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(Minckley 1982, Tyus et al. 1982, Carlson and Muth 1989). The numerous nonnative species <br /> have begun to overshadow the 14 native fish species in the basin. <br /> Nonnative fishes compete with native fishes in several ways and include predation,habitat <br /> degradation, competition for resources,hybridization or disease transmission (Martinez et al. <br /> 2014). The capacity of a particular area to support aquatic life is limited by physical habitat <br /> conditions and increasing the number of species in an area usually results in smaller populations <br /> of most species. The size of each species population is controlled by the ability of each life stage <br /> to compete for space and food resources and to avoid predation. Some life stages of nonnative <br /> fishes appear to have a greater ability to compete for space and food and to avoid predation in the <br /> existing altered habitat than do some life stages of native fishes. Tyus and Saunders (1996) cite <br /> numerous examples of both indirect and direct evidence of predation on eggs and larvae by <br /> nonnative species. <br /> The Service has begun discussions about the potential downlisting of Colorado pikeminnow,but <br /> the biggest obstacle may become the existing and future threat of invasive ecological impacts by <br /> nonnative aquatic species,particularly predatory sport fishes. The most problematic nonnative <br /> fish species in the basin have been identified as northern pike, smallmouth bass and channel <br /> catfish ktalurus punctatus, although other nonnative percid, ictalurid, cyprinid, centrarchid and <br /> catastomid species continue to be problematic as well (Martinez et al. 2014). Arguably the <br /> biggest efforts of the Recovery Program today center on the control of nonnatives species. <br /> Threats from pesticides and pollutants include accidental spills of petroleum products and <br /> hazardous materials; discharge of pollutants from uranium mill tailings; and high selenium <br /> concentration in the water and food chain (Service 2002a). Accidental spills of hazardous <br /> material into occupied habitat can cause immediate mortality when lethal toxicity levels are <br /> exceeded. Researchers now speculate that mercury may pose a more significant threat to <br /> Colorado pikeminnow populations of the upper Colorado River basin than previously recognized <br /> (Service 2014b). Osmundson and Lusk(2012)have recently reported elevated mercury <br /> concentrations in Colorado pikeminnow muscle tissue; the highest concentrations were from the <br /> largest adults collected from the Green and Colorado River sub-basins. <br /> To summarize, Colorado pikeminnow habitat loss and degradation from dams and diversions <br /> constructed decades ago generated some of the early, primary impacts to the species. Most of <br /> the long-term impacts from these structures continue and are unlikely to change significantly in <br /> the near term. In the remaining suitable habitats, nonnative fish species pose a significant <br /> ongoing threat and challenge to recovery. Contaminants, including mercury and selenium, pose <br /> a threat as well,but the magnitude of this threat is in need of further investigation. <br /> 2.2 Razorback Sucker <br /> 2.2.1 Species description <br /> Like all suckers (family Catostomidae meaning"down mouth"),the razorback sucker has a <br /> ventral mouth. It is a robust, river catostomid endemic to the Colorado River Basin (Sigler and <br /> Sigler 1996; Service 2002b) and is the largest native sucker to the western United States. The <br /> 15 <br />