Because methylmercury is formed in aquatic systems and cannot be easily removed from organisms, it bioaccumulates in aquatic food chains from bacteria to plankton, to large invertebrates, and from there to herbivorous fish and then to piscivorous fish.
This is because methylmercury has a half-life of about 72 days in aquatic organisms so that it accumulates in these food chains.
Organisms, including humans, fish-eating birds, mammals such as otters, squid, and whales, that eat fish, ingest the methylmercury that builds up in their bodies.
Fish and other aquatic animals such as molluscs, oysters and eels are the only major source of human exposure to methylmercury.
The methylmercury content depends on the type of fish, its age and size and the type of water in which it was found. In general, fish-eating fish such as white shark, swordfish, sailfish, larger tuna, broad-mouthed, Atlantic salmon and pike contain higher levels of methylmercury than herbivorous or smaller fish such as tilapia, herring and eel.
Older and larger fish usually have higher concentrations of methylmercury within a given fish species. Fish that grow up in water with a higher acidity are also more likely to bioaccumulate larger concentrations of methylmercury.
In Iraq, in the 1960s and 1970s, wheat was treated with methylmercury as a preservative. This wheat was then sown and fed to animals and consumed by humans.
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