Abstract
There is a growing body of literature that suggests riverine fish are some of the most threatened taxa on a global scale. Similarly, the literature suggests less‐altered tributaries may offer refugia for large‐river specialists. The greater Mississippi River basin, including the Missouri River system, has been subjected to anthropogenic changes in flow, habitat fragmentation and turbidity, leading to declines in multiple native fish species. Small‐bodied fish species are exceptionally susceptible to changes in habitat, resulting in a heightened risk of extinction. The sturgeon chub Macrhybopsis gelida belongs to the family Leucisidae and is a benthic, small‐bodied minnow native to the greater Mississippi River basin and is highly adapted for life in large, turbid and free‐flowing rivers. Sturgeon chub distribution, habitat use and associations with other species are well studied in large mainstem rivers, but remain poorly understood in tributaries, which may serve as refugia as mainstem populations decline. Evidence of recent declines in the upper Missouri River highlights the need to better understand the role of these tributary systems. We therefore assessed the distribution and abundance of sturgeon chub populations in western Missouri River tributaries of South Dakota, evaluated broad‐scale abiotic and biotic influences on their distributional patterns and characterized their habitat associations. Sturgeon chub were captured in the Cheyenne ( n = 81), White ( n = 331) and Little White ( n = 71) rivers but were absent from the Little Missouri and Grand rivers. Sturgeon chub distributions were limited to lower areas of all rivers (i.e. closer proximity to mouths) where stream width, turbidity, discharge and observed habitat complexity (i.e. greater diversity of mesohabitats present) were highest. Furthermore, sturgeon chub were often found in association with high abundances (≥10% of catch) of flathead chub Platygobio gracilis and Hybognathus spp. (plains minnow Hybognathus placitus and western silvery minnow Hybognathus argyritis ). Sturgeon chub generally used main‐ or secondary‐flowing macrohabitats and were predominantly found in channel border or thalweg mesohabitats in most sampled reaches. Mesohabitats with greater bottom velocities [mean = 0.34 m/s, standard deviation (SD) = 0.11] and higher percentages of gravel (mean = 44, SD = 24) in the substrate were more likely to contain sturgeon chub in the White and Little White rivers, whereas greater depths (mean = 0.49 m, SD = 0.19) were associated with their presence in the Cheyenne River. Our study highlights the importance of tributaries as a surrogate to the mainstem Missouri River regarding sturgeon chub persistence that may be applicable to other imperilled fish species.