| Summary: | Longnose Sucker (Catostomus catostomus) and Lake Chub (Couesius plumbeus) were once thought to exist throughout the Black Hills, South Dakota; however, recent surveys have documented dramatic declines. To inform future conservation, I provide: (1) updated regional and local distributions, (2) descriptions of occurrence patterns and size structure, (3) assessments of the influence of stream characteristics on relative abundances, and (4) describe the interrelatedness and regional population genetic structure for both species. Longnose Sucker were restricted to two major tributaries of the Redwater subbasin, and more often captured in stream reaches with steep banks and more even substrate composition. Additionally, greater species richness and higher width-depth ratios were important stream characteristics. Lake Chub were most often captured in riffle macrohabitats within a single river system in the Middle Cheyenne – Elk subbasin. Stream reaches with fewer trout and greater native species richness were important factors in Lake Chub occurrences. Reduced representation genome sequencing suggests that Longnose Sucker move throughout the Redwater subbasin. Genetically distinct Lake Chub populations are found in the Middle Cheyenne – Elk Subbasin. These results provide crucial data to better understand the resiliency, redundancy, and representation of these fishes and inform their conservation in the Black Hills of South Dakota.
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