Mercury in Fishes from 21 National Parks in the Western United States— Park Variation in Concentrations and Ecological Risk

This report is from the United States Geological Survey and the National Park Service, Air Resources Division. It looks at mercury concentration in fish located in 84 different sites across 21 national parks around the US. The study of “unprecedented spatial breadth” found that mercury levels can vary greatly between different parks, and even within parks themselves, marking the need for more localized monitoring. In some cases, high mercury levels were found in extremely remote areas, indicating that in addition to local monitoring, more research should be carried out into the transport of mercury through the ecosystem.

There is no doubt that the spread of mercury is a huge environmental problem: “mercury (Hg) is among the most widespread environmental contaminants […] More than 16 million lake acres and 1 million river miles are under fish consumption advisories due to Hg contamination in the United States, and 81 percent of all fish consumption advisories were issued because of Hg contamination.” Due to the toxicity of mercury – it is a potent neurotoxin for both wildlife and humans, and can become even more toxic when it is found in water – and because of how it bioaccumulates up the food chain with devastating results, scientists are increasingly concerned with how it travels through ecosystems. With this study in particular, researchers were interested in finding out more about the prevalence of mercury contamination in remote areas, comparing Hg levels both among and within particular western national parks in the United States. The study was comprehensive: researchers sampled 86 individual sites from 21 national parks in 10 states.

For animal advocates, it is worth noting that measurements of mercury contamination – and more accurately, how it bioaccumulates in fish – were obtained by catching fish in the field, after which they “were immediately placed on wet or dry ice until transport to the laboratory, where they were stored at -20°C until processing and analysis. […] In the laboratory, we thawed each fish to room temperature, and […] from each fish with greater than 100 mm Standard Length, we dissected 5–10 g of skinless axial muscle using nitric acid and deionized-water-rinsed scalpels, scissors, and forceps.” The results of these tests at different sites yielded a thorough view of how mercury has spread in the particular parks that were studied. In the words of the researchers, mercury contamination is “widespread, yet highly variable.” The concentrations they found “ranged over two orders of magnitude” and “importantly, this variation in Hg concentrations not only occurred across parks, but also among different sites within parks.” These findings indicate that, at the very least, Hg monitoring needs to be done on as localized a basis as possible, in as many sites as possible to get a truly accurate picture of a particular region: “monitoring programs should not focus on a small number of ‘indicator’ sites and we recommend that where feasible, newly established monitoring or investigation efforts focus on sampling at the largest spatial scale possible within a park to better understand this variability.”

The researchers hope that their work will show that an even broader toxocological assessment of “key ecological endpoints” such as Capitol Reef, Zion, Lassen Volcanic, Yosemite, Wrangell-St. Elias, Lake Clark, and Glacier national parks are worth conducting. “Direct assessments of the physiological, behavioral, and reproductive responses of wildlife and fish in these parks will be tremendously valuable […] and provide a baseline from which meaningful benchmarks for evaluating the integrative risk of Hg toxicity to whole parks or ecosystems could be developed.” Knowing the full picture of mercury contamination could have implications for all kinds of policy decisions moving forward, and might affect how national parks and other areas are protected from such contamination in the future.

Original Abstract:
Mercury (Hg) is a global contaminant and human activities have increased atmospheric Hg concentrations 3- to 5-fold during the past 150 years. This increased release into the atmosphere has resulted in elevated loadings to aquatic habitats where biogeochemical processes promote the microbial conversion of inorganic Hg to methylmercury, the bioavailable form of Hg. The physicochemical properties of Hg and its complex environmental cycle have resulted in some of the most remote and protected areas of the world becoming contaminated with Hg concentrations that threaten ecosystem and human health. The national park network in the United States is comprised of some of the most pristine and sensitive wilderness in North America. There is concern that via global distribution, Hg contamination could threaten the ecological integrity of aquatic communities in the parks and the wildlife that depends on them. In this study, we examined Hg concentrations in non-migratory freshwater fish in 86 sites across 21 national parks in the Western United States. We report Hg concentrations of more than 1,400 fish collected in waters extending over a 4,000 kilometer distance, from Alaska to the arid Southwest. Across all parks, sites, and species, fish total Hg (THg) concentrations ranged from 9.9 to 1,109 nanograms per gram wet weight (ng/g ww) with a mean of 77.7 ng/g ww. We found substantial variation in fish THg concentrations among and within parks, suggesting that patterns of Hg risk are driven by processes occurring at a combination of scales. Additionally, variation (up to 20-fold) in site-specific fish THg concentrations within individual parks suggests that more intensive sampling in some parks will be required to effectively characterize Hg contamination in western national parks.

Across all fish sampled, only 5 percent had THg concentrations exceeding a benchmark (200 ng/g ww) associated with toxic responses within the fish themselves. However, Hg concentrations in 35 percent of fish sampled were above a benchmark for risk to highly sensitive avian consumers (90 ng/g ww), and THg concentrations in 68 percent of fish sampled were above exposure levels recommended by the Great Lakes Advisory 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 Hg concentration exceeding the benchmark (950 ng/g ww) at which no human consumption is advised. Zion, Capital 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, yet their Hg concentrations were as high or higher than those in the largest, long-lived predatory species, such as lake trout. Future targeted research and monitoring across park habitats would help identify patterns of Hg distribution across the landscape and facilitate management decisions aimed at reducing the ecological risk posed by Hg contamination in sensitive ecosystems protected by the National Park Service.