The U.S. Geological Survey has released a study showing that every fish tested from a sample of 291 streams country-wide had mercury contamination. A quarter of the fish studied had mercury levels that exceeded those set by the US EPA for the health of people who eat average amounts of fish, and two-thirds of the fish had levels of mercury that exceeded the EPA's levels of concern for fish-eating mammals.

Some of the highest levels of mercury contamination were found in Southeast "blackwater" streams, areas with abundant wetlands as compared to the rest of the country. High or elevated levels were also found in the Northeast and Upper Midwest regions in relatively undeveloped watersheds, and Western areas affected by mining.

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

Mercury is emitted into the atmosphere from different sources, and then enters natural waterways by precipitation. Natural features of watersheds enhance the conversion of mercury to a toxic form. That toxic form is taken up easily by organisms in the water, leading to contamination in fish.

For more information, read the abstract of the study or the complete findings at the USGS website.