Mercury Toxicity and Bioaccumulation in Fish and Wildlife

Mercury is a highly toxic element that is found both naturally and as an introduced contaminant in the environment. Although its potential for toxicity in highly contaminated areas is well documented, research has shown that mercury can be a threat to the health of people and wildlife in many environments that seem otherwise unpolluted.

The toxic effects of mercury depend on its chemical form and the route of exposure. Methylmercury [ CH3Hg ] is the most toxic form. It affects the immune system, alters genetic and enzyme systems, and damages the nervous system, including coordination and the senses of touch, taste, and sight.

The transformation from elemental mercury to methylmercury is aided by sulfate-reducing bacteria and other microbes that thrive in near the sediment-water interface or in algal mats of streams, rivers, lakes, and oceans. In these habitats methylmercury is transported through the food chain, from the lowest level, such as bacteria and plankton, to fish and then to fish-eating wildlife and people. Because animals accumulate methylmercury faster than they eliminate it, higher concentrations of mercury are consumed at each successive level of the food chain. This process, known as mercury biomagnification, can result in toxic effects in consumers at the top of aquatic food chains in locations remote from point sources with very low atmospheric deposition rates.

USGS scientists have been tracking mercury and studying its effects on wildlife in the environments that are known to favor the production of methylmercury, such as the Florida Everglades and coastal wetlands of the San Francisco Bay. Below are samples of mercury research conducted within the USGS Contaminant Biology Program. For a more comprehensive picture of USGS interdisciplinary research on mercury, visit the Web sites of related USGS programs, featured in the right-hand box, as well as Mercury in the Environment, a Web page presenting USGS mercury studies by the integrated science approach.

Bioaccumulation of mercury in fish and fish-forage organisms

• Bioaccumulation of Mercury by Fish and Fish-forage Organisms in Camp Far West Reservoir, Yuba and Placer County (WFRC)
  
  
• Determination of Total Mercury in Fillets of Sport Fishes Collected from Folsom Reservoir, California, 2006, Open-file report 2006-1078 (CERC)
  
  
• Determination of Total Mercury in Fillets of Sport Fishes Collected from Folsom and New Melones Reservoirs, California, 2004, Open-file report 2007-1077 (CERC)
  
  
• Determination of Total Mercury in Whole-Body Fish and Fish Muscle Plugs Collected from the South Fork of the Humboldt River, Nevada, September 2005, Open-file report 2007-1059 (CERC)
  
  
• Mercury Contamination in Fillets of Selected Sport Fishes from Lake Natoma, Sacramento County, California (WFRC)
  
  

Effects of mercury on birds

• Determination of Source and Bioavailability of Mercury to Bald Eagles in Maine (LSC)
  
  
• Effects of Mercury on Fish-eating and Aquatic Birds Nesting along the Mid-to-Lower Carson River and Willamette River Headwaters (FRESC) (double-crested cormorant, snowy egret, and black-crowned night-heron)
  
  
• Mercury Correlations Among Six Tissue Types in Waterbirds in San Francisco Bay (Davis Field Station, WERC)
  
  
• Mercury in Birds of the San Francisco Bay-Delta: Trophic Pathways, Bioaccumulation, and Ecotoxicological Risk to Avian Reproduction - In support of adaptive restoration of the Bay-Delta ecosystem, scientists from the USGS Western Ecological Research Center (WERC) together with their colleagues from the U.S. Fish and Wildlife Service (USFWS) study methylmercury effects in three distinct foraging guilds of estuarine waterbirds:
  • surface-feeding birds or recurvirostridae (American avocets and black-necked stilts);
  • diving birds feeding on bottom-dwelling organisms or benthivores (surf scoters); and
  • obligate fish-eating birds or obligate piscivores (Forster’s terns and Caspian terns).
  Birds of each guild differ by their feeding method, diet preferences, and habitat use, so each guild can be considered as a unique foraging pathway within the Bay-Delta ecosystem for mercury bioaccumulation. To learn about some of the project's findings see:

• Mercury in Forster’s Terns in Relation to Space Use of San Francisco Bay Habitats, publication brief for resource manager.
  
  
• Mercury Contamination and Effects on Avocet and Stilt Chick Survival - publication brief for resource managers, presenting findings and management implications of a study, which examined factors influencing mercury concentrations in shorebird chicks and related these concentrations to the survival rate of chicks from hatching to fledging.
  
  
• Mercury in Birds of the San-Francisco Bay-Delta: Trophic Pathways, Bioaccumulation, and Ecotoxicological Risk to Avian Reproduction (PDF), 2006 Annual Administrative Report  to California Bay-delta Authority, USFWS, California Department of Fish and Game, USGS
  
  
• Mercury Contamination in Waterbirds Breeding in San Francisco Bay  - a popular article by the project scientists, providing overview of the issue.
  
  
• Mercury in Tree Swallows and Feathers of Other Species That Line Tree Swallow Nests at Acadia National Park, Maine (PWRC)
  
  
• Tree Swallows as Indicators of Mercury Bioaccumulation in the North Fork of the Holston River, VA (UMESC)
  
  

Development of the scientific basis for assessment, restoration, and monitoring of mercury-contaminated habitats

• Bioavailability and Potential Effects of Mercury and Other Selected Trace Metals on Biota in Plow Shop and Grove Pond, Fort Devens, MA (PWRC)
  
  
• Cycling and Speciation of Mercury in the Food Chain of South Florida (FISC)
  For a broad picture of USGS mercury studies in Florida visit Mercury page on South Florida Information Access web site
  
  
• Determination of Source and Bioavailability of Mercury to Bald Eagles in Maine (LSC)
  
  
  
• Mercury Contamination from Historical Gold Mining in California (WERC)
  
  
• Mercury at the Oat Hill Extension Mine and James Creek, Napa County, California: Tailings, Sediment, Water, and Biota, 2003-2004, Open-file report 2007-1132 (WERC)
  
  
• Passive Integrated Mercury Sampler (PIMS). Download information sheet. (102.87 KB, PDF) (CERC)