How Does So Much Mercury End Up in Fish?

We've all heard the warnings about mercury in fish. Physicians and health advocates say people should only consume high-mercury count fish on a very limited basis, while children and pregnant women shouldn't eat these species at all. But few people seem to really understand how the heavy metal winds up in fish in the first place. For those curious about how water-dwelling creatures can contain so much of a land-based element, read on.

So let's begin with how mercury moves from the land into water systems. This biggest creator of mercury pollution is (surprise!) coal-fired power plants. Coal naturally contains mercury, so when power plants burn the material, it releases mercury into the air. Mercury settles in water or on land and eventually runs into waterways. Other sources of mercury pollution include chlorine chemical plants (which use mercury to convert salt to chlorine and lye), automobile scraps, waste incinerators, forest fires, and some manufacturing processes. Mercury also forms when oil and natural gas is burned, and a small amount of mercury occurs naturally in rocks, soil, and water.

So now that you understand how mercury makes its way from land to water, let's look at how the heavy metal winds up in so many species of fish. As mercury settles into waterways, tiny bacteria convert the substance into methylmercury, which is then consumed by plankton. Small fish eat the plankton (and in turn, all the methylmercury the plankton ate). Once it enters a fish's body, methylmercury binds to proteins like muscle tissue. Larger fish then eat the smaller fish, absorbing all their methylmercury. By the same process, methylmercury moves higher and higher up the food chain. The largest, oldest fish in the waterway therefore typically contain the highest mercury levels.

Because methylmercury binds to protein tissues, there's no way to clean or cook mercury out of a fish's body. So when humans consume these fish, they're also eating up all that mercury. That's a problem because mercury can cause numbness, fatigue, and blurred vision in adults. In children, especially fetuses, the health consequences are more drastic: Mercury exposure can cause abnormal brain development and learning disabilities.

Changing this situation at the regulatory level has proven problematic: Coal-fired power plants could install pollution-control devices to remove mercury before its spewed into the air, but so far, many of them haven't embraced this technology. Chlorine chemical plants could opt for cleaner, mercury-free technology, but according to the Natural Resources Defense Council (NRDC), seven U.S.-based plants still use mercury, with many more international plants using the heavy metal. When we're talking about mercury in oceans, the problem is truly a global one, which means it needs international regulations.

In the meantime, consumers can limit their mercury intake by controlling what's on their dinner plates. Stay informed about which fish contain high mercury counts, and make sure to avoid eating these species. Fish like bluefin tuna, swordfish, and shark often contain dangerously high levels of the heavy metal, and many other species should be avoided or eaten in very limited amounts, too. Use fish guides provided by the NRDC (pdf) or Environmental Defense Fund (pdf) to learn which fish are safe to eat. You can also use the NRDC's Mercury Calculator to estimate your average daily mercury dose.

Photo Credit: macglee via Wikimedia Commons