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Toxic Fish: How Mercury Builds Up in the Marine Ecosystem via Biomagnification

By mining and burning fossil fuels (especially coal), toxic nerve-damaging mercury is making its way into our seas and onto our plates. In some areas where mining has been releasing mercury for hundreds and even thousands of years it has accumulated in dangerous concentrations and flowed into rivers and the ocean. Sadly this means that eating fish– which is a good source of protein, low in saturated fats and our best source of Omega-3 – can harm animals who eat it such as seals, sharks, birds and us, by exposing us to high levels of the toxic chemical. How does the mercury get into fish and why do some fish have more mercury in them than others?

Mercury occurs naturally in very low concentrations in the sea, emitted by volcanos and released by erosion, but our activities have released a lot more. These still relatively safe concentrations of mercury become magnified as mercury is absorbed and then accumulated up the food chain.
Mercury and its compounds are absorbed by algae and other microbes, who are the base of the food chain. These algae and other microbes are eaten by other organisms, who are then eaten by other organisms higher in the food chain and so on. Fish easily absorb mercury and other metal pollutants such as copper and cadmium but can’t get it out of their bodies very easily as they are soluble and can’t be excreted.  Instead of being pooed out like many other pollutants it accumulates in the fish’s muscles and other tissues. This results in accumulation of mercury in the marine food chain.
This accumulation is magnified higher up the food chain, as anything that eats these fish consume the higher concentrations of mercury the fish has accumulated over its life time. This means top-level predators such as seals, sharks, tuna, eagles (and humans) can have much higher amounts of mercury in their tissues than we would expect from living in or near polluted areas alone. Each step up the food chain can lead to ten times higher concentration of mercury. For example a Cod had approximately 95% lower methyl mercury concentrations than did pilot whale in the same environment. This happens in similar ways for plastic microbeads and other pollutants and is called biomagnification.

Mediterranean fish, like this Striped Sea Bream, have been exposed to Mercury pollution from gold mining at  least since Roman times. Mercury still pours into the sea from some of these old mines today. Greece (2016) 

The risk globally is currently is very low so don’t panic. Only certain types of fish, including mackeral and tuna, have dangerous levels of mercury and only if you eat them nearly every day. (Though the Atlantic Mackeral caught here in Cornwall,  contain a lot less Mercury than the Spanish Mackeral eaten elsewhere and are very safe and healthy to eat). Though groups who traditionally eat large amounts of seafood, such as the Japanese, are at a higher risk. However, because of Mercury Biomagnification the NHS advices that pregnant/nursing women and young children limit their intake of certain fish, including canned tuna and oily fish, and advice not to eat at all shark, swordfish or marlin (which Isn’t the norm anyway). See here for more advice.  For the rest of us, unless you are eating oily fish nearly every day, the risk is really low so go ahead and enjoy your fish and chips! 

Global governments are well aware of the threats of mercury and are trying to clean up industries act. But what about the pollutants already in the sea? Well some Australian scientists have invented a cheap material made from industrial waste products and orange peel that can remove mercury from water and soils. In the near future, we may be able to scrub our oceans clean. But for now the best solution is to stop making the problem worse.