Large-scale factors associated with sicklefin chub distribution in the Missouri and lower Yellowstone rivers

The conservation and management of imperiled fishes requires information on the factors influencing their distribution. The sicklefin chub Macrhybopsis meeki is an imperiled, small-bodied, riverine fish largely restricted to the main-stem Missouri, lower Yellowstone, and middle Mississippi rivers. We tested the association of sicklefin chub presence/absence with physical habitat. temperature, turbidity, flow, and piscivore abundance between July and October 19961997 at a riverscape spatial scale encompassing about 48-192-km-long segments over nearly 2,000 km of the Missouri and lower Yellowstone rivers. Four of the 64 variables examined were significantly associated with sicklefin chub presence/absence: distance to upstream impoundment, flow constancy (a measure of flow variability), mean turbidity, and percent of total annual flow occurring in August. The frequency of occurrence of sicklefin chub was highest when study segments were over 301 km downstream from an impoundment, flow constancy was 0.56 or lower (indicating variable flow), mean summer-early autumn turbidity was 80 nephelometric turbidity units or greater, and less than 10% of total annual flow occurred in August. Two complimentary models were developed from these results: a univariate model containing only a distance-to-upstream-impoundment term, and a multiple model that included mean turbidity and August flow. Models were evaluated for accuracy with an independent data set from 1998. The univariate model was a more accurate predictor of sicklefin chub presence/absence in 1998, with correct predictions in 79% of the 14 segments sampled, compared with 69% for the multiple model. These results demonstrate, at a riverscape spatial scale, the importance of maintaining or restoring long, free-flowing reaches containing a natural range of variability in seasonal low flows and turbidity to conserve a small-bodied, obligate-riverine fish.