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~ Arizona Game and Fish Department. Young-of-year humpback chubs, including larvae, use backwaters, eddies, and runs (reviewed by Miller and Hubert 1990), Holden (1977) sampled those habitats in Desolation and Gray Canyons on Green River in proportion to their occurrence and reported young-of-yearhumpback chubs (30 to 70 mm total length) preferred backwaters with no current, a fIrm silt bottom, and 0.6 m maximum depth. Deeper eddies and slow runs also received some use. Kaeding and Zimmerman (1982) commonly collected young-of-year and juvenile life stages from shoreline/run and shoreline/eddy LCR habitats. Valdez (1990) collected larvae and young-of- year humpback chubs from backwaters (most occurrences), along shorelines, and in isolated pools in Cataract Canyon on the Colorado River. Valdez et al. (1990), using humpback chub data from the Green River (1500 observations), found larvae (< 21 mm) at a mean depth of 0.4 m (range 0.03 to 2.5 m) and in water velocities less than 0.03 mls (range <0.0 to 0.09 m/s). Young-of-year were found in mean water depths of 0.6 m and mean velocities 0.06 m/s. Juvenile humpback chub are frequently found in eddy habitats and also in other low to moderate velocity areas (Kaeding and Zimmerman 1982, Tyus and Karp 1989). Valdez et al' (1992a) captured juvenile humpback chub along Grand Canyon mainstem shoreline habitats with talus slopes, earthen banks with root wads, and near large standing boulders. Pockets of sand were prevalent in those habitats, but few juvenile humpback chubs were captured on sand beaches. Including data from upper basin studies, Valdez et al. (1992a) report for humpback chub a "... transition in habitat use with size or age from shallow, protected areas such as backwaters or . stable shorelines to areas with moderate depth and velocity and uneven substrate." RAZORBACK SUCKER General Status The razorback sucker was listed as an endangered species throughout its range on October 23, 1991 (U.S. Fish and Wildlife Service 1993a, 1993b). Once one of the most abundant and widespread fIshes of the mainstem rivers in the Colorado River Basin, successful reproduction of this monotypic species is now virtually nonexistent in all natural riverine environments (reviewed in Minckley et al. 1991, U.S. Fish and Wildlife Service 1993a). Archaeological and historical records reported by the above reviewers identifIed lower basin occurrence of razorback suckers in Arizona in the San Pedro River, the Gila River near Phoenix, the upper Verde River, Colorado River near Lee's Ferry, and the Salton Sea (California) and Colorado River delta (Mexico). The razorback sucker now inhabits about 25 % of its former range, and of the four "big river" endangered fIsh, only the bony tail chub is believed to be more rare (U.S. Fish and Wildlife Service 1991b). In the upper basin, the species persists in the San Juan and Colorado River arms of Lake Powell, the Grand Valley area of the Colorado River, and the lower Yampa River and Green River, with the largest concentration in this last area. In the lower basin, the formation of Lake Mohave on the Colorado River in 1954 retained a large number of razorback suckers that now comprise. the largest population of the species which Marsh and Minckley 13 October 1993 Draft biological opinion 2,21-93-P-167 12 I: " ~ ~ ~ . " ~ [j ~ , I -$ -' ~I ~ g w k I c. ~ ~ ~ ~ ~ k ~ .): <; ; i~ M ';7 U:: ~J, ~'; :-.: ~' .. c ~,' :1 "i .~ f'; ~., ".' , '. ~ ~.', ~