WASHINGTON – No fish can escape mercury pollution. That's the take-home message from a federal study of mercury contamination released Wednesday that tested fish from nearly 300 streams across the country.

The toxic substance was found in every fish sampled, a finding that underscores how widespread mercury pollution has become.

But while all fish had traces of contamination, only about a quarter had mercury levels exceeding what the Environmental Protection Agency says is safe for people eating average amounts of fish.

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, including bass, trout and catfish, from 291 streams nationwide.

"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 consumed by eating fish 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 here and abroad 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 the Carolinas, Georgia, Florida and Louisiana, where bacteria in surrounding forests and wetlands help in the conversion. The second-highest concentration of mercury was detected in largemouth bass from the North Fork of the Edisto River near Fairview Crossroads, S.C.

"Unfortunately, it's the case that almost any fish you test will have mercury now," said Andrew Rypel, a post-doctoral researcher at the University of Mississippi who has studied mercury contamination in fish throughout the Southeast. He said other research has shown mercury in fish from isolated areas of Alaska and Canada, and species that live in the deep ocean.

Mercury was also found in high concentrations in western streams that drain areas mined for mercury and gold. The most contaminated sample came from smallmouth bass collected from the Carson River at Dayton, Nev., an area tainted with mercury from gold mining. At 58 other streams, mostly in the West, the acidic conditions created by mining could also 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.

"This is showing that the problem is much more widespread," said Sonya Lunder, a senior analyst for the Environmental Working Group, which has pushed for stronger advisories on consumption of mercury-laden fish and controls on the sources of mercury pollution. "If you are living in an area that doesn't have a mercury advisory, you should use caution."

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.

The EPA also has also proposed a new regulation to clamp down on emissions of mercury from cement plants.