Population demographics of six freshwater mussel species (Bivalvia: Unionidae) in the upper Clinch river, Virginia and Tennessee
Growth rates, age-frequency distributions, and mortality rates of six mussel species were examined at four sites in the Clinch River, Virginia and Tennessee, to identify potentially impacted sites in the upper river. The bioaccumulation of copper in mussel shells also was examined as a possible contributing factor to the declining mussel fauna.
Higher growth rates observed at Hackneys site, river kilometer (RK) 433.7, may have been due to the discharge of domestic sewage from the town of Cleveland into the Clinch River. However, a sewage treatment facility was constructed in 1986 which removed the source of enrichment. Growth rates of female Lampsilis fasciola were significantly less than growth rates of male L. fasciola after 3 years of age, probably due to the onset of sexual maturity.
Age-class distributions revealed an absence of recruited juveniles at the Slant site (RK 359.3) after 1977 to 1979 for four mussel species examined. Substantial erosion of stream banks in tributaries of the Clinch River and deposition of sediment in the Clinch River were observed at Slant, indicating a potential cause of decline. Mean annual mortality rates of adult mussels were extraordinarily high for all species at all sites, which confirms the overall decline of freshwater mussels in the upper Clinch River.
There were no statistically significant differences in accumulation of copper in shells of L. fasciola among sites or sexes. Although Appalachian Power Company’s Clinch River Plant (CRP) had a history of high copper levels in the effluent discharge, it is apparently not the cause of reduced recruitment at the Slant site, located 72 km downstream. As indicated by this research, water quality or habitat conditions of the upper Clinch River continue to be insufficient to sustain freshwater mussels. The CRP has improved the effluent discharge to contain less than 12 μg Cu/L, which should improve conditions directly downstream; however, erosion of stream banks, sewage treatment facilities, and agricultural and urban runoff, continue to contribute to the demise of a rich freshwater mussel fauna.