Biology of the Flathead Chub in the Lower Yellowstone River, Montana

Abstract

The flathead chub (Platygobio gracilis) (Cyprinidae), which has declined in much of the Missouri River drainage, was investigated in 1997 by sampling with stationary nets at a mainstem irrigation diversion dam on the lower Yellowstone River, Montana. Total lengths of 1,327 chubs sampled ranged from 32 mm to 304 mm. Mean length of 71 known females (181 mm TL) was significantly greater than mean length of145 knoum males (138 mm TL, P < 0.05). Ages of 281 fish of both sexes ranged from one to seven years. Maximum ages of males and females were 5 and 7 years, respectively. Mean number of eggs per female (mean length, 186 mm TL; mean weight, 68.5 g) was 6,981; eggs were typically of two or more sizes per fish, indicating non-synchronous development. One large female had 36,150 eggs, of which 28,200 were fully developed. The presence of numerous chubs of diverse sizes and ages at the freeflowing, turbid lower Yellowstone River contrasts with declines in abundance at other more altered locations in the Missouri River basin.