In its recent publication, “Data on Mercury in Water, Bed Sediment, and Fish from Streams Across the United States, 1998–2005,” (pdf) the United States Geological Survey (USGS), a division of the Department of the Interior (DOI), released information related to mercury contamination in 291 streams and rivers across the country. 

mercury in freshwater fishThe USGS study’s findings are alarming: every fish sampled in 291 streams across the country were found to be contaminated with mercury, a contaminant that can react with certain bacteria in water and form methylmercury, a dangerous poison. According to the National Institutes of Health:

Unborn babies and young infants are very sensitive to methylmercury’s effects. Methylmercury causes central nervous system (brain and spinal cord) damage. How bad the damage is depends on how much poison gets into the body. Many of the symptoms of mercury poisoning are similar to those seen in cerebral palsy. In fact, methylmercury is thought to cause a form of cerebral palsy.

Populations that regularly consume a large amount of fish are more at risk for developing methylmercury poisoning because their potential exposure to the poison is much greater than populations with lower fish consumption rates. According to a press release issued by the DOI in conjunction with the release of the USGS report:

About a quarter of these fish were found to contain mercury at levels exceeding the criterion for the protection of people who consume average amounts of fish, established by the U.S. Environmental Protection Agency. More than two-thirds of the fish exceeded the U.S. EPA level of concern for fish-eating mammals.

Secretary of the Interior Ken Salazar commented, “This study shows just how widespread mercury pollution has become in our air, watersheds, and many of our fish in freshwater streams. This science sends a clear message that our country must continue to confront pollution, restore our nation’s waterways, and protect the public from potential health dangers.”  

The report revealed that tea-colored or “blackwater” streams in “relatively undeveloped forested watersheds containing abundant wetlands” in the Carolinas, Georgia, Florida, and Louisiana yielded fish with some of the highest levels of mercury. Coincidentally, similar relatively undeveloped watersheds in the Upper Midwest and Northeast also contained fish with high levels of mercury.  

According to USGS, atmospheric mercury is the main source of mercury in most of the streams studied, while coal-fired power plants are the largest source of mercury emissions in the United States. Gold and mercury mining also contribute to the level of mercury in some streams, particularly on the West Coast. 

USGS scientist Barbara Scudder stated, “This study improves our understanding of where mercury ends up in fish in freshwater streams. The findings are critical for decision-makers to effectively manage mercury sources and to better anticipate concentrations of mercury and methylmercury in unstudied streams in comparable environmental settings.”

Additional studies by USGS will continue to shed light on mercury contamination in our streams, and will hopefully shed more light on how much and which kinds of fish are safest for the general population, but most importantly for at-risk populations like pregnant women, children, and populations who depend on fishing for a large portion of their diet, such as Native American tribes.