Question: I’m keen for my children to eat fish (especially oily fish), but am concerned about mercury levels. Do you have any good guidelines around fish for kids – serving sizes, species, servings per week, etc?
Answer: Mercury is a naturally occurring element in the Earth’s crust, soil, air and water. But it is also a widespread, persistent pollutant that can cause adverse health effects if eaten in large quantities, so New Zealand has recently instigated a monitoring programme to assess levels and routes of mercury exposure in children and adults.
Humans cannot create or destroy mercury, but we can scatter it around our ecosystem where it shouldn’t be. For example, when we burn coal, mercury is released into the atmosphere, settling into water or onto land, where it’s washed into the water. Micro-organisms convert mercury into highly toxic methylmercury, which then accumulates in fish and shellfish and ends up inside people who consume these marine foods. An estimated 80-90% of organic mercury in the human body originates from fish and shellfish intake.
Indeed, the first biomonitoring survey of New Zealanders’ blood mercury, published in 2021, found that fish and shellfish consumption strongly predicted levels of the metal. Notably, 93% of children (aged 5-18 years) and 99% of New Zealand adults had detectable mercury in their blood. Interestingly, boys had mercury levels 40% higher than girls. And children and adults who ate fish three or more times a week (compared with once a week) had 2.7 and 1.7 times higher mercury levels respectively.
The researchers noted there was currently no New Zealand blood mercury notification level (that is, a level at which medical or governmental intervention is required). So they compared New Zealand data with overseas measures. They found none of the Kiwi survey participants exceeded Health Canada’s reference values (ie, the normal acceptable range) for mercury. However, 9% of adults, 4% of children, and 6% of women of childbearing age exceeded Germany’s reference values, suggesting there was potential for adverse health effects among some New Zealanders at our present blood mercury levels. In particular, neurodevelopmental effects in children were highlighted as of primary concern – affirming your concerns about mercury intake in your children.
High mercury consumption is a problem because methylmercury induces toxic effects in several organ systems, particularly the nervous system, which is especially sensitive during its critical growth in the womb. Methylmercury exposure in utero can negatively affect cognitive thinking, memory, attention, language, fine motor and visual-spatial skills.
Typically, most people are not exposed to enough mercury to cause harm and our bodies can successfully excrete any mercury build-up over time. And of course, fish consumption is linked to significant health benefits: it is an excellent source of protein, omega-3 fatty acids, iodine, selenium and vitamins. So how much fish is safe to eat?
The Ministry for Primary Industries (MPI) advises pregnant women and people who eat a lot of fish to be careful about their intake – limiting different categories of fish known to have high mercury levels (see box). For instance, predatory fish, such as sharks and swordfish, typically have higher mercury levels. Likewise, freshwater fish, such as trout, that live in lakes and rivers supplied by geothermal water can also have higher mercury levels because volcanic emissions often contain the element.
For those in the general population who eat fish more than four times a week, the MPI advice is to select from species in its unrestricted consumption category for some of those meals.
Otherwise, no guidelines exist for other adults or children. However, all New Zealanders would be protected by following the advice for pregnant women, MPI says.
Australian officials differ in providing specific advice for pregnant women and children up to six years of age – given that young children are more susceptible to mercury while their brain is rapidly developing.
Ultimately, if intake guidelines are followed, the significant health benefits of eating fish outweigh the risks. And as New Zealanders typically eat less than one serving of fish per week, most of us would benefit from eating more of it.
New Zealand fish intake recommendations for pregnant women:
Pregnant women should limit billfish, sharks and fish from geothermal regions to one serving every 1-2 weeks. In contrast, fish with moderate mercury levels, such as albacore tuna and snapper, and farmed salmon can safely be eaten up to three to four times a week during pregnancy. And other fish, such as flounder, hoki and tarakihi, accumulate only low levels of mercury and fall into MPI’s unrestricted category, which everyone can eat without restriction.
Australian fish intake recommendations for pregnant women, children and others:
Pregnant women and children up to 6 years of age should limit orange roughy or catfish intake to one serve a week, with no other fish that week, or one portion of shark or billfish and no other fish that fortnight. One serving is 150g for adults and 75g for children up to 6 years. The general Australian population should also limit the intake of shark and billfish to one serving a week and no other fish that week.
MPI’s website has advice on mercury levels in fish and consumption limits for particular species. Click here to find out more.
