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INTitOD11CTION neral Deacri_ t~ The humpback chub ,Gila cy~ha Miller) is one of North America's most bizarre animals. A prominent nuchal hump, flattened head (concave posteriorly), long fleshy snout; and small eye give it as almost grotesque appearance. Yet, these features combine to provide this minnow with unique adaptations which enable it to survive in one of the world's most severe aquatic. ecosystems - the Colorado: River. The humpback. chub was one of the last large fish species to be discovered in North America. A specimen caught is the Grand Canyon, and another specimen and ahead from unknown localities, formed the basis for describing this unique creature (Miller 1946)- Recent publications have more fully described. the intraspecific variation of this species (floldea:•and Stalnaker 1970;-Minckley 1973; Suttkus and Clemmer 1977). The following summary (Hinckley 1973) aptly describes the humpback chub; "Body streamlined; skull concave on dorsum. Napwhich often produced"at occiput into a truncate, prominent hU°mp. Caudal projects forward to bverhan g occiput in large adults. peduncle thin, somewhat pencil-Mike but not greatly elongated, its length divided by length of head Iess than 1.0; head length divided by caudal peduncle less than 5.0. Squsmation often incomplete, or scales ,embedded deeply (especially on hump). -Figs large, falcate. Origin of dorsal fin about esuusuallatg between snout and caudal fin base. Dorsal fin ray y-.• anal fin rays 10 or more. Mouth inferior, overhung by snout. Pharyngeal arch small, its lower ramus short, teeth usually 2, 5-4,2.". Maximum length of humapb8ck-chubs is about 45 cm and adults tend to be olivaceous or brownish on the back sad silvery on the sides and belly. Characters which distinguish it from the closely related bonytail chub (G. ele ana and roundtail chub (G: robusta include: 1) the prominent nuchal hump-with lateral grooves that exEead posteriorly'aloag the hump, 2) the flat head with fleshy snout and small eyes, 3)'dorsal rays typically 9 and anal rays typically 10 (occasionally 11), 4) a caudal peduncle depth intermediate between those of the bonytail (narrow)