The fish on your dinner plate may have more mercury than it used to

Changes in industry fishing practices are driving the rise

We’ve all read and heard the warnings about consuming fish: to limit our intake as fish and some seafoods contain mercury, a very strong neurotoxin. That health concern may be increasing.

Research published in the Proceedings of the National Academy of Sciences (PNAS) in September, indicates that recent trends in the fishing industry are increasing mercury exposure for those of us who eat fish and seafood.

It all boils down to two findings: (1) mercury concentrations in the oceans increase in lower latitudes, the part of the globe around the tropics, and (2) in recent years, large commercial fishing operations have begun harvesting more fish in the open waters of the tropics.

Scientists have thought for a while, and research seems to back up this claim, that micronutrients contained in fish, such as omega-3 fatty acids, offset the uptake of mercury when people eat fish. Making matters worse in the current situation is that fish living in warmer waters have lower concentrations of these beneficial micronutrients in their tissues.

The authors of the recent PNAS paper used fisheries data from the United Nations Food and Agriculture Organization, as well as data on mercury concentrations from previous research.

Additionally, the authors found that mercury exposure is highest for subsistence fishers, many of whom practice traditional, preindustrial fishing methods associated with native peoples. For many of these people, the fish they catch make up a large portion of their diets.

Most anthropogenic mercury in the environment is now and has been generated in the global north, the industrial centers such as the United States, Europe, and now China. Like climate change, this is another example of less-developed countries of the global south and tropics paying the price for the industrialism of the global north.

How mercury winds up in your fish and chips

Mercury is a metal that naturally occurs in the Earth’s crust. It is liquid at room temperature, and most folks are familiar with its use in common thermometers. Whether as a pure element or in various compounds, mercury is highly toxic in all its forms. In seafood, mercury is in the form of methylmercury, an organic/metallic compound that is a powerful neurotoxin. Eating enough food with methylmercury can lead to muscle weakness and lack of coordination, impaired speech, hearing, and walking; a “pins and needles” feeling in hands, feet, and around the mouth; and a loss of peripheral vision.

How exactly mercury winds up in the food chain has remained a mystery, although recent research indicates that processes by phytoplankton in the oceans play a key role.

Mercury can wind up in the environment from natural occurrences, such as volcanic eruptions. Since the dawn of the industrial revolution, humans have added more mercury to the atmosphere. Coal can have traces of mercury that are released to the atmosphere during combustion. Other human sources include chlorine alkali processing, metal processing, and waste incineration. Our industrial activities have doubled, possibly tripled the amount of mercury in the atmosphere, which is increasing by about 1.5 percent a year.

From the atmosphere, mercury is deposited directly into bodies of water or deposited on land, where it is washed from watersheds into bodies of water, where it is consumed by phytoplankton and other organisms.

Once it enters an organism, it tends to stay in that organism’s tissues. As predators, such as tuna, consume more and more organisms containing mercury, the neurotoxin accumulates in the predators’ tissues. These are the mercury-bioaccumulated fish we have for dinner.

To reduce the mercury in our fish, the authors of the paper call on nations to reduce industrial emissions, which would also be very good for human health and protecting our planet from further warming. The authors don’t make any suggestions for consumers. If you're concerned about your consumption of fish with mercury, you may want to check out this site from Johns Hopkins University, which lists fish with higher and lower mercury content.