Mercury levels in excess of U.S. Environmental Protection Agency health thresholds for potential impacts to fish, birds, and humans have been detected in fish in some of the most remote national park lakes and streams in the western United States and Alaska.

 Mercury is harmful to human and wildlife health. It arises from natural sources, such as volcanic eruptions, and from human sources such as burning fossil fuels in power plants. Mercury is distributed at local or regional scales as a result of current and historic mining activities. Human activities have increased levels of atmospheric mercury at least three fold during the past 150 years.  

This study of mercury in fish is the first of its kind to incorporate information from remote places at 21 national parks in 10 western states, including Alaska. Western parks were selected for the study because of the significant role that atmospheric mercury deposition plays in remote places, and the lack of broad-scale assessments on mercury in fish in remote areas of the west.

Mercury concentrations in fish sampled from these parks were generally low but elevated in some instances. This study examines total mercury in fish, of which 95 percent is in the form of methylmercury, the most dangerous form to human and wildlife health.

“Although fish mercury concentrations were elevated in some sites, the majority of fish across the region had concentrations that were below most benchmarks associated with impaired health of fish, wildlife, and humans. However, the range of concentrations measured suggest that complex processes are involved in driving mercury accumulation in these environments and further research is needed to better understand these processes, and assess risk,” said USGS ecologist Collin Eagles-Smith, the lead author of the publication.

Between 2008 and 2012, NPS resource managers collected more than 1,400 fish from 86 lakes and rivers, and USGS scientists measured mercury concentrations in fish muscle tissue. Sixteen fish species were sampled, with a focus on commonly consumed sport fish found across the study area such as brook, rainbow, cutthroat, and lake trout. Smaller prey fish consumed by birds and wildlife were also sampled.

Scientists compared fish mercury concentrations among sites within an individual park, as well as from one park to other parks, and identified areas with elevated mercury levels. They also compared the mercury concentrations in the fish to a range of health benchmarks including human health guidelines established by the EPA for fish consumption, and wildlife risk thresholds that indicate the potential for toxicity and impairment in fish and fish-eating birds.

The authors found that mercury levels varied greatly, from park to park and among sites within each park. In most parks, mercury concentrations in fish were moderate to low in comparison with similar fish species from other locations in the Western states. Mercury concentrations were below EPA’s fish tissue criterion for safe human consumption in 96 percent of the sport fish sampled.

The average concentration of mercury in sport fish from two sites in two Alaskan parks exceeded EPA’s human health criterion. Mercury levels in individual fish at some parks from other states including California, Colorado, Washington, and Wyoming also exceeded the human health criterion.

Neither the USGS nor the NPS regulate environmental health guidelines. The NPS is coordinating with state officials in the 10 study states regarding potential fish consumption advisories. State fish consumption guidelines consider both the risks associated with mercury exposure and the benefits of fish consumption, such as improved cardiac health from increased omega-3 fatty acid consumption or potential reduced intake of unhealthy fats due to food substitutions. According to the Centers for Disease Control and Prevention, exposure to high levels of mercury in humans may cause damage to the brain, kidneys, and the developing fetus. Pregnant women and young children are particularly sensitive to the effects of mercury.

Mercury at elevated levels can also impact wildlife. High mercury concentrations in birds, mammals, and fish can result in reduced foraging efficiency, survival, and reproductive success. Mercury concentrations in fish exceeded the most conservative fish toxicity benchmark at 15 percent of all sites, and levels exceeded the most sensitive health benchmark for fish-eating birds at 52 percent of all sites.

Mercury threatens natural resources, including wildlife, which the NPS is mandated to protect. “This is a wake-up call,” said NPS ecologist Colleen Flanagan Pritz, a co-author of the report. “We need to see fewer contaminants in park ecosystems, especially contaminants like mercury where concentrations in fish challenge the very mission of the national parks to leave wild life unimpaired for future generations."