Mercury was found in every fish caught in 291 U.S. streams and rivers by U.S. Geological Survey scientists, and 25 percent of the fish had levels of mercury considered unsafe for people who consume average amounts of fish.
The USGS study found that some of the highest levels of mercury were in fish taken from "blackwater" streams in the Southeast, Bloomberg news reported. High levels of mercury were also found in fish in the Northeast and Upper Midwest, and in waterways in the West that flow near gold and mercury mines.
Coal-fired power plants produce most the mercury found in streams, rivers and fish, the USGS said.
"This science sends a clear message that our country must continue to confront pollution," Interior Secretary Ken Salazar said in a news release, Bloomberg reported.