Large fish that are high in the food chain, like the five listed above, have the largest levels of mercury in their flesh. They absorb it from water in the form of methyl mercury, which is a compound that’s even more toxic than the original substance. Methyl mercury is dangerous to humans because it accumulates in the organism and can cause damage to the brain and the nervous system, especially that of children and fetuses. It can also affect the kidneys and the liver.
Another fish known to contain high levels of mercury is the ever popular tuna which should also be avoided. Choose canned light tuna though and you should be safe.
Some low-mercury fish including trout, catfish, crab, wild salmon (not farmed), haddock, sardine, herring, shrimp and pollock are quite safe and healthy for anyone. Including them in your diet instead of the five high-mercury fish should bring you fresh health benefits.