Researchers found mercury in every fish tested in a nationwide stream survey, with some of the higher concentrations showing up in mining areas of the West.

In about a quarter of the fish, levels of the toxic metal exceeded federal standards for people who eat an average amount of fish.

“This study shows just how widespread mercury pollution has become in our air, watersheds, and many of our fish in freshwater streams,” U.S. Interior Secretary Ken Salazar said in a press release.

The study, conducted by the U.S. Geological Survey, sampled 34 fish species at 291 stream sites across the country from 1998 to 2005.

The most contaminated sample came from smallmouth bass in the Carson River in Dayton, Nev., a historic gold mining area.