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The flathead chub, Hybopsis gracilis (Richardson), occurs in the Plains Region of Canada and the United States, in four major drainage systems: Mackenzie River, which discharges into the Arctic Ocean; Saskatchewan River, which discharges into Hudson Bay via Nelson River; and Missouri-Mississippi System and Rio Grande, both draining into the Gulf of Mexico. Each of these systems is occupied in part only. In the Mackenzie Basin, H. gracilis has been reported as far north as Fort Good Hope (Walters

Ten meristic characters and seventeen measurements of body-parts (the latter expressed as proportions of standard length) have been analyzed. They are: number of rays in the dorsal, anal, caudal, pectoral and pelvic fins; number of scales in the lateral line, before the dorsal fin, around the body and around the caudal peduncle; number of vertebrae; body-depth, depth of caudal peduncle, length of caudal peduncle, predorsal length, length of depressed anal and dorsal fins, length of pectoral and

Hybopsis gracilis (Richardson) Flathead Chub (Synonymy under accounts of subspecies) Description. --Pharyngeal teeth 2,4-4,2, hooked; dorsal fin of moderate size, falcate, first principal ray longest, extending beyond posterior rays in depressed fin, its origin usually slightly in front of insertion of pelvic fin, approximately equidistant from tip of snout and base of caudal fin, rays 8, rarely 9; pectoral fin strongly falcate, rays 14-20, usually 16-18; pelvic rays 8, rarely 9; anal fin falcat

Two subspecies of H. gracilis are recognized by us: one northern and eastern, characteristically inhabiting large rivers ( H. g. gracilis ), and one southern and western, characteristically inhabiting small streams ( H. g. gulonella ). Other scientific names that have been applied to this fish in the past are listed in the synonymy. H. g. gulonella is a chubby, deep-bodied fish, whereas H. g. gracilis is long and slender. The head of the creek subspecies is deeper and longer than that of H. g. g

Habitat The species inhabits alkaline streams with shifting sand bottoms where the waterlevel fluctuates considerably with heavy rains and melting snow. The flathead chub is found in silty water and often is the predominant species in streams that have high turbidity. The remarkable ability of this fish to withstand exceedingly high turbidity is illustrated by its predominance in the Little Missouri River, which has an average concentration of suspended silt two and one-half times that of the Mi

Hybopsis gracilis is highly variable in several morphological characteristics, including size and shape of head, body, and fins, and number of scales, vertebrae, and fin-rays. The variations are correlated in a way that indicates the existence of two subspecies. One of these, H. g. gracilis , attains large size, and has 1) a slender, streamlined body, 2) a depressed head that is acutely wedge-shaped in profile, 3) strongly falcate fins with the dorsal and pelvic fins originating anteriorly, and