Food Focus November 2008

Mercury in fish

Fish is a nutritious food – it is an excellent source of protein, is low in saturated fat and contains the essential omega-3 fatty acids, iodine and some vitamins

While it is important to enjoy the health benefits of eating fish, most fish also contain mercury and it is advisable to keep exposure to mercury within safe limits. It is particularly important that women who are pregnant, or planning to be, monitor their fish intake as there are unresolved issues around levels of mercury in some fish and its potential impact on the growing foetus.

Why mercury is a concern

Mercury occurs naturally in the environment and our most common exposure to it is through fish and other seafood. Most people are not exposed to levels high enough to harm the nervous system as the body excretes mercury over time. However, unborn babies are potentially more sensitive to the harmful effects and their exposure to mercury should be limited. It is also recommended that heavy consumers of fish restrict consumption of certain species high in mercury.

Mercury levels in fish vary considerably between species depending on habits and feeding patterns. Most species accumulate only low levels of mercury over their lives but predatory fish at the top of the food chain, such as shark and swordfish, tend to accumulate higher levels of mercury. Older fish can also have higher mercury levels. Freshwater fish, such as trout, that live in lakes and rivers supplied by geothermal water may also accumulate higher levels of mercury, as mercury is commonly found in volcanic emissions.

Which fish to eat when pregnant

Eating fish during pregnancy is recommended as part of a well-balanced diet because it is a nutritious food for you and your growing baby. To ensure your exposure to mercury is within safe limits, it is recommended that women who are pregnant or considering pregnancy, limit their consumption of fish containing higher levels of mercury and eat a variety of fish where possible.

For the many commonly eaten fish species in New Zealand there is little concern about mercury levels and they 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. Mixed fish (eg, battered fish and fish fingers) are also low in mercury and can be eaten without restriction.

Fish species to be mindful of during pregnancy are some of the longer lived and larger fish, because they tend to accumulate more mercury in their lifetime, and consumption should be limited to three to four servings per week (an average portion size is 150g). 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 that pregnant women would be wise to eat no more than one serving of per week or fortnight, and not at all if you are already eating 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.

Although the mercury content of fish is not affected by processing techniques such as cooking, canning or freezing, the tuna species and small fish commonly used in canning are short-lived species that accumulate only low amounts of mercury.

Mercury in fish and seafood – advice for pregnant women (FoodSmart Website)