According to the US Environmental Protection Agency (USEPA), all fish contain highly toxic mercury. This mercury is especially dangerous for children and pregnant women. The Agency has therefore issued a list of fish, which it has divided into three categories according to how often they can be eaten. Fish in the first category can be eaten 2 to 3 times a week, fish in the second category once a week and fish in the third category should be avoided altogether. But why do fish contain mercury at all?
The first famous and most serious case of mercury poisoning from fish is the so-called Minamata disease, in which up to 50,000 people were poisoned in Japan in the 1950s and 5,000 of them died. Behind the poisoning was the plastics factory Chisso Corporation, which was discharging wastewater into the bay. The wastewater contained so-called methylmercury, which settled at the bottom of the bay. So the mercury got into the fish. Humans first observed strange behavior and unexplained deaths of cats and birds. Only later did the poisoning affect the people around the bay, whose diet was largely made up of fish. People began to experience symptoms such as muscle weakness, hearing loss, loss of peripheral vision and speech, paralysis, or death. Newborns were often very deformed and mentally underdeveloped.
Mercury enters the water in three ways: by human influence, by re-emission and naturally. Natural influences include volcanic activity and geothermal vents and account for about 10% of total pollution. Human influences account for about 30% and include, in particular, coal combustion, cement production, oil refining and small-scale gold mining. Re-emissions then account for 60% of total pollution, but most mercury from re-emissions is of human origin.
The reason for mercury poisoning is the so-called biomagnification in the food chain. Algae are the first to absorb mercury often. Algae are then eaten by small fish and then by larger and larger ones. Larger fish are eaten by other animals, such as humans. The problem with mercury is that it is almost eliminated from the body. Any fish that eats another fish will also receive the mercury stored in it. The larger the fish and the higher it stands in the food chain, the greater the concentration of mercury in the body. Therefore, one should avoid eating especially swordfish, sharks, mackerel or bigeye tuna. In contrast, animals with very low mercury concentrations include shrimp, clams, oysters, sardines, salmon, anchovies or trout.
If you eat fish often, there is a mercury calculator in which you enter what fish and how much you eat, and you get a percentage of mercury exposure relative to the USEPA limit. The calculator can be found here.