US study shows mercury contamination in fish

Scientists detected mercury contamination in every fish sampled in 291 streams across the US, according to a U.S. Geological Survey study.

About a quarter of these fish were found to contain mercury exceeding the standard fixed by Environmnetal Protection Agency for people to eat fish. More than two-thirds of the fish exceeded the EPA level of concern for fish-eating mammals.

“US must continue to confront pollution, restore our nation’s waterways, and protect the public from potential health dangers,” said Secretary of the Interior Ken Salazar

Some of the highest levels of mercury in fish were found in the tea-colored or “blackwater” streams in North and South Carolina, Georgia, Florida and Louisiana, areas associated with relatively undeveloped forested watersheds containing abundant wetlands compared to the rest of the country. High levels of mercury in fish also were found in relatively undeveloped watersheds in the Northeast and the Upper Midwest. Elevated levels are noted in areas of the Western US affected by mining.

Mercury, a neurotoxin, is one of the most serious contaminants threatening the US' waters. The main source of mercury to natural waters is mercury that is emitted to the atmosphere and deposited onto watersheds by precipitation. However, atmospheric mercury alone does not explain contamination in fish in the streams. Naturally occurring watershed features, like wetlands and forests, can enhance the conversion of mercury to the toxic form, methylmercury. Methylmercury is readily taken up by aquatic organisms, resulting in contamination in fish.

“This study improves our understanding of where mercury ends up in fish in freshwater streams,” said USGS scientist Barbara Scudder. “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.”

The USGS studied mercury contamination in fish, bed sediment and water from 291 streams across the nation, sampled from 1998 to 2005. Atmospheric mercury is the main source to most of these streams, coal-fired power plants are the largest source of mercury emissions in the US, but 59 of the streams also were potentially affected by gold and mercury mining.

Since USGS studies targeted specific sites and fish species, the findings may not be representative of mercury levels in all types of freshwater environments across the US.