Mercury from volcanoes, coal burning power plants, and mines gets into the water. In the water, the mercury is turned into a form that can be absorbed by fish. Small fish become contaminated with mercury, which are then eaten by larger fish. The largest fish have more mercury because they eat lots of smaller fish and they tend to live longer, so they build up more mercury in their bodies over time. Mercury attaches to the protein in the muscle, or fillet, of fish, and cannot be removed by cooking or cleaning the fish.