Mercury levels in fish

Fish is a healthy food but can contain the contaminant mercury. Some fish species contain more mercury than others. Follow our advice to keep your exposure to mercury within safe limits.

What is mercury?

Mercury is a contaminant that occurs naturally in seawater, rivers and lakes. It can be toxic if eaten in large quantities. Mercury is not found in most foods but can be present in fish.

Why mercury is in fish

Mercury occurs naturally in environments that fish live in. Fish accumulate mercury in the form of methylmercury from the water they live in and the food they eat.

Mercury levels in fish vary between species depending on habits and feeding patterns. Most species build up only low levels of mercury over their lives.

Fish are likely to have higher mercury levels if they:

  • are predatory
  • live for a long time
  • live in lakes or rivers supplied by geothermal water.

When to be aware of mercury in fish

Fish is a nutritious food. It is low in saturated fat, and is an excellent source of:

  • protein
  • essential omega-3 fatty acids
  • iodine
  • some vitamins.

Most people are not exposed to high enough levels of mercury to cause harm. Our bodies excrete any mercury build up over time.

However, some types of fish contain high levels of mercury. If you're pregnant or eat a lot of fish, you need to be more careful.

Eating fish when pregnant

Fish is a nutritious part of a well-balanced diet during pregnancy. However, eating fish with high mercury levels can come with health risks. Mercury exposure can harm the development of foetuses and children. Pregnant women or women considering pregnancy should limit consumption of some fish.

Fish to be aware of include:

  • cardinalfish
  • dogfish (excluding rig)
  • Lake Rotomahana trout
  • lake trout from geothermal regions
  • school shark (greyboy, tope)
  • marlin (striped)
  • southern bluefin tuna
  • swordfish.

How much fish can pregnant women safely eat?

Some fish species can be eaten without restriction but others should be eaten less often.

Check the list of safe food, including seafood for pregnant women

Who to contact

If you have questions about mercury levels in fish, email

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