A matter of scale

Eating fish and shellfish can raise mercury levels in our bodies, so it's wise to follow the advice on how often to serve them up.

- Jennifer Bowden

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: M 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 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 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. Microorganisms 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.