Elevated levels of mercury are being found in fish in some of the remotest areas of the earth, including lakes in national parks in the western U.S. and Alaska, according to a study by the National Park Service (NPS) and the U.S. Geological Survey (USGS).  The first-time research involved collection of 1,400 fish from 86 lakes and rivers in 21 national parks.  Sixteen fish species were sampled, with a focus on commonly consumed sport fish such as brook, rainbow, cutthroat, and lake trout.  Smaller prey fish consumed by birds and wildlife were also sampled. 

Mercury is among the most widespread environmental contaminants, distributed at a global scale from natural sources such as volcanic eruptions and from anthropogenic sources such as coal-fired power plants.  Mercury emitted into the atmosphere can depose to distant land areas and water bodies.  Pollution at local or regional scales is traced to current and historic mining activities. 

When reactive forms of inorganic mercury are deposited or transported to aquatic habitats, microbial processes can convert the substance to organic methylmercury, which efficiently biomagnifies through food webs and is a potent neurotoxin.  In the United States more than 16 million lake acres and 1 million river miles are under fish consumption advisories because of mercury contamination, and 81 percent of all fish consumption advisories were issued because of mercury contamination.

Risk to humans

The researchers found that total mercury concentrations in the study areas ranged from 9.9 to 1,109 nanograms per gram wet weight (ng/g ww), with a mean of 77.7 ng/g ww.  Across all fish sampled, only 5 percent had mercury concentrations exceeding a benchmark of 200 ng/g ww associated with toxic responses of the fish themselves.  However, mercury concentrations in 35 percent of fish sampled were above a benchmark for risk to highly sensitive avian consumers (90 ng/g ww), and mercury concentration in 68 percent of fish sampled were above exposure levels recommended by the Great Lakes Advisor Group (50 ng/g ww) for unlimited consumption by humans. 

Of the fish assessed for risk to human consumers (that is, species that are large enough to be consumed by recreational or subsistence anglers), only one individual fish from Yosemite National Park had a muscle mercury concentration exceeding the benchmark of 950 ng/g ww, at which no human consumption is advised.  Zion, Capitol Reef, Wrangell-St. Elias, and Lake Clark National Parks all contained sites in which most fish exceeded benchmarks for the protection of human and wildlife health. 

“This finding is particularly concerning in Zion and Capitol Reef National Parks because the fish from these parks were speckled dace, a small, invertebrate-feeding species,” report the researchers.  “Yet their mercury concentrations were as high as or higher than those in the largest, long-lived predatory species, such as lake trout.”

Conservation a concern

The reporters note that other research has found high mercury concentrations in fish in other remote areas.  For example, total mercury concentrations in arctic char and lake trout from lakes in Nunavut, Canada, have up to 1,400 ng/g ww.

“Because many remote and protected areas provide refuge from development and human disturbance, they often support high densities of rare and sensitive wildlife species,” state the researchers.  “As a result, the ecological consequences of elevated mercury in these locations could be particularly harmful from a conservation perspective.”

According to the authors, future targeted research and monitoring across park habitats would help identify patterns of mercury distribution across the landscape and facilitate management decisions aimed at reducing the ecological risk posed by mercury contamination in sensitive ecosystems protected by the NPS.

The NPS/USGS report