Multiscale Processes Regulate Brassy Minnow Persistence in a Great Plains River

Brassy minnow Hybognathus hankinsoni, a threatened species in Colorado, live in harsh, fluctuating stream environments subject to summer drying and winter freezing in the western Great Plains and yet may be capable of rapid dispersal and reproduction during the wet season. We studied brassy minnow populations at multiple spatial scales in the Arikaree River basin of eastern Colorado to determine the underlying mechanisms driving local extinction and colonization and regional persistence. Habitat units in three 6.4-km segments, arrayed across a gradient of stream intermittency from perennial to seasonally dry, were sampled five times in 2000 and 2001, the two driest summers on record. Logistic regression showed that brassy minnow were more likely to persist through summer drying in deeper pools connected to other habitat units and were more likely to persist in pools in the more perennial segments. The main cause of fish extirpation was pool drying, which logistic regression showed was more likely in shallower pools and in the drier segments. Despite poor adult survival in the drier segments, larval brassy minnow were widely distributed in all segments in early summer of both years, indicating substantial movement for recolonization. The dynamic nature of plains streams and the large spatial scales over which brassy minnow carry out their life history require research and management at the intermediate segment scale for effective conservation.