A federal study of mercury contamination released Wednesday found the toxic substance in every fish tested at nearly 300 streams across the country, a finding that underscores how widespread mercury pollution has become.

WASHINGTON — A federal study of mercury contamination released Wednesday found the toxic substance in every fish tested at nearly 300 streams across the country, a finding that underscores how widespread mercury pollution has become.

The study by the U.S. Geological Survey is the most comprehensive look to date at mercury in the nation's streams. From 1998 to 2005, scientists collected and tested more than a thousand fish from 291 streams nationwide. While all fish had traces of mercury contamination, only about a quarter had levels exceeding what the Environmental Protection Agency says is safe for people eating average amounts of fish.

"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," Interior Secretary Ken Salazar said in a statement.

Mercury can damage the nervous system and cause learning disabilities in developing fetuses and young children. The main source of mercury to most of the streams tested, according to the researchers, is emissions from coal-fired power plants. The mercury released from smokestacks rains down into waterways, where natural processes convert it into methylmercury — a form that allows the toxin to wind its way up the food chain into fish.

Some of the highest levels in fish were detected in the remote blackwater streams along the coasts of North and South Carolina, Georgia, Florida and Louisiana, where surrounding forests and wetlands help in the conversion.

Mercury was also detected in high concentrations in western streams that drain areas mined for mercury and gold. At about 59 of the streams, mostly in the West, mining could be contributing to the mercury levels, the researchers said.

"Some ecosystems are more sensitive than others," said Barbara Scudder, the lead USGS scientist on the study.

All but two states — Alaska and Wyoming — have issued fish-consumption advisories because of mercury contamination. Some of the streams studied already had warnings.

Earlier this year, the EPA, which regulates air pollution, proposed a new regulation to clamp down on mercury emissions from cement plants.

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On the Net:

U.S. Geological Survey: http:water.usgs.gov/nawqa/mercury/