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?).
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.
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.
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.
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.
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.
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.