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xxii . Prologue et al. 1967). The humpback chub is a relatively large North American minnow reaching a maximum total length of 480 mm and a weight of 1,165 g (this study). Humpback chub have a laterally-compressed and tapering fusiform body, short narrow caudal peduncle with deeply forked tail fin, and large falcate paired fins. Adults have a narrow flattened head, with small eyes and a long fleshy snout and inferior subterminal mouth. Subadults are olivaceous above with silvery sides fading to a creamy white belly, while adults are light olivaceous and slate-gray dorsally and laterally, with a white belly tinged with light orange and yellow. Dorsal and anal fins typically have 9 and 10 principal rays, respectively; caudal peduncle length divided by head length is typically less than 1.0, and head length divided by caudal peduncle depth is usually less than 5.0. Scales are deeply embedded, isolated dorsally and imbricated laterally and ventrally, with the head and nuchal hump naked. The pharyngeal arch is small. with a short lower ramus and deciduous teeth in a typical pattern of 2,5-4,2. Spawning adults during March-June are tinged with rosy red on the gill coverings, paired fIns, and belly, and pimple-like nuptual tubercles develop on the head and paired fIns. The head is narrow and flattened and may be dorsally concave. The eyes are small and the snout is long and fleshy with an inferior subterminal mouth. The paired fins (pectoral and pelvic) are large and falcate. Critical habitat for the humpback chub and three other mainstem species (Colorado squawfish, bonytail, razorback sucker) was designated on March 21, 1994 (50 FR 13374). For the humpback Final Report chub, critical habitat includes 610 km (379 mi)in seven reaches of the Colorado River Basin representing about 28% of historic habitat. Critical habitat for humpback chub in Grand Canyon includes 280 km of the Colorado River from Nautaloid Canyon (RM 35).to Granite Park (RM 209) and the lower 12.9 kIn (8 mi) of the Little Colorado River (LCR). In addition to Grand Canyon in the lower Colorado River basin, humpback chub remain in five canyon regions in the upper Colorado River basin (Black Rocks, Westwater Canyon, Cataract Canyon, Desolation/Gray canyons, Yampa Canyon). Specimens and historic records (Gaufin et al. 1960, Hagen and Banks 1963, Holden and Stalnaker 1975) indicate that the species was extirpated from at least seven additional canyon regions in the upper basin (Flaming Gorge, Lodore Canyon, Whirlpool Canyon, Split Mountain Canyon, Moab Canyon, Debeque Canyon, Narrow Canyon). Reasons for decline and major threats faced by the species today include inundation of habitat behind mainstem dams, coldwater releases below dams, modified habitat from channel geomorphic changes, altered flow regimes, altered food bases, invasion by non- native fishes, alien parasites and diseases, and introgressive hybridization with native congeneric species (Valdez and Clemmer 1982). Bonytail (Gila eleaans) . The bonytail is the rarest of the big river fishes of the Colorado River. FeWer than 10 individuals have been caught in the upper basin in the last decade and small numbers of adults persist in Lake Mohave, Nevada-Arizona (Kaeding et al. 1986, Minckley et aI. 1989, Valdez et alI995). It was listed as an Bonytail