratio was not attained by that prey species. Prey species Prey size at age 1 TL (mm) Weight (g) smallmouth bass SBFs (103 prey/km) Age 1 Size thresholds Northem p Age 1 ike SBFs (103 prey/km) Size thresholds Bluehead sucker 81 7.2 23.5 93.6 (15.2-626) 2.4 0.7 (0.1-4.1) Colorado pikeminnow 71 2.3 72.0 207 (29.9-1,566) 7.3 1.3 (0.1-8.7) Plannelmouth sucker 73 2.8 59.2 183 (26.4-1,384) 6.0 1.2 (0.1-7.7) Humpback chub 76 3.5 47.8 168 (24.8-1,246) 4.9 1.1 (0.1-7.1) Razorback sucker 78 5.8 29.0 106 (t6.7-731) 3.0 0.8 (0.1-4.6) Roundtail chub 55 1.3 127 164 (24.2-1,203) 12.9 1.1 (0.1-6.9) Speckled dace 69 1.4 117 301 (45.Z-2,160) 11.9 (12.9) Mottled sculpin 32 0.4 470 110 (5.6-852) 47.9 (4.6)