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1946 JOHNSON ET AL. 1,200 - Yartq~a River ~~~`~ t,000 --- carlander(t988.1877j ~~ ~ ~ 444 f ~ 800 ~ normum ,~ gifts ~-.. ~ ~ ~ ~ 400 ~ F~ ~f - ' j ~ ~~ Jf ~~' ~ 1 _ `. 1 `' ~ ~ ~ 200 ~ `~, calh h mss ~ smalEmouth bass Age 9 11 13 1b Ficuet=. 2.-Observed growth of channel catfish (1979- 1988; Tyus and Nikirk 1990), northern pike (1987-1991; Martinez 1995; Nesler 1995), and smallmouth bass (2003; Martinez 2004) in the Yampa River, Colorado. "Typical" growth trajectories across the species' ranges (Cazlander 1969, 1977) are shown for comparison. Results For channel catfish marked for the population estimate (n = 364 fish), the arithmetic mean TL was 472 mm (SE = 4.7) and the geometric mean weight was 1,101 g; this body size corresponded to an age of 14 years. For northern pike (n = 295 fish), mean TL was 560 mm (SE = 9.0) and geometric mean weight was 990 g, corresponding to an age of 4 years. For smallmouth bass (n =1,400 fish), mean TL was 241 mm (SE = 2.2) and geometric mean weight was 290 g, corresponding to an age of 4 years. smallmouth bass were substantially more numerous (N = 267 fish/km; 95% CI = 234-304 fish/km) than northern pike (5 fish/km; 95% CI = 4.3- 6.1 fish/km) or channel catfish (68 fish/km; 95% CI = 34-166 fish/km). Channel catfish from the Yampa River grew considerably more slowly than the national average (Carlander 1969), and this disparity increased with age (Figure 2). By age 8, channel catfish were approximately 200 mm smaller than the length at age reported for channel catfish elsewhere. The growth of northern pike as reported by Nesler (1995) was similar to the national average (Carlander 1969), as was the growth of smallmouth bass (Carlander 1977). In 2003 and 2004, mean W was 118 for channel catfish, 93 for northern pike, and 102 for smallmouth bass. The channel catfish diet (n = 32 stomachs examined; 30 were. nonempty) included very few fish (0.2% by mass); only 6.6% of stomachs with food contained any fish remains. Most of the diet consisted of virile crayfish Orconectes virilis (53.7%) and plant matter (36.0%); insects (primarily Ephemeroptera, Trichop- tera, and Plecoptera) made up the remainder of the diet (10.1%). Northem pike (n = 45 stomachs; 33 were nonempty) were primarily piscivorous; fish constituted 72.2% of the diet (by mass). Northem pike also ate crayfish (24.5%) and a few insects (3.3%). Northem pike preyed nearly equally on catostomids, centrarch- ids, cyprinids, and salmonids. smallmouth bass (n = 178 stomachs; 149 were nonempty) consumed mainly crayfish (51.5% by mass). Aquatic insects, including Ephemeroptera (80% by number), Plecoptera (15%), Hemiptera (Corixidae and Notonectidae: 3%), and Trichoptera (<1%), made up 42.8% of the diet. Only 5.7% of the diet consisted of fish; because of the advanced state of digestion in the samples, most fish remains could not be definitively identified below the family level. Small cyprinids and smallmouth bass each constituted 40% of the fish prey, and suckers and sunfish made up the remainder. In contrast, fish (minnows and suckers) were the primary prey (67.7%) of smallmouth bass collected from the upper Colorado River (n = 325 stomachs), while crayfish (12.9%) and insects (19.4%) made up much smaller proportions of the diet than was observed in the Yampa River. Northern pike and smallmouth bass consumed fish that were similar in size relative to predator size. The median prey :predator size ratio (•I'L) of fish in stomachs was 0.22 for northern pike and 0.21 for smallmouth bass. Northem pike consumed proportion- ately larger prey (p95 = 0.50) than smallmouth bass (p95 = 0.39), but the minimum prey size consumed was similar between the two species (PS = 0.12 for northern pike and 0.11 for smallmouth bass). Per-capita consumption of fish was highest for northern pike (2.7 kg/year; Table 2). Fish constituted a small fraction of the diet in channel catfish simulations; thus, consumption of fish by the average channel catfish was predicted to be only 3.3 g/year. Per-capita consumption of fish by smallmouth bass was estimated at 0.06 kg/year. At the population level, realized fish consumption by smallmouth bass .(mean = 15.2 kg •km-' •year'; 95% CI = 13.3-17.1 kg • km-' •year ') was similar to fish consumption by northern pike (mean = 13.7 kg• km ' •year'; 95% CI = 11.4-16.0 kg • km-' •year ') and was about 65 times higher than fish consumption by channel catfish (mean = 0.22 kg • km ' •year' ; 95% CI = 0.05-0.40 kg• km-' •year '). Potential piscivory by channel catfish was not different from realized consumption, because we assumed that no diet shift would occur. Potential piscivory by the smallmouth bass population was 168.5 kg•km-' •year-' (95% CI = 147.0-189.9 kg • km-' •year '), or about 10 times higher than that by the northern pike population (17.2 kg • km-' •year '; 95% CI = 14.2-20.1 kg• km-' •year'). Combined