Mercury is a metal that when present in the water of rivers, lakes and seas contaminates fishes, crustaceans, molluscs and other living beings. The mercury accumulates along the food chain and in each following trophic level the amount of the metal within the individuals is higher. When humans eat contaminated animals they also become contaminated and severe nervous system injuries may emerge. The main sources of mercury pollution are gold mining and the use of derived substances in industry and agriculture.