Where does the mercury in our fish come from?

By: Dana Sackett

The answer to the question, “Where does the mercury in our fish come from?” can be a bit complicated. While most mercury in our air and water originates from burning coal and other industrial sources, the form of mercury released from these sources is usually inorganic, meaning not the form that we are overly concerned about. However, after that inorganic mercury finds its way onto land and into the water, some can be transformed by bacteria into an organic form called methylmercury. Methylmercury is highly toxic, particularly to developing children, and can enter the food web at the bottom, then be transferred through diet to high levels in top predators. For this reason, mercury has more consumption advisories than any other contaminant when it comes to fish (see Mercury in Fish, What Should I Eat?).

Source: http://whyfiles.org/201mercury/3.html

So when scientists talk about sources of mercury in fish, we could be talking about the original source of inorganic mercury emissions into the air and water, the source of the more toxic methylmercury (where the conversion from the less toxic to the more toxic form occurs), or the specific prey fish are eating. For example, scientists have found that those bacteria that convert mercury from the less to the more toxic form work better in places where the water is acidic and the oxygen in the water is low; meaning sites that have low oxygen or are acidic, or both, are often a source of methylmercury to the food web.

Nearby human sources, such as coal burning and mining, can contaminate water sources with mercury. After conversion by bacteria, methylmercury can be efficiently absorbed in the bodies of fish. Through the process of biomagnification, mercury levels in each successive predatory stage increase. Source: https://en.wikipedia.org/wiki/Mercury_in_fish

For example, research we conducted in Hawaii, found that some of the methylmercury in several deeper water bottomfish originated from a deep ocean source, likely the oxygen minimum zone (an area of low oxygen in the ocean between 400-1000m). An area that is expanding because of climate change. Complicating things even more, a recent study found a new potential source of methylmercury to the environment, seals and sea lions.

Figure 3
The relationship between fish tissue mercury in several deeper water bottomfish and several explanatory factors from the (a) Main Hawaiian Islands, which has freshwater and coastal sources of methylmercury, and the (b) Northwestern Hawaiian Islands, which does not have freshwater and coastal sources of methylmecury. The similarity in these two relationships suggests that freshwater and coastal sources of methylmercury were not a source of mercury in fish tissue. Source: Sackett et al. 2015

The reason seals and sea lions are sources of methylmercury to an entire ecosystem: fur. Because seals and sea lions are high-up on the food web, consuming many predatory fishes, they have often accumulated a lot of mercury. One way their body gets rid of that mercury is through their fur. This would not be a problem, except that when hundreds of seals or sea lions congregate in an area and molt each year they are infusing that ecosystem with fur full of methylmercury, essentially recycling and moving the methylmercury these animals have accumulated over time from the ocean to coastal sites where they congregate.

Methylmercury in nearshore unfiltered seawater samples. (A) Methylmercury concentrations for the Año Nuevo mainland rookery during the 2012 molting season (M1–M6), 2013 breeding season (B1–B6), and Central California comparison sites. Source: Cossaboon et al. 2015.

This study estimated that the entire Northern elephant seal population (~210-240K) is likely redistributing a total of 10-12 kilograms of mercury accumulated from ocean prey to nearshore waters. That is a lot when you consider that 0.3 micrograms of mercury per kilogram of fish tissue is the level at which the EPA becomes concerned about human consumption. And that is just the Northern elephant seals, California sea lions, harbor seals, Northern fur seals and others annually molt in nearshore rookeries as well.

Illustration of the conversion of inorganic mercury to methylmercury by anaerobic bacteria, biomagnification of methylmercury at successive marine trophic levels, and then reintroduction of methylmercury to the base of the food chain via top-down contamination. Images are courtesy of the Environmental Molecular Sciences Laboratory (EMSL), a Department of Energy Office of Science user facility at the Pacific Northwest National Laboratory (anaerobic bacteria); Richard R. Kirby, Secchi Disk project (phytoplankton); Maria Grazia Mazzocchi, Stazione Zoologica Anton Dohrn, Italy (zooplankton); and FishWatch, National Oceanic and Atmospheric Administration (prey fish). Source: Cossaboon et al. 2015.

Because the form of the mercury being released into these coastal environments from fur is almost entirely methylmercury (no conversion by bacteria needed) it can directly enter back into the food web, accumulating to higher potentially toxic levels in top predatory fish. Additionally, the authors of this study point out that because marine mammals forage in areas that have lots of fish, they often hang-out in the areas that have the most productive human fisheries.

A Northern elephant seal rookery. Photo by Mike Baird. Source: https://www.flickr.com/photos/mikebaird/6815150423

This study highlights the need to fill in gaps of how mercury is being moved and recycled in the environment, and more importantly highlights the growing need to reduce industrial pollutants that may be affecting the environment in ways we still don’t understand. After all, the original primary source of mercury in our fish is from industrial pollution.



Cossaboon JM, Ganguli PM, Flegal AR. 2015. Mercury offloaded in Northern elephant seal hair affects coastal seawater surrounding rookery. www.pnas.org/cgi/doi/10.1073/pnas.1506520112

Sackett DK, Aday DD, Rice JA, Cope WG. 2009. A statewide assessment of mercury dynamics in North Carolina water bodies and fish. Transactions of the American Fisheries Society 138:1328-1341.

Sackett DK, Drazen JC, Choy CA, Popp B, Pitz GL. 2015. Mercury sources and trophic ecology for Hawaiian bottomfish. Environmental Science and Technology 49:6909-6918.


2 Comments Add yours

  1. Tom Swihart says:

    I have recently been diagnosed with 95percent menthylmurcury in my body. I don’t eat a lot of fish but I do consume large amounts of codliver oil as well a krill oil capsules. Not sure if that’s my source. I will continue to check it out. In meantime, I have given up all things with anything to do with fish. I’m currently on a regimen of things to help get the menthylmurcury out of my body. Will continue to visit this site for any info I can get. Thanks

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