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The Clinch and Powell rivers, Tennessee (TN) and Virginia (VA), upstream of Norris Reservoir, TN, are known for high freshwater mussel species diversity and endemism. Collectively, these rivers harbored at least 56 species historically and 49 are extant, many of which now survive only in the Clinch or Powell rivers or a few other streams. Among an unparalleled 24 federally endangered mussel species known from these rivers, 20 species are considered extant. We sampled 0.25 m-2 quadrats at six Clinch River sites and four Powell River sites for a total of 4–6 sample years at each site. Overall trends were highly significant in the Clinch River, with mean mussel density at combined sites in each state increasing from 16.5 m-2 to 41.7 m-2 (p < 0.0001) at sites in TN but declining from 12.0 m-2 to 3.3 m-2 (p < 0.001) at sites in VA. Cumulative species richness was 39, ranging from 36 in TN to 27 in VA. Greater density in the Clinch River, TN, was due primarily to increases in Epioblasma capsaeformis, Medionidus conradicus, and Ptychobranchus subtentus, which were rare or undetected at most sites in 1979, but increased five- to ten-fold by 2004. Conversely, at Pendleton Island, VA, which was the best site for mussels in the river circa 1980, the decline in density was highly significant, from 24.6 m-2 in 1979 to 4.6 m-2 (p < 0.001) in 2004. In the Powell River, there was also a highly significant decline in mean mussel density at combined sites from 8.7 m-2 to 3.3 m-2 (p < 0.001), with a total of 33 species documented. Though species diversity remains relatively high, our results confirm that mussel populations have declined in large reaches of each river over the 25-year study period.

freshwater mollusk biology and conservation

Quantitative Monitoring of Freshwater Mussel Populations from 1979–2004 in the Clinch and Powell Rivers of Tennessee and Virginia, with Miscellaneous Notes on the Fauna