Bioaccumulation of Mercury in Fish Varied by Species and Location in the Chesapeake Bay Watershed—Summary of Existing Data and a Roadmap for Integrated Monitoring

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Fish mercury data from State monitoring programs and research studies within the Chesapeake Bay were compiled and summarized to provide a comprehensive overview of the variation in fish mercury concentrations among species and habitats within the watershed. These data are put into context with existing health benchmarks for humans, birds, and fish. Scientists also provide a roadmap for an integrated monitoring program. 

Background and Issue. The Chesapeake Bay, along the mid-Atlantic coast of North America, is the largest estuary in the United States and supports ecologically diverse freshwater and estuarine habitats. The Chesapeake Bay watershed is also home to economically important industrial, agricultural, and urban infrastructure and land uses. Contaminants in the watershed are a concern because of their potential to have adverse effects on fish, wildlife, and human health. Mercury and polychlorinated biphenyls in fish tissues are the primary contaminants that result in fish consumption advisories in the bay and its watershed.  

Chesapeake Bay: A Landsat 8 Surface Reflectance Mosaic

Chesapeake Bay is the Nation's largest estuary. The USGS provides scientific information for this vital ecosystem. (Public domain.)

Identifying and prioritizing mitigation strategies to reduce exposure and health risks posed by mercury requires an understanding of its occurrence and of the processes that control bioaccumulation through the food chain. Various jurisdictions regularly monitor fish mercury concentrations in the watershed to inform impairments of waterbodies and fish consumption advisories, offering an untapped resource to understand larger scale patterns of mercury concentrations and potential risks to fish, wildlife, and humans.

U.S. Geological Survey (USGS) scientists compiled fish mercury data from State monitoring programs and recent research efforts into an integrated dataset for the freshwater habitats across the watershed. The dataset consisted of nearly 8,000 mercury measurements in fish tissue from 600 locations, which were collected between 1990 and 2017 in freshwater portions of the watershed. The study did not include mercury concentrations in fish from tidal waters. Using these data, scientists provided a comprehensive overview of the variation in fish mercury concentrations among 61 fish species and two freshwater habitats (rivers and streams; ponds and lakes) across the watershed and put those data into context with existing benchmarks of health impairment to humans, wildlife, and fish.

The study findings highlight that mercury concentrations in fish were variable across areas and among species within the freshwater portions of the watershed (landlocked individuals not migratory estuarine individuals). These results indicate that local variation in sources, environmental conditions, and species ecology contributes to mercury accumulation across the watershed.

The USGS has prepared a map-based narrative, which was released in April 2021, to further communication the results of the study. View this story map for a visulaization of the study results.  

Specific findings include the following:

  • Mean concentration of mercury in all fish species was 0.22 microgram per gram, with a range that spanned four orders of magnitude.
  • For individual fish species residing in the freshwater portions of the, the highest concentrations of mercury were measured in striped bass (Morone saxatilis), bowfin (Amia calva), walleye (Sander vitreus), largemouth bass (Micropterus salmoides), flathead catfish (Pylodictis olivaris), and smallmouth Bass (Micropterus dolomieu). The lowest concentrations were present in several trout species including brook trout (Salvelinus fontinalis) and creek chub (Semotilus atromaculatus).
  • Mean concentrations of mercury in fish were greatest in the Susquehanna watershed, followed by the Potomac watershed and Coastal drainages, with lowest mean concentrations in the York, Rappahannock and James watersheds.
  • Fish mercury concentrations were lower, but the proportion of fish exceeding health guidelines were comparable or higher in Chesapeake Bay fish in comparison to other regions with extensive mercury data syntheses (Northeastern United States, Canada, and the Western United States).
  • Forty-five, 48, and 35 percent of fish mercury concentrations in the present study exceeded benchmarks for human, avian, and fish health risks, respectively
  • The pattern of fish mercury concentrations was not consistent with regional pattern in atmospheric mercury wet deposition, which supports previous findings that biogeochemical and ecological drivers are important determinants for fish mercury bioaccumulation that confound the linkage with inorganic mercury loading from the atmosphere.
Brook trout

Brook trout, as shown in the image, had some of the lowest mercury concentrations in their tissues. (Credit: R. Hagerty. Public domain)

Implications and Next Steps.  One of the benefits of this study is the understanding it provides of mercury bioaccumulation across numerous fish species and habitats within the watershed.  An additional benefit could, in the future, be the ability to answer broader questions about factors driving mercury bioaccumulation in fish. In this context, the authors provide a roadmap for an integrated monitoring program that could allow agencies to meet their current goals while also providing the ability to answer broader questions to streamline additional mitigation actions if needed to minimize exposures and health risks.

This study was a coordinated effort between the USGS Contaminant Biology and Toxic Substances Hydrology Programs and the USGS Chesapeake Bay studies to document the extent, severity, and biological impacts of chemical contaminants in the watershed and produce science to develop strategies to reduce identified impacts.

The USGS is continuing research on contaminant sources and pathways in the environment and the potential effects on fish, wildlife, and humans through its Chesapeake Science Plan.  For more information on Chesapeake Activities contact Scott Phillips.

This research was funded by the USGS Environmental Health Program (Contaminants Biology and Toxic Substances Hydrology) of the U.S. Geological Survey Ecosystems Mission Area, and Environments Programs. We also acknowledge the efforts of the state agencies who collected and provided data to support the analyses.
Paper: Willacker Jr., J.J., Eagles-Smith, C.A., Blazer, V.S., 2020, Mercury bioaccumulation in freshwater fishes of the Chesapeake Bay watershed: Ecotoxicology, v. 29, pp. 459-484,