I live on an island with a thriving local fishing industry so seafood makes up a large part of my diet. I am naturally concerned by any information that seems to suggest that the consumption of seafood may become a dangerous habit. The growing weight of evidence to suggest that some species of fish may contain potentially harmful levels of organic mercury is almost enough to put me off my dinner. The United Nations Food and Agriculture Organisation (UNFAO) believe that around 1 billion people rely on fish as their primary source of animal protein. In 2010, fish provided more than 2.9 billion people with almost 20% of their animal protein.
So, how does mercury find its way into fish?
If you could sum up the relationship between people, the sea, mercury and seafood it would be best described as complicated. Natural process such as: volcanic activity, the erosion of certain rocks and emissions from soil all contribute to the levels of mercury found in sea water. Human processes including but not limited to: mining, poor sewage treatment and coal fired power stations all add mercury to the oceans as well. Mercury enters the oceans at its surface either from the air or by being carried in water. It travels easily from its source and is dispersed around the globe by ocean currents. Areas far from any source of mercury pollution are, unfortunately, still as risk from mercury pollution. The North Pole and the Arctic circle is an area where, due to ocean currents and winds, many of the pollutants released around the globe end up. As a result, the levels of mercury in the top 100 meters of the globe’s oceans has increased by a factor of 3.4 since the industrial revolution.
This mercury enters the oceans in an inorganic form which is harder to digest and therefore doesn’t build up as readily in the fish that end up on our plates. However, certain bacteria in the oceans are able to digest inorganic mercury and release organic mercury which can build up in the tissues of living organisms. This organic mercury can end up concentrated at the top of food webs as predators eat prey. This process is called bio-accumulation. What this means is that pollutants may only be present in small amounts in prey species but as these support fewer numbers of predators the pollutants become more and more concentrated as you move up the food chain. The levels of mercury in a top predator such as tuna are 10 million times higher than those in the surrounding sea water. You can probably see why it is potentially dangerous to eat certain species of fish.
Its not all doom and gloom. The levels of mercury pollution in the oceans can most likely be tackled with some pretty simple solutions. Coal fired power stations can be fitted with filters that remove the majority of the mercury and other pollutants from the exhaust gases these produce. Sewage can be chemically treated so that pollutants are removed before the sewage is either released into the oceans, used for fertiliser, or used to produce power. The most effective way to achieve this would be to work in partnership with developing economies to help them leapfrog straight to cleaner technologies. Understandably, this is a pretty optimistic outlook and whilst the solutions seem simple they will be invariably difficult to implement politically.
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