Mercury in fish

Mercury is a naturally occurring element found in air, water and food. For most people, the main source of exposure is food, especially fish. The mercury in fish is mostly in the more toxic methylmercury form, which binds to proteins in the body such as those in muscle tissue. Everyday food processing, preparation and cooking do not significantly reduce the amount of mercury in fish.

Mercury in the environment#

Mercury occurs in the environment in three forms: metallic (elemental), inorganic and organic. The organic form, particularly methylmercury, is the most dangerous. Methylmercury in fish mainly comes from mercury in ocean sediment that is transformed by microorganisms. Fish absorb this organic mercury through their gills as they swim and through their digestive tracts as they feed.

Mercury levels differ from one species to the next, depending on factors such as the type of fish, its size, location, habitat, diet and age. Predatory fish that are large and near the top of the food chain tend to contain the most mercury.

• Fish with higher mercury levels include shark, ray, swordfish, barramundi, gemfish, orange roughy, ling and southern bluefin tuna.
• Fish with lower mercury levels include canned tuna, salmon, and shellfish such as prawns, lobsters and oysters.

Fish as part of a healthy diet#

Fish is an important part of a healthy diet. It is high in protein, low in saturated fat, high in unsaturated fat, and rich in omega-3 oils. For most fish, mercury is not a health risk when the fish is eaten as part of a normal, varied diet.

For healthy adults who are not pregnant, and for older children aged 6 years and over, fish with high mercury levels should not be eaten more than once a week.

Mercury and the unborn baby#

Babies developing in the uterus appear to be the most vulnerable to the effects of mercury on the nervous system, which may slow development in the early years. Mercury from fish eaten by a pregnant woman can raise her own mercury levels, and this can be passed through the placenta to her developing baby.

The effects on the brain and nervous system may not be noticed until developmental milestones, such as walking and talking, are delayed. Memory, language and attention span may also be affected.

Studies of the brain development of children whose mothers ate significant amounts of high-mercury fish during pregnancy have been carried out in several countries. An international expert committee convened by the Food and Agriculture Organization (FAO) and the World Health Organization (WHO) reviewed these studies and recommended reducing the amount of fish known to contain mercury in the diet, particularly for pregnant women. This position has since been reaffirmed. Some research also suggests that mercury levels in certain fish, particularly some shark species, can be even higher than those seen in the areas first studied.

Dietary advice on mercury has been updated over time and now covers pregnant women, women intending to become pregnant within the next 6 months, young children and the general population.

How much fish to eat#

The guidance below sets out safe amounts of fish for different groups. (As a rough guide, 150g is about 2 frozen crumbed fish portions, and 75g is about 3 fish fingers.)

Pregnant women and women planning pregnancy (1 serve = 150g):

• 2 to 3 serves per week of any fish or seafood other than orange roughy (sea perch), catfish, shark (flake) or billfish (swordfish, broadbill, marlin); OR
• 1 serve per week of orange roughy (sea perch) or catfish, and no other fish that week; OR
• 1 serve per fortnight of shark (flake) or billfish (swordfish, broadbill, marlin), and no other fish that fortnight.

Children up to 6 years (1 serve = 75g):

• 2 to 3 serves per week of any fish or seafood other than orange roughy (sea perch), catfish, shark (flake) or billfish (swordfish, broadbill, marlin); OR
• 1 serve per week of orange roughy (sea perch) or catfish, and no other fish that week; OR
• 1 serve per fortnight of shark (flake) or billfish (swordfish, broadbill, marlin), and no other fish that fortnight.

Rest of the population (1 serve = 150g):

• 2 to 3 serves per week of any fish or seafood other than shark (flake) or billfish (swordfish, broadbill, marlin); OR
• 1 serve per week of shark (flake) or billfish (swordfish, broadbill, marlin), and no other fish that week.

It is suggested that pregnant women eat 2 to 3 serves of fish each week for their own good health and that of their developing baby, while being careful about which fish they choose, since some types contain high levels of mercury that can harm the developing fetus.

Mercury, pregnancy and breastfeeding#

Women should not be worried if they have had the occasional meal of high-mercury fish. It is only a potential problem when that type of fish is eaten regularly, because this causes a build-up of mercury in the mother’s blood.

Methylmercury from fish eaten during pregnancy seems to pose a health risk to the baby only while it is in the womb. Once the baby is born, mercury levels in the mother’s milk are not high enough to be a risk to the infant.

It is worth remembering that the body does clear mercury over time. People only exceed safe mercury levels if they regularly eat a lot of high-mercury fish over many months.

Key points#

• Unborn babies are at the greatest risk from mercury.
• Mercury occurs in the environment in three forms; the organic form, methylmercury, is the most hazardous.
• Mercury from most fish is not a health risk when the fish is eaten as part of a normal diet.
• Unborn babies are at increased risk from mercury, so pregnant women should be selective about the fish they eat.
• Once the baby is born, the levels of mercury in the mother’s milk are not high enough to be a risk to the infant.

Where to get help#