It's ironic that the one of the healthiest foods in the world is also one of the most polluted. But such is the case with seafood, which can have such high levels of industrial chemicals and heavy metals that regularly eating some kinds can lead to long-term health problems. And the problem is likely going to get worse, according to a new study published in the journal Nature Geoscience.
The researchers were trying to get a better handle on why there's so much mercury in fish, which is the primary source of the neurotoxic heavy metal in the average person's diet; mercury has also been linked to heart and immune system problems. It's well known that mercury levels are highest in large predators, such as tuna and swordfish, because the big guys feed on smaller fish and pollutants like mercury accumulate as you move up the food chain. And four years ago, these same researchers determined that large fish swimming in deeper waters have higher levels of mercury than predatory fish that swim near the surface do.
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The question was why. Turns out that it's all about bacteria—and coal-fired power plants in developing nations.
The scientists studied fish living in the North Pacific, which lies in the path of winds that blow from China, India, and other developing Asian nations with lots of coal-fired power plants. By analyzing the chemical makeup of the mercury in fish tissue, they were able to fingerprint the metals as coming from the atmosphere, not from volcanic eruptions or oceanic sediments, as some have argued. The researchers noted the mercury they found in fish was "a near perfect match" with the forms of mercury in the atmosphere, which are known to travel long distances from their original source.
Once the mercury is deposited, bacteria living near the surface of water convert the metal to its more toxic form, methylmercury, which is what builds up in fish. Sunlight can destroy as much as 80 percent of the harmful heavy metal living on or near the surface, which is why surface sea dwellers contain less mercury, but, the researchers found, those same bacteria can hitch a ride on dead or decaying organic matter, and as a result, harmful methylmercury winds up in sun-starved deeper waters.
And those levels are likely to increase. According to the study, by 2050 mercury levels in the North Pacific will be double levels found in 1995 because of increasing emissions from coal-fired power plants in those same Asian countries—assuming the countries don't institute emissions controls or switch to less-polluting sources of energy.
So practice the precautionary principle! You can't spot mercury in fish, so you need to be well armed to know which fish to avoid and which fish are safer. According to the Natural Resources Defense Council (NRDC), these fish varieties have the highest levels of mercury:
Eat three servings or less per month:
Bluefish
Grouper
Mackerel (Spanish, Gulf)
Sea Bass (Chilean)
Tuna (Canned Albacore)
Tuna (Yellowfin)
Avoid eating:
Mackerel (King)
Marlin
Orange Roughy
Shark
Swordfish
Tilefish
Tuna (Bigeye, Ahi)
NRDC makes this list available as a downloadable wallet card, which you can find here. To find fish that are low in mercury and high in omega-3s, check out this list of the 10 Healthiest Fish on the Planet.