NaturalNews) In a new study conducted by the U.S. Geological Survey (USGS),

 every single fish tested from 291 freshwater streams across the United States was found to be contaminated with mercury.

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

Mercury is a potent neurotoxin that builds up in the food chain at ever higher concentrations in predators such as large fish and humans. It is especially damaging to the developing nervous systems of fetuses and children, but can have severe effects on adults, as well. The pollutant enters the environment almost wholly as atmospheric emissions from industrial processes, primarily the burning of coal for electricity. It then spreads across the plant and settles back to the surface, eventually concentrating in rivers, lakes and oceans, where it enters the aquatic food chain.

The number one cause of human mercury poisoning in the United States is the consumption of fish and shellfish.

Researchers tested the water, sediment and fish of the 291 streams between 1998 and 2005. Fish tested were mostly larger species near the top of the food chain, such as largemouth bass.

All fish were contaminated with mercury, more than 66 percent of them at levels higher than those set by the Environmental Protection agency as a "level of concern for fish-eating mammals," according to Reuters. More than 25 percent of the fish were contaminated at levels higher than those set as the threshold for human consumption.

The study is the first to focus on mercury contamination of streams, rather than lakes, reservoirs, wetlands or oceans. The researchers found the highest mercury concentrations in fish from the coastal blackwater streams of the Southeast. Apparently the combination of pine forests and wooded wetlands found in these regions transforms mercury very effectively into its more toxic organic form (methylmercury). Mercury concentrations were also high in streams fed from areas with a history of mining