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near term. In the remaining suitable habitats, nonnative fish species pose a significant ongoing <br /> threat and challenge to recovery. Contaminants, including mercury and selenium,pose a threat <br /> as well, but the magnitude of this threat is in need of further investigation. <br /> 2.3 Humpback Chub <br /> 2.3.1 Species description <br /> The humpback chub is a medium-sized freshwater fish of the minnow family endemic to the <br /> Colorado River basin. The species evolved around 3 to 5 million years ago (Sigler and Sigler <br /> 1996). The pronounced hump behind its head gives the humpback chub a striking, unusual <br /> appearance. It has an olive-colored back, silver sides, a white belly, small eyes, and a long snout <br /> that overhangs its jaw (Sigler and Sigler 1996). This fish can grow to nearly 500 mm (20 in.) <br /> and may survive more than 30 years in the wild(Service 2002c). The humpback chub does not <br /> have the swimming speed or strength of species such as the Colorado pikeminnow. Instead, it <br /> uses its large fins to "glide" through slow-moving areas, feeding on insects. <br /> Historic distribution is surmised from various reports and collections that indicate the species <br /> inhabited canyons of the Colorado River and four of its tributaries: the Green,Yampa, White, <br /> and Little Colorado Rivers. Presently the species occupies about 68 percent of its historic <br /> habitat. Historic to current abundance trends are unclear because historic abundance is unknown <br /> (Service 2002c). <br /> The Office of Endangered Species first included the humpback chub in the List of Endangered <br /> Species on March 11, 1967 (32 FR 4001). Subsequently, it was considered endangered under <br /> provisions of the Endangered Species Conservation Act of 1969 (16 U.S.C. 668aa) and was <br /> included in the United States List of Endangered Native Fish and Wildlife issued on June 4, 1973 <br /> (38 FR No. 106). It is currently protected under the Endangered Species Act of 1973 as an <br /> endangered species throughout its range (ESA; 16 U.S.C. 1531 et. seq.). The Service finalized <br /> the latest recovery plan for the species in 2002 (Service 2002c),but is currently drafting an <br /> updated revision. <br /> Separate, objective recovery criteria were developed for each of two recovery units (the Upper <br /> Colorado and Lower Colorado River Basins as delineated at Glen Canyon Dam)to address <br /> unique threats and site-specific management actions necessary to minimize or remove those <br /> threats. This biological opinion's focus is on the Upper Colorado River Basin recovery unit and <br /> will therefore describe the status of the humpback chub in that unit. <br /> 2.3.2 Life History <br /> Like other large desert river fishes,the humpback chub is an obligate warm-water species that <br /> requires relatively warm temperatures for spawning, egg incubation, and survival of larvae. <br /> Unlike Colorado pikeminnow and razorback sucker, which are known to make extended <br /> migrations of up to several hundred miles to spawning areas, humpback chubs do not appear to <br /> make extensive migrations. Instead, humpback chub live and complete their entire life cycle in <br /> canyon-bound reaches of the Colorado River mainstem and larger tributaries characterized by <br /> 23 <br />