Historical assessment of freshwater mussel communities in the Duck River, Tennessee: seven decades of change in a biodiversity hotspot

The Duck River in Tennessee supports one of the most biologically diverse freshwater mussel communities in North America, historically harboring over 70 species. This comprehensive historical assessment, based on a database of over 7,000 mussel records spanning from the early 1800s to 2023, documents significant changes in mussel community structure while highlighting the continued global importance of this system for freshwater mussel conservation (Womble and Rosenberger 2025). Sixty-two mussel species remain extant in the Duck River watershed, with the core area of diversity concentrated within approximately 50 river miles between Columbia and Lillard Mill (less than 18% of the river’s total length). Reported species richness has declined at 85% of surveyed sites, including at Lillard Mill and Columbia, which have decreased by 14 and 11 species, respectively. Mussel species extirpated from the Duck River are predominantly either extinct, such as the Yellow Blossom (Epioblasma florentina) and Turgid Blossom (Epioblasma turgidula), or federally endangered taxa that are naturally rare across their range, including the Cracking Pearlymussel (Hemistena lata), Golden Riffleshell (Epioblasma aureola), and Littlewing Pearlymussel (Pegias fabula). Several species that are common in other systems, such as the Lilliput (Toxolasma parvum) and Flat Floater (Utterbackiana suborbiculata), may persist in the Duck River at densities below detection thresholds, particularly in the lower mainstem where survey coverage has been limited. Significant data gaps identified in the lower Duck River limit our understanding of mussel community conditions across the entire watershed, and expanded survey efforts would enhance the assessment of drainage-wide trends.
Data indicate that mussel species richness declines began in the Duck River as early as the 1960s, coinciding with other enigmatic declines of freshwater mussels throughout the Southeastern United States (Haag 2019). Rebounds of mussel communities were observed in the early 2000s, and species richness was higher than in the late 1970s and late 1980s (Ahlstedt et al. 2017). While overall species richness was lower in the early 2000s than in the 1920s, site-specific declines in richness are not statistically significant (Ahlstedt et al. 2017), indicating a potential return toward historical species richness for some river reaches. This recovery has been attributed to reservoir release improvements at Normandy Dam, reduction of point source pollution, farmland acquisition and removal, and channel restoration from past destabilization events (Ahlstedt et al. 2017). Mussel community conditions in some river reaches, such as Shelbyville and above Normandy Dam, have not rebounded.
While available data provide insights into general temporal trends in mussel density and abundance in the Duck River, inconsistencies in survey methods and effort limit direct comparisons for most of the watershed. Quantitative surveys since 1979 suggest that mussel densities generally increased in the core area of mussel diversity from the late 1970s to the early 2000s (Ahlstedt et al. 2017). However, long-term monitoring by the Tennessee Wildlife Resources Agency (TWRA) from 2010 to 2020 indicates that densities in this area have declined since 2000. Despite this, some species, such as the Round Hickorynut, have experienced localized population increases (Wisniewski 2020). Trends in mussel abundance outside of the core area are mixed: while some sites (e.g., below Columbia Dam, Leftwich, Tugas Bend) show documented increases in mussel abundance, others, like Littlelot, have experienced what may be precipitous declines. 
Due to inconsistencies in the collection methods of freshwater mussel data in the Duck River, direct temporal comparisons of population metrics such as richness and abundance cannot be obtained in a statistically robust manner drainage-wide. We used the Mussel Assemblage Health Index (MAHI; Haag et a. 2024) to supplement our assessment of mussel community changes throughout the Duck River, allowing us to depict mussel community health at four long-term monitoring sites (Columbia, Venable Spring, Hooper Island, Lillard Mill). MAHI integrates fundamental aspects of mussel communities (species richness, abundance, recruitment, and population growth) to assess overall assemblage health. The tool provides an objective assessment of mussel health in the Duck River that is independent of regional context and individual perception, while still incorporating expert judgment that is often challenging to quantify in analyses. All four sites analyzed met or exceeded the minimum MAHI score for “healthy” assemblages, reflecting the robustness of Duck River mussel communities relative to overall mussel assemblage health in eastern North America. However, intermediate scores and declining densities in 3 of 4 sites highlight the importance of continued monitoring of these communities. 
Given increasing anthropogenic stressors, such as water withdrawal demands, land use impacts, and drought events, the development of longer-term datasets using standardized, comparable, and validated data spanning 30 years or more would considerably improve our ability to distinguish natural population variability from anthropogenic impacts (Vaughn and Atkinson 2025). The documented susceptibility of mussel populations to dynamic lags, where environmental impacts may not manifest for several decades, underscores the importance of continued monitoring efforts and mechanistic studies to understand cause-and-effect relationships in freshwater systems (Strayer 2024). Our findings indicate that standardized, consistent monitoring approaches are critical for detecting temporal trends in mussel communities and evaluating conservation effectiveness. This analysis demonstrates the value of integrating multiple assessment frameworks, such as MAHI, with traditional monitoring datasets to provide more comprehensive evaluations of assemblage condition. The concentration of diverse mussel communities within a relatively confined geographic area (approximately 50 river miles) represents both a conservation opportunity and a vulnerability that warrants attention. Prioritizing survey methods that reduce bias and allow for the estimation of abundance will improve the accuracy and comparability of future mussel assemblage assessments. These efforts are especially urgent given the increasing threats to mussel populations in the Duck River (Ahlstedt et al. 2017; Stephenson 2024).