Mercury contamination found in a quarter of U.S. freshwater fish exceeds federal safe levels for human consumption, according to a study released today by the U.S. Geological Survey.

The agency examined mercury in fish, sediment and water drawn from 291 rivers and streams between 1998 and 2005, finding 25 percent carried mercury at levels above the safe standard for human consumption (0.3 parts per million wet weight), while all of the fish had detectable mercury levels.

"This study shows just how widespread mercury pollution has become in our air, watersheds, and many of our fish in freshwater streams," Interior Secretary Ken Salazar said in a statement. "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."

Atmospheric deposition of mercury is responsible for the contamination of most waterways, but that alone does not account for all of it, USGS noted. Wetlands, forests and organic soils can enhance the conversion of mercury to highly toxic methylmercury, which accumulates in the food chain and can cause serious public health problems.

Overall, the scientists found concentrations in fish corresponded with increasing concentrations of methylmercury in the water.

Some of the highest levels of mercury in fish were found in "blackwater" streams in North and South Carolina, Georgia, Florida and Louisiana, all of which have large, undeveloped, forested watersheds. Some high levels were also found in the western United States, which the scientists attributed to mining.

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

Gavin Gibbons, a spokesman for the National Fisheries Institute, emphasized that the study does not address commercial fish, which mostly come from the oceans or aquaculture.

"To suggest that this study in any way represents a health hazard for normal consumption of commercial seafood would be a distortion," Gibbons said. "For those who rely on subsistence fishing or those who enjoy recreational fishing, it highlights the need to check with local and regional fish advisories. This is not a study that should have consumers in any way concerned about the commercial fish they regularly enjoy."

Click here to read the study.

Reprinted from Greenwire with permission from Environment & Energy Publishing, LLC., 202-628-6500