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NONNATIVE FISH PREDATION THREAT WY ~~ Yampa R. ` ~', ~-' ~~~~©~~~ ~ ~'c- ~ ~~ ®~~ was'- ~ '~., ~ ~ ~~ . A a ~~7 ~~m~"F~4 1943 t~yammg --_ __ _ _ _ ____ Colorado ` ~ , ~ ~ ~ ~ ~:; t~ ~ t ~ "'VVV [.3 i ~J ibcd ~ F ~ .. t _ ti c`~~ i f;r;onurne ~~ ~ ' N 1- ~ j I_ i ~ ~ ~E~ ~. I ~ ~ .:~~~ C .' (~ E'= S?EA~AABGI ~ «J~LLe6~ F`.SSa`~~~ CUCJB}.1:- ! 1RK Q -" ~~ ~' RtiAYF? ~`~"'~ ~_ ~ f ~. RK 224 t RiCRK84~~ ~ ~Rf4'l65 ~ ~~~ ~-~..~ t i-i~ .~ _,. ,RK 89 RFE 191 ~~' i"t`ztl=' ~~ ~ ~ ~ a $ `'~ t FicuRe 1.-Maps of (A) the upper Colorado River and some of its tributaries in eastern Utah, southwest Wyoming, and northwest Colorado (shaded rectangle =portion of the Yampa River shown in B; shaded oval =Grand Valley reach [GVR]) and (B) the Yampa River, showing river kilometer (RK; RK 0 =confluence with the Green River) designations for locations mentioned in the text. The lower bound of the study area was at approximately RK 80, but sampling was concentrated between RK 165 and 191. Upstream boundaries of endangered species critical habitat for endangered fishes were at RK 72 for bonytails and humpback chub, RK 89 for razorback suckers, and RK 224 for Colorado pikeminnow (Roehm 2004). these actions could be explained and justified to the public. Study Area The Yampa River, located in northwest Colorado (Figure 1), is bounded by the Continenta] Divide to the east (elevation = 3,712 m above sea level [ASL]) and terminates at its confluence with the Green River in Dinosaur National Monument (1,548 m ASL). Climatic conditions vary with elevation, but the area is characterized by relatively cool, dry summers (July mean air temperature = 19.5°C at river kilometer [RKM] 126 and 224) and cold winters. Average summer water temperature at RKM 126 during May- August 1996-2002 was 17.5°C, and the mean peak water temperature was 22.0°C (July) during that period (USGS 2006a). The Yampa River displays a snowmelt hydrograph (Van Steeter and Pitlick 1998) typical of the region (Doff and Ward 1989); peak flows occur during spring (median ~ 280 m3/s; USGS 2006b), and mean annual flow is approximately 42 m;/s (USGS 2006a). The fish assemblage was historically limited to 12 species (Table 1). Today, four of the species are federally listed as endangered and another two are listed as species of special concern by the state of Colorado. The lower 80-km reach of the Yampa River is designated as critical habitat for all four of the endangered fishes, and Colorado pikeminnow critical habitat extends upstream to RKM 224 (Figure 1). Intentional and unintentional nonnative fish stocking and immigration are common, and the assemblage is currently dominated by species in a variety of trophic and thermal guilds. Several nonnative game fishes provide popular recreational fisheries. Channel catfish were introduced into the basin in 1892 and have been abundant in the Yampa River for decades (Holden and Stalnaker 1975; Tyus and Nikirk 1990). Northern pike were first stocked in the Yampa River basin in 1977 (Hawkins et al. 2005) and became abundant in the main-stem Yampa River beginning in the mid-1980s (Tyus and Beard 1990). smallmouth bass were introduced into the basin in the late 1970s, when they were stocked into Elkhead Reservoir (on Elkhead Creek, a Yampa River tributary; P.J.M., unpublished data). smallmouth bass were rarely found in the Yampa River until the early 1990s (Tyus et al. 1982; Nesler 1995), when a rapid drawdown of Elkhead Reservoir in the winter of 1991-1992 introduced many into the river (Martinez 2003). Several subsequent yeazs of low river flows, which were probably favorable for recruitment, were thought to have increased the primarily downstream distribution and abundance of smallmouth bass (Martinez 2005). Methods Sampling locations were designated according to their distance upstream (km) from the Yampa River's confluence with the Green River (RKM 0; Figure lb). Most of our work focused on a 26-km reach of the Yampa River from the Duffy Mountain boat ramp (RKM 165) to just above Milk Creek (RKM 191); this reach is considered to be the epicenter of the smallmouth bass population's recent rapid expansion. Channel catfish, northern pike, and smallmouth bass were collected for population estimates by boat