Scientists have detected mercury contamination in every one of hundreds of fish sampled from 291 freshwater streams, according to a US government study.

More than a quarter of those fish contained concentrations of mercury exceeding levels set by the Environmental Protection Agency for the protection of people who eat average amounts of fish, the US Geological Survey report said.

More than two-thirds exceeded the EPA-set level of concern for fish-eating mammals.

"This study shows just how widespread mercury pollution has become in our air, watersheds, and many of our fish in freshwater streams," Interior Secretary Ken Salazar said in a statement. The USGS is part of the Interior Department.

The neurotoxin enters the environment chiefly as an air pollutant spewed into the atmosphere by industrial emissions, then falls back to the surface in precipitation and particulate matter carried over long distances.

The main source of atmospheric mercury, according to the EPA, is coal-fired power plants. Conducted from 1998 through 2005, the USGS study is the first comprehensive survey of mercury contamination in the water, sediments and fish of rivers and creeks throughout the United States.

Most previous studies have focused on lakes, reservoirs and wetlands. Mercury contamination in ocean species such as tuna has also received widespread attention.

Some of the highest levels of mercury in the latest study were found in the coastal "blackwater" streams of North and South Carolina, Georgia, Florida and Louisiana - relatively undeveloped areas marked by abundant pine forests and wooded wetlands.