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<br />The Grand Canyon is the location of the species' (1) original description - a specimen taken at <br />or near mouth of Bright Angel Creek at River Mile (RM) 87.8 (Miller 1946); (2) oldest record- <br />4,000 years B.C., from archeological remains in. Stanton Cave at RM 31.6 (Euler 1978); (3) <br />current southern distribution - at RM 222 (Valdez et al. 1992a), and (4) the largest population <br />(Marsh and Douglas 1992). <br /> <br />The Grand,Canyon population of the humpbaClcchubis foundin'Marble Canyon and Grand <br />Canyon, including several tributaries to.themainstem river. Recent main stem studies have found <br />the humpback chub to be more abundant in the reaches immediately upstream and downstream <br />of the Little Colorado River (LCRHKaeding and ZimmermanJ983; Maddux et al. 1987; Valdez <br />et al. 1992a). The Grand Canyon population of humpback chub may be composed of a resident <br />LCR group that is supplemented by individuals in the spring and summer that overwinter in the <br />mainstem or there may be a more complex' relationship between use of the mainstem andLCR <br />depending on environmental conditions in each river (Angradi et al: 1992). The number of <br />main stem humpback chub that migrate to the LCR is not known. <br /> <br />The possibility exists that humpback chub in the middle and lower Grand Canyon may represent <br />a separate population. The genetic identity of the humpback chub throughout the Grand Canyon <br />is being investigated in a basin-wide taxonomic study of the genus Gila (U.S. Fish and Wildlife <br />Service 199Ia). <br /> <br />Humpback chub have been aged to 22 years using otoliths, but the relationship of age to length <br />is not well known (Hendrickson 1993). In warm water of the LCR, humpback chub were found <br />to grow to approximately 100 millimeters (mm) the first year and then 250-300 mm in three <br />years (Kaeding and Zimmerman 1983). Few individuals are found greater than 450 mm total <br />length (Kubly 1990; Valdez et al. 1992a). The size of the Grand Canyon population has been <br />estimated by several researchers, and all researchers have recognized the inherent limitations of <br />those numbers. Kaeding and Zimmerman (1982) estimated 7,000 to 8,000 humpback chub <br />greater than 200 mm in the LCR; AGFD estimated 5,000 to 18,000 humpback chub in the lower <br />reach of the LCR from 1987-1989 (Kubly 1990); Valdez et al. (1992a) estimated 2407 humpback <br />chub greater than 175 mm near the LCR with a wide confidence interval of 1,102 to 6,564; and <br />Douglas and Marsh (1992), working with humpback chubs greater than l?Omm total length in . <br />the LCR, placed 2031 new passive integrated transponder (PIT) tags, recaptured 1814 old PIT <br />tags, and recaptured 6 I old Carlin or floy tags for a total of 3906 individuals humpback chubs. <br /> <br />Adults <br /> <br />Locations of humpback chub listed above identify the species' canyon-bound macrohabitat which <br />usually inCludes swift water and boulders. Within the;cai'iyons, Miller and Hubert's (1990) <br />review found shoreline-eddy habitats were the most frequently utilized by adult humpback chub, <br />with backwater-eddy and pools also identified. Kaedinget al';':(I990) considered the similarity <br />of humpback chub habitats to be "... dynamic flow. vectors thatresult from water moving rapidly <br />among large, angular boulders and shoreline rock outcrops" . and suggested that"... the unusual <br />morphology of this species is an adaptation to life in such habitats." Substrates ranged from <br />bedrock and boulders to sand and silt (Miller and Hubert 1990). Humpback chub select low <br /> <br />15 December 1994 final biological opinion 2-21-93-F-167 <br /> <br />8 <br />