Flathead Chubs, Platygobio gracilis, in the upper Missouri River: The biology of a species at risk in an endangered habitat

Abstract

Flathead Chub, Platygobio gracilis, populations have declined in portions of their range where natural flood-pulses and functioning backwaters have been eliminated. Observations of these declines have led ecologists to consider both the species and their at-risk habitats. Backwater and sandbar habitats in the Missouri River, North Dakota were sampled during a high flow (1997) and an average flow (1999) year. Of the 817 larval fishes captured in the backwaters, none were Flathead Chubs. Significantly fewer (P<0.01) Flathead Chub adults and juveniles were captured in the backwaters than from sandbar habitats during all sample periods. Male chubs were all mature at 110-mm total length (TL) and age 2, whereas females were not all mature until age 3 and 170-mm TL. In 1997, Ostracoda, Hemiptera, and Copepoda dominated the diets, whereas in 1999, Coleoptera, Trichoptera, and Hymenoptera were utilized in significantly greater amounts (P<0.05). Flathead Chub maximum age was 5 and the mean back-calculated total lengths at age were 104-, 153-, 186-, and 223-mm TL for ages 1-4. Incremental growth analyses indicated that greater increases in total length were attained in 1997 than in 1999; however, body condition was significantly higher in 1999 (P<0.006). The contradiction between growth and condition cannot be explained; however, it could be energetically advantageous to maintain a more fusiform body shape during high flows. Although the backwaters were not important physical habitat for Flathead Chubs, backwater productivity may have critically contributed to the prey base during the high flow period of 1997.