Characterization of the native freshwater mussel assemblage across a reach of the Rappahannock River, Virginia

Abstract

Native freshwater mussels occur in rivers of all sizes, yet much of the literature focuses on small- to medium-sized rivers, likely because of logistic challenges of sampling deep, fast, and turbid large rivers. As large-river surveys become more common, the information resulting from a systematic sampling design can provide baseline data on population sizes to inform decision making by resource managers. We collected baseline data on mussel resources in the Rappahannock River, Virginia, in August 2024. This Atlantic coast river, which drains into the Chesapeake Bay, is ∼320 km long, up to 1.6 km wide, and up to 35 m deep. We conducted a systematic quantitative survey across the 75-km reach of the river from Fredericksburg to Leedstown, Virginia. Divers obtained 187 whole substrate samples to estimate the species richness, density, population size, size distribution, community composition, and longitudinal spatial distribution of the native mussel assemblage. Six species were found, and the community was dominated by Eastern Elliptio ( Elliptio complanata ). Mean reach density (± 1 standard error) and population size was 1.25 (0.21) mussels m 22 and 19.3 (3.3) million mussels, respectively. The presence of few fresh dead mussels indicated low to moderate natural mortality. Shell length ranged from 32 to 122 mm, and multiple size classes were observed in E. complanata . Species richness and density had similar longitudinal patterns, with the highest values in the middle of the sampled reach (river kilometers 60–95). These data provide resource managers with baseline information on the spatial distribution, density, length demography, diversity, and evenness of native freshwater mussels across this reach of the Rappahannock River. Identification of mussel occurrence, density, and evidence of reproduction can be used by managers to aid conservation and management decisions such as prioritizing areas for reintroductions and habitat restoration.