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3.2.2 Razorback sucker <br /> Less is known about the numbers of the other three endangered fish within the Yampa and White <br /> Rivers. The Yampa River at the mouth of Yampa Canyon was an historical site for razorback <br /> sucker reproduction, and in fact,was the first such spawning site described in the Upper <br /> Colorado River Basin (McAda and Wydoski 1980, Bestgen 1990). More recently, only a few <br /> razorback larvae have been captured in the lower Yampa River in 2000, 2008, and 2011 <br /> (Bestgen et al. 2012). Although substantial numbers of razorback sucker do not occur in the <br /> Yampa River, scattered individuals can occasionally be found (Bestgen et al. 2012). <br /> Razorback suckers are not stocked into the Yampa River or White Rivers. They are, however, <br /> stocked into the Green River and can swim up and into the Yampa or White River. A few <br /> substantial captures of adult razorback suckers occurred in the lower White River in 2011. A <br /> passive integrated antenna array near the Bonanza Bridge (installed September 2012) <br /> demonstrated that razorback sucker and Colorado pikeminnow use the Utah portion of the White <br /> River in higher numbers than previously thought. However, a recent expansion of smallmouth <br /> bass in the White River is a cause for concern for this native fish stronghold(Recovery Program <br /> 2015). In 2011,researchers documented spawning by razorback sucker in the White River for <br /> the first time (Bestgen et al. 2012). <br /> The current and increasingly most significant threat to the razorback sucker in the action area is <br /> from nonnative species,which is discussed in the Status of the Species section. See also the <br /> discussion regarding nonnative species in the Colorado pikeminnow Status of the Species and <br /> Baseline sections above, as this threat is similar for all endangered fish in the upper Colorado <br /> River basin, particularly regarding predation from nonnative predators. <br /> 3.2.3 Humpback chub <br /> The Yampa River humpback chub population exists in the lower Yampa River Canyon and into <br /> the Green River through Split Mountain Canyon. This population is small,with an estimate of <br /> about 400 wild adults in 1998-2000. Sampling during 2003-2004 caught only 13 fish, too few to <br /> estimate population size (Finney 2006). In 2007, the Recovery Program brought 400 young-of- <br /> year Gila spp. caught in Yampa Canyon into captivity as a research activity to determine the best <br /> methods for capture,transport, and holding at two different hatchery facilities. Approximately <br /> 15 percent of the Gila species were tentatively identified as humpback chub by physical <br /> characteristics. Geneticists at Southwest Native Aquatic Resources and Recovery Center <br /> (SNARRC), Dexter,NM,have since provided preliminary results indicating that the Yampa fish <br /> in captivity were hybrids between humpback chub and roundtail chub. These fish were <br /> considered unsuitable for broodstock and were released into the Green River in Dinosaur <br /> National Monument. Currently, it is not known if pure humpback chubs occur in Yampa <br /> Canyon. The Recovery Program (2015) states that a small population of humpback chub <br /> historically existed in the Yampa River in Dinosaur National Monument(Service 2002a), but is <br /> now believed to be reduced to a few individuals. <br /> The current and increasingly most significant threat to the humpback chub in the action area is <br /> from nonnative species,which is discussed in the Status of the Species section. See also the <br /> 37 <br />