Is the mercury in fish dangerous?

As humans we come across mercury surprisingly often in our day-to-day lives. The silvery liquid metal is hiding in thermometers, lighting, and batteries. It’s poisonous and there are concerns that it is also becoming increasingly common in fish. Surveys have shown that two-thirds of the British public take their fish and chips with salt and vinegar, and over a third like theirs with a side of mushy peas. Unsurprisingly, there were no recorded requests for a portion of mercury.

Mercury consumption can do nasty things to you. It can cause sensation, speech and co-ordination difficulties, and in very severe cases, paralysis and death. Consequently, official bodies such as the US Food and Drug Administration (FDA) now issue guidelines warning of the potential effects of consuming a lot of fish.

Bruce the Shark and friends from Finding NemoFor a solution, just look to the words a wise shark once said: “fish are friends, not food” (‘Bruce’, Finding Nemo). Sadly, for those of us who want to continue eating fish (and not take anecdotes from lying, fictitious cartoon sharks), it is important to know how to avoid mercury in what we eat and where it comes from.

A silver cloud looming over the oceans

As strange as it sounds, the mercury in the ocean gets there from the air. Burning fossil fuels, gold mining and cement production all cause mercury to go airborne. A small amount comes from volcanic eruptions. The atmospheric mercury levels are very low, so there’s no need to fret over the prospect of giant silver clouds unleashing torrential apocalyptic mercury rain (for now). It does however eventually come to rest on land, rivers or oceans. And once in our waterways it can be ingested by fish, algae and other organisms.

Fish eating fishCertain forms of algae convert mercury into a particularly toxic form called methylmercury. This poison then moves up the food chain, being excreted into the water by the algae and ingested by plankton. Algae and plankton are eaten by small fish, e.g. salmon, pollock, and perch. These fish are then in turn eaten by larger fish like trout and tuna. Finally, higher predators such as you, me and Bruce the shark, eat these fish. The biggest fish accumulate methylmercury faster than they can remove it (a phenomenon known as bioaccumulation) and it is possible that mercury poisoning might drive some species to the brink of extinction.

Metallic swordfish

For us humans, our mercury exposure will depend on the amounts and types of fish we eat. The people at most risk are young children, pregnant and nursing mothers, and from an economic standpoint, the owners of your local fish and chip shop.

Generally speaking, few of us eat enough fish to get poorly from it but there is health advice for reducing mercury exposure:

  • Shark, swordfish and marlin: do not eat these if you are pregnant or trying to get pregnant. All other adults, including breastfeeding women, should eat no more than one portion per week. These fish can contain more mercury than other types of fish, and can potentially damage a developing baby’s nervous system.
  • Oily fish: if you are trying for a baby, pregnant or breastfeeding, you should have no more than two portions of oily fish a week. (A portion is about 140 grams.)
  • Canned tuna: if you are trying for a baby or are pregnant, you should have no more than four cans of tuna a week. This is because tuna contains higher levels of mercury than other fish. If you are breastfeeding, there is no limit on how much canned tuna you can eat.

How can we be mercury free?

Koh Khai NokIn order to combat rising mercury levels, the United Nations, EU and Environmental Protection Agency have now imposed measures to tackle mercury pollution. These include limits on mercury emission,  requirements for mercury filters to be installed where dangerous smoke is being released, and ‘encouragement’ to companies to use alternative industrial methods that are not mercury intensive.

Despite these measures, it will probably take many years before these changes filter through the oceans, plankton and fish – and there may be quite a wait before we see any benefits. Hopefully, with advances in technology we will see the near elimination of mercury release in industry.

Photo credits: Gray Catbird of Pixar Wiki (under CC BY-SA licence), Food Chain by Susan Sermoneta and Koh Khai Nok by Christopher Lance (Flickr CC). Koh Khai Nok

Article by Jack Williams

August 12, 2015

Jack is researching body clock disruption at the University of Manchester. Ironically, much of his research must be done at night, making him a human lab rat to his own project. Away from his research he plays almost every sport under the sun, in the hope that eventually he will find one he’s good at.

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