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been documented recently, but appears to be the most limiting factor for re-establishing a <br /> self-sustaining population in the wild. <br /> 2.2.4 Threats <br /> According to the 2002 Recovery Goals for the species, the primary threats to razorback sucker <br /> populations are streamflow regulation and habitat modification (including cold-water dam <br /> releases,habitat loss, and blockage of migration corridors); competition with and predation by <br /> nonnative fish species; and pesticides and pollutants (Service 2002b). No new threats have <br /> emerged since the completion of this document. The Service's status review of razorback sucker <br /> completed in 2012 (Service 2012b)reported that 85 percent of the downlisting recovery factor <br /> criteria(Service 2002b)have been addressed to varying degrees; however, nonnative fish species <br /> continue to be problematic. <br /> Many researchers believe that nonnative species are a major cause for the lack of recruitment and <br /> that nonnative fish are the most important biological threat to the razorback sucker(e.g.,McAda <br /> and Wydoski 1980, Minckley 1983, 59 FR 54957, Service 2002b,Muth et al. 2000). There are <br /> reports of predation of razorback sucker eggs and larvae by common carp, channel catfish, <br /> smallmouth bass, largemouth bass,bluegill, green sunfish, and red-ear sunfish (Marsh and <br /> Langhorst 1988, Langhorst 1989). <br /> Marsh and Langhorst(1988) found higher growth rates in larval razorback sucker in the absence <br /> of predators in Lake Mohave, and Marsh and Brooks (1989)reported that channel catfish and <br /> flathead catfish were major predators of stocked razorback sucker in the Gila River. Juvenile <br /> razorback sucker(average total length [TL] 171 mm [6.7 in.]) stocked in isolated coves along the <br /> Colorado River in California, suffered extensive predation by channel catfish and largemouth <br /> bass (Langhorst 1989). <br /> Carpenter and Mueller(2008)tested nine non-native species of fish that co-occur with razorback <br /> sucker and found that seven species consumed significant numbers of larval razorback suckers. <br /> The seven species consumed an average of 54—99 percent of the razorback sucker larvae even <br /> though alternative food was available (Carpenter and Mueller 2008). Lentsch et al. (1996) <br /> identified six species of nonnative fishes in the upper Colorado River Basin as threats to <br /> razorback sucker: red shiner, common carp, sand shiner, fathead minnow, channel catfish, and <br /> green sunfish. Smaller fish, such as adult red shiner, are known predators of larval native fish <br /> (Ruppert et al. 1993). Large predators, such as walleye,northern pike (Esox lucius), and striped <br /> bass, also pose a threat to subadult and adult razorback sucker(Tyus and Beard 1990). Until <br /> recently, efforts to introduce young razorback sucker into Lake Mohave have failed because of <br /> predation by nonnative species(Minckley et al. 1991, Clarkson et al. 1993, Burke 1994,Marsh et al. <br /> 2003). <br /> Overall, the threats to the razorback sucker from nonnative fish are similar to those facing the <br /> Colorado pikeminnow, as described above. See the discussion on threats to the Colorado <br /> pikeminnow above for further information,particularly regarding the threat to all endangered <br /> fish due to predation from nonnative species. One threat from nonnative species peculiar to the <br /> razorback sucker is from hybridization. While hybridization between native and endangered <br /> razorback sucker may occur in the wild at a low level (Buth et al. 1987), the mass release of any <br /> 21 <br />