Fish is an excellent source of protein, omega-3 fatty acids and nutrients such as iodine, selenium and vitamin D. UK guidance recommends eating at least two portions of fish a week, one of them oily. But talking about fish also means talking about mercury.
Why do fish contain mercury?
Mercury occurs naturally in the environment and is also released by industry. In water it can be converted into methylmercury, which builds up through the food chain. Methylmercury accounts for around 90% of the mercury found in fish and shellfish, and larger predatory fish tend to contain the most.
Which fish contain more mercury?
Higher levels: swordfish, marlin and shark. Fresh tuna can also contain more than other fish.
Lower levels: salmon, trout, mackerel, herring, sardines, anchovies, cod, haddock, hake, prawns, crab, cockles, mussels, oysters, scallops and squid.
Why does it matter?
According to the World Health Organization, mercury can be toxic to the nervous system and other organs. Because methylmercury can cross the placenta, there is specific advice for pregnancy and for children.
UK advice from the NHS
- Everyone: eat a variety of fish, including at least one portion of oily fish a week.
- Pregnant women and those trying for a baby: avoid shark, swordfish and marlin; limit tuna to no more than 4 cans or 2 fresh tuna steaks a week; have no more than 2 portions of oily fish a week.
- Children under 16: avoid shark, swordfish and marlin.
Please check the latest NHS guidance, as recommendations may be updated.
Simple ways to reduce exposure
- Vary the fish you eat – choose mostly lower-mercury species.
- Eat a varied diet overall, as different foods have different contaminant profiles.
- Wash fruit and vegetables before eating.
What about food supplements?
Legal maximum levels for mercury also apply to food supplements. Because mushrooms naturally absorb substances from their environment, we test every batch of our mushroom supplements for heavy metals, including mercury, to confirm that results are within legal limits.
A little history
Concern about mercury in fish dates back to the Minamata disaster in Japan in the 1950s and 1960s, when industrial mercury pollution of Minamata Bay seriously affected the local population. In the 1970s, FAO/WHO experts carried out the first international risk assessment of mercury intake.