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.