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Mercury in fish and seafood – advice for pregnant women

Mercury occurs naturally in the environment and most seafood contains small amounts of mercury in the form of methyl-mercury. Our common exposure to mercury is through fish and seafood. Read our recommendations on the consumption of seafood to keep your exposure to mercury within safe limits during pregnancy.

Remember that you should not eat raw seafood during pregnancy – it should be cooked until steaming hot and eaten straight away. See our food safety in pregnancy’ page for more details about what is safe to eat during pregnancy.

Food safety when you are pregnant

Why mercury is a concern during pregnancy

Fish and seafood are highly nutritious foods for everyone, particularly for pregnant mothers because omega-3 fatty acids are important for your baby’s central nervous system development. Fish and seafood are low in saturated fat and is also an excellent source of protein, iodine and some vitamins.

There are, however, unresolved issues about the levels of mercury in some fish and seafood types and its potential impact on the growing foetus.

At high levels of exposure, mercury can harm the nervous system. Most people are not exposed to levels high enough to be of concern, and the body over time excretes mercury so accumulation is also not a problem. However, a developing foetus is potentially more sensitive to the effects of mercury. Therefore, women of childbearing age are given precautionary advice to take care to moderate their consumption of some types of fish and seafood that are known to have high levels of mercury.

This advice, and the table lower down this page provides recommendations on consumption of fish and seafood in New Zealand, based on the best scientific knowledge currently available. The advice will allow you to get the many health benefits from continuing to eat fish and seafood while keeping your exposure to mercury within safe limits. Calculations for weekly intakes are based on the World Health Organization (WHO) determinations for safe levels of methyl-mercury consumption, and take into account other dietary sources of methyl-mercury.

Because monitoring is ongoing, the table will be updated as new information is obtained.

What pregnant women should be aware of with mercury

For the many commonly eaten fish and seafood species in New Zealand there is little concern about mercury levels and these species can be eaten freely. Included are skipjack tuna, tarakihi, blue cod, hoki, john dory, monkfish, warehou, whitebait, flat fish like flounder, as well as mussels, pacific oysters and small, canned fish such as sardines. (See the table lower down this page for more details). The Total Diet Survey 2003/04 showed that mixed fish (eg, battered fish and fish fingers) are also low in methyl-mercury and can be eaten without restriction.

Total Diet Survey 2003/04

Seafood types to be mindful of during pregnancy are some of the longer lived and larger fish, and consumption of these types should be limited to three to four servings (each of about 150g) per week. These include salmon, albacore tuna, mackerel, gemfish, orange roughy, ling, skate, kahawai, hapuka (groper), bluenose, ghost sharks, bass, oreo dories, red cod, ribaldo and rig (spotted dogfish or lemonfish).

There are a small number of species where it would be wise to eat no more than one serving per week or fortnight during pregnancy, and not at all if consuming other types of fish or seafood. These include dogfish (apart from rig), school shark, swordfish, marlin, cardinal fish, and fish such as trout caught in geothermal regions (mercury is produced from volcanic emissions). Pregnant women should also limit their intake of bluff oysters and queen scallops due to high cadmium concentrations.

There is no reason to be concerned about breastfeeding. The critical period of mercury exposure for your baby is while it is still developing in the womb. Once the baby is born, the risk is much lower and is the same as the risk for adults, therefore no additional precautions are necessary.

Mercury levels in canned or cooked fish compared with fresh fish

Canned or cooked fish and seafood are not a higher risk than fresh. While the mercury content of fish or seafood is not affected by processing techniques such as cooking, canning or freezing, the tuna species commonly used in canning and small fish that are canned (including sardines, pilchards and herring) are short-lived species which accumulate only low amounts of mercury.

What if I only like eating one type of fish or seafood?

The advice for pregnant women relates only to fish or seafood with higher levels of mercury. If your favourite type is one of these then consider moderating your intake or eating a variety of species known to have lower levels of mercury.

How fish oil products are affected

Fish oil products and supplements are not a major source of mercury in the diet and there is no recommendation to restrict intake.

Do these recommendations apply in Australia?

Australia has its own species specific recommendations for which fish types are safe to eat during pregnancy. Many of the fish species sold in Australia are different to those available in New Zealand (such as barramundi and catfish). Most come from different fishing grounds to those sold here and have different exposure levels. In addition, some fish with apparently the same common name are in fact different species. For example ’orange roughy' in Australia is also known there as ’seaperch'; seaperch sold in New Zealand is an unrelated species.

Heavy metals in our diet

Recommended weekly servings of some fish and seafood species, for pregnant women

This table outlines different types of fresh seafood, and the recommended intake per week for pregnant women who wish to have the benefits of fish and seafood while limiting their exposure to methyl-mercury.

May 2008