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.
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.
At about 59 of the streams, mostly in the West, mining could be contributing to the mercury levels, the researchers said.
Earlier this year, the Obama administration said it would begin crafting a new regulations to control mercury emissions from power plants after a federal appeals court threw out plans drafted by the Bush administration and favored by industry.
The Bush rule would have allowed power plants to buy and sell pollution credits, instead of requiring each plant to install equipment to reduce mercury pollution.
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