2022-12-19_GENERAL DOCUMENTS - C1981010

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2.3.4 Threats The humpback chub was designated as an endangered species prior to enactment of the ESA, and therefore a formal listing package identifying threats was not assembled. Construction and operation of mainstem dams,nonnative fish species, and local eradication of native minnows and suckers in advance of new human-made reservoirs in the early 1960's were recognized as early threats (Service 2002c). According to the 2002 Recovery Goals for the species,the primary threats to humpback chub are streamflow regulation, habitat modification, predation by non-native fish species,parasitism, hybridization with other native Gila species, and pesticides and pollutants (Service 2002c). No new threats have emerged since the completion of this document. The Service's status review of humpback chub completed in 2011 (Service 201 lb) reported that 60 percent of the recovery factor criteria(Service 2002c)have been addressed to varying degrees; however,nonnative fish species and issues dealing with the potential chemical contamination of the river from spills and pipelines continue to be problematic. Overall, the threats to the humpback chub from nonnative fish are similar to those facing the Colorado pikeminnow, as described above. See the discussion on threats to the Colorado pikeminnow above for further information, particularly regarding the threat to all endangered fish due to predation from nonnative species. To summarize, humpback chub habitat loss and degradation from dams and diversions constructed decades ago posed some of the early, primary impacts to the species. Most of the long-term impacts from these structures continue and are unlikely to change significantly in the near term. In the remaining suitable habitats, nonnative fish species pose a significant ongoing threat and challenge to recovery. Contaminants, including mercury and selenium, may pose a lesser threat as well, but the magnitude of this threat is in need of further investigation. 2.4 Bonytail 2.4.1 Species description The bonytail is a medium-sized freshwater fish in the minnow family, endemic to the Colorado River Basin. The species evolved around 3 to 5 million years ago (Sigler and Sigler 1996). Individuals have large fins and a streamlined body that typically is very thin in front of the tail. They have a gray or olive colored back, silver sides, and a white belly (Sigler and Sigler 1996). The mouth is slightly overhung by the snout and there is a smooth low hump behind the head that is not as pronounced as the hump on a humpback chub. A very close relative to the roundtail chub (Gila robusta), bonytail can be distinguished by counting the number of rays in the fins,with bonytail having 10 dorsal and anal fin rays (Sigler and Sigler 1996). The fish can grow to be 600 mm (24 in.) and are thought to live as long as 20 to 50 years (Sigler and Sigler 1996). Little is known about the specific food and habitat of the bonytail because the species was extirpated from most of its historic range prior to extensive fishery surveys,but it is considered adapted to mainstem rivers, residing in pools and eddies,while eating terrestrial and aquatic insects (Service 2002d). Bonytail were once widespread in the large rivers of the Colorado River Basin (Service 2002a). The species experienced a dramatic, but poorly documented, decline starting in about 1950, 28