Mercury occurs naturally in small amounts in the environment
. But industrial pollution can produce mercury that accumulates in lakes, rivers and oceans.
Microorganisms in these bodies of water
convert the inorganic mercury to a highly toxic form, called methyl mercury. Methyl mercury enters the aquatic food
chain by binding with particles and sediment eaten by fish. Fish eat these microorganisms. Because they eliminate mercury at a very slow rate, mercury tends to accumulate in their tissues and organs.
Large fish that are higher in the food
chain — such as shark, tilefish, swordfish and king mackerel — tend to have higher levels of methyl mercury than do smaller fish. Larger fish eat the smaller fish, gaining higher concentrations of the toxin. The longer a fish lives, the larger it grows and the more mercury it can collect.