Avoiding Fishy Mercury

Mercury Bioaccumulation in Fish

Fish consumption is generally very healthy. They contain high quality protein and other essential nutrients, are low in saturated fat, and contain omega-3 fatty acids, a type of essential fatty acid that promotes healthy cardiovascular systems.

In a recent article I discussed different fish based on their environmental impact and fishing practices and suggested Eco-friendly fish for your consumption. Today I want to consider mercury levels in fish and its health effects, especially in kids and pregnant women.

Mercury is a naturally occurring element, which is found in soil, rocks, lakes, streams and oceans. In addition to natural sources, mercury is released into the atmosphere and water from man made sources, such as coal generated power plants, mining operations and paper processing plants.

It is first released into the air and then enters the water with precipitation. Once in the water, methane-generating bacteria turn the mercury into methyl mercury, a highly toxic form of mercury. Fish consume methyl mercury through their diet and absorb it from the water. Predatory fish (fish that eat other fish) and older fish generally contain higher levels of methyl mercury than vegetarian or smaller fish.

Mercury bio-accumulates in fatty tissues. This means is that when a larger fish eats a smaller fish, it accumulates the level of methyl-mercury that the smaller fish contained. When it eats another smaller fish, it accumulates some more methyl mercury. The more fish it consumes, the more methyl-mercury it accumulates, and the level does not drop. Then along comes an even bigger fish and eats the fish that ate the smaller fish. This large predatory fish accumulates all the mercury of the fish it just ate and so the vicious circle continues.

And then when we humans eat a juicy fillet of that large fish, we consume all that accumulated mercury.

That’s why predatory long lived fish have the highest concentrations of mercury in their tissues, and those are the ones that we should avoid.

Coal Burning Power Plant (UWEC)

Mercury can cause damage to the nervous system if consumed in sufficient amounts over a period of time. When you eat fish that contains methyl mercury, it is absorbed through the intestine and spread throughout the body. It affects the nervous system because it easily enters the brain. In pregnant women, methyl mercury can cross the placenta affecting the growing fetus. Methyl mercury is also passed through breast milk, increasing the risk of delays in brain development. The child may experience delayed motor skills and learning problems.

Most governmental, health and environmental organization recommends pregnant women, women of childbearing age and children to limit or stop the consumption of predatory fish such as tuna, shark, grouper and swordfish. For the remainder of the population, the standard recommendation is to consume these fish no more than once every two weeks to a month (depending on body weight).

Following you’ll find a list of fish with high, medium and low levels of MERCURY:

HIGH: Swordfish(*), Marlin, Tuna(*), Shark(*), Grouper (*), King Mackerel.

MEDIUM: Bass, Cod (*), Halibut, Lobster, Mahi Mahi(*), Snapper.

LOW: Sardines, Oysters, Salmon, Crab, Tilapia, Shrimp (*), Trout, Herring, Mackerel (not king), Clams.

(*) Highly Environmentally Destructive Practices

SOURCES

EPA

NRDC

University of Wisconsin-Eau Claire