Spatio-temporal patterns of the decline of freshwater mussels in the little South Fork Cumberland River, USA

The Little South Fork Cumberland River, Kentucky and Tennessee, USA, was a globally important conservation refugium for freshwater mussels (Mollusca: Unionidae) because it supported an intact example (26 species) of the unique Cumberland River mussel fauna including imperiled species. We used previous surveys and our 1997-1998 survey to reconstruct the historical fauna, to describe spatio-temporal patterns of density and number of species, and to evaluate the probable sequence and cause of observed mussel declines. We were specifically interested in better understanding how mussel assemblages respond to chronic disturbances, and how these changes manifest in persistence patterns. Density and numbers of species declined steadily from 1981 to 1998, but declines occurred first in the lower river (early 1980s), followed by declines in the upper river (late 1980s to early 1990s). Of the total species recorded from the Little South Fork, 17 (65%) are seemingly extirpated and five others appear near extirpation. Declines are associated with at least two, temporally distinct major insults. Lower river declines are associated with surface mining, whereas, oil extraction activities are implicated in upper river declines. Regardless of causal factors, species persistence was primarily a function of predecline population size with only the most numerous and widespread species surviving. At this time, the river appears lost as a conservation refugium for mussels despite its remoteness, predominantly forested watershed, and several layers of existing statutory and regulatory environmental safeguards. We suggest that the river could be restored and mussels reintroduced if an interagency task force is formed to identify and mitigate specific stressors now affecting most mussel species in the river.