Mercury contamination in fish

In the U.S. large scale mining and processing of gold ore is a now a relatively minor source of direct mercury emissions to air thanks to effective regulations. Globally, however, large scale gold mining still emits substantial quantities of mercury to the environment. In addition, the largest use of mercury in the world is artisanal and small scale gold mining. Approximately 10-20 million miners around the world, especially in Asia, Africa and South America, use mercury to bind with gold contained inside ore, and then burn off the mercury, leaving just the gold behind. This low-tech practice releases a significant quantity of mercury to the air, causes severe damage to soils, water bodies and wildlife near the mining sites, and results in heavy mercury exposures to the miners and their families, and adds to the global pool of mercury in the environment.
http://www.nrdc.org/health/effects/mercury/sources.asp