How Does Mercury Get Into Fish?
Mercury is naturally occurring in the earth’s crust and is released into the environment with natural events such as volcanic activity. But human activities like coal burning, gold mining, and chloralkali manufacturing plants currently contribute the vast majority of the mercury released into our environment. Atmospheric inputs of mercury have increased threefold in the past two centuries and anthropogenic or man-made sources now are estimated to make up two-thirds of the mercury in the atmosphere. , 
Mercury commonly occurs in three forms: elemental, inorganic and organic. The elemental form is liquid at room temperature and, unlike most metals, can volatilize at environmental temperatures. This form what is used in some glass thermometers and instrument gauges. When coal is burned elemental mercury and complexed inorganic mercury are released into the atmosphere and can be carried long distances in the wind before it is deposited in the soil or water. When mercury is deposited in aquatic environments, some microorganisms, including sulfate-reducing bacteria, convert the inorganic form of mercury into methylmercury and release it from their cells.
Both inorganic and methylmercury are taken up by phytoplankton (single-celled algae at the base of most aquatic food chains). When the phytoplankton are consumed by small animals (usually zooplankton), the methylmercury is assimilated and retained by the animals but the inorganic mercury displays low assimilation and is readily lost from the animals in waste products.
Small fish that eat the zooplankton are therefore exposed to food-borne mercury that is predominantly in the methylated form. These fish are consumed by larger fish, and so on. Because the methylmercury is highly assimilated and lost extremely slowly from fish, there is a steady build-up of this form of mercury in aquatic food chains, such that long-lived fish at the top of the food chain are highly enriched in methylmercury. Methylmercury therefore displays clear evidence of biomagnification, where its concentrations are higher in predator tissue than in prey tissue. This is almost unique among metals and is somewhat similar to biomagnification of organic pollutants like chlorinated hydrocarbons.
Most of the methylmercury in fish is found in the fillet, the part most commonly consumed by people. (For more basic information on mercury in our environment see this United States Geological Survey fact sheet and our Resources page.)
 Morel et al., Annu. Rev. Ecol. Syst., 29: 543-66, 1998.
 See the Environmental Protection Agency white paper “ Control of Mercury Emissions from Coal Fired Electric Utility Boilers: An Update” for more information on current and projected mercury emissions and pollution control systems. Many countries outside the U.S. still use coal burning to produce energy without pollution controls; thus, this is a concern that must be addressed globally.