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<br />The Grand Canyon is the location of the species' (1) original description - a specimen taken off <br />the 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 humpback chub is found in Marble Canyon and Grand <br />Canyon, including several tributaries to the mainstem river. Recent mainstem studies have found <br />the humpback chub to be more abundant in the reaches immediately upstream and downstream <br />of the Little Colorado River (LCR) (Kaeding and Zimmerman 1983, 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 />main stem or there may be a more complex relationship between use of the mainstem and LCR <br />depending on environmental conditions in each river (Angradi et al. 1992). How many <br />mainstem 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 1991a). <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). As a long-lived species, humpback chub were found to <br />grow to approximately 100 mm the first year and then 250-300 mm in three years (Kaeding <br />1983). Few individuals are found greater than 450 mm total length (Kubly 1990, Valdez et al, <br />1992a). The size of the Grand Canyon population has been estimated by several researchers, <br />and all researchers have recognized. the inherent limitations of those numbers. Kaeding and <br />Zimmerman (1982) estimated 7,000 to 8,000 humpback chub greater than 200 mm in the LCR; <br />Arizona Game and Fish Department 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 (1993), working with humpback chubs greater than 150 mm total length in <br />the LCR, placed 2031 new passive integrated transponder (PIT) tags, recaptured 1814 old PIT <br />tags, and recaptured 61 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 canyons, 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. Kaeding et al. (1990) considered the similarity <br />of humpback chub habitats to be' .., dynamic flow vectors that result 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 />13 October 1993 Draft biological opinion 2,21,9J.F-167 <br /> <br />9 <br />