Levels of a Common Environmental Toxin May Be Rising in Our Favorite Fish

A study on the safety of popular seafood is raising debate.


Climate change is threatening the most beautiful locales on Earth, from magical cloud forests to rare desert trees. Unfortunately, it’s also ruining some of the world’s favorite foods, too.

New research published on Wednesday in Nature suggests that popular food fish like tuna and cod, which already contain significant levels of methylmercury (MeHg), will have even greater concentrations of the neurotoxin in coming decades. While MeHg poses health risks to all humans, it’s considered especially serious for fetuses, who can experience lifelong nervous system damage from exposure in the womb.

The international team of researchers behind the paper show that climate change and overfishing will contribute to higher levels of methylmercury in three large, predatory fish that are popular among human seafood eaters: the spiny dogfish (Squalus acanthias), the Atlantic cod (Gadus morhua), and the Atlantic bluefin tuna (Thunnus thynnus). The team argues that rising ocean temperatures will result in greater methylmercury concentrations and that overfishing of these three predators’ main diets have forced them to switch to prey species that contain more methylmercury.

“Climate change is likely to exacerbate human exposure to MeHg through marine fish, suggesting that stronger rather than weaker regulations are needed to protect ecosystem and human health,” write the study’s authors, led by Harvard researchers Amina Schartup, Ph.D., and Elsie Sunderland, Ph.D..

The tuna is an apex predator, so all the mercury in its prey accumulate in its body.

How Does Mercury Get Into These Fish?

Mercury generally exists in the ocean as MeHg, which scientists suspect is formed by anaerobic microbes reacting with mercury in rivers, lakes, and oceans. A naturally occurring substance, mercury is produced by volcanos and forest fires as well as by fossil fuel combustion, mining, and paper pulp production. So while some mercury in natural ecosystems has existed for a long time, human activities have accelerated its release into the oceans. Global agreements have limited mercury emissions somewhat in recent years, but they have not stopped completely.

The reason mercury is a concern in fish like tuna, cod, and dogfish is that as it moves up the food chain, it becomes more and more concentrated. Small fish have some of the substance in their bodies, and when predators eat them, the methylmercury stays in their tissues and accumulates. This process, called biomagnification, means that top-level predators become a sort of mercury magnet, building up high levels over their lifetime.

In the ocean, methylmercury doesn’t seem to harm organisms, even as it accumulates in their body tissues. But for humans consuming them, there are concerns.

Cod, a cold water fish, is also a top predator, making it a mercury sink.

What Are the Health Risks of Mercury?

For adults, methylmercury poisoning can cause minor central nervous system issues including vision problems, lack of coordination, and muscle weakness. While the symptoms are worrisome, mercury toxicity is easily treated with chelation therapy, which uses drugs that bind to mercury and carry it out of the body.

For fetuses, mercury exposure is considered to be a lot more serious. It can cause deficits in cognition, motor skills, memory, and vision, according to the US Environmental Protection Agency.

Some experts dispute the dangers of methylmercury exposure, though.

In an invited commentary, Emeir McSorley, Ph.D., a senior lecturer in human nutrition at Ulster University who wasn’t involved in the new study, writes that a cohort of hundreds of women and children exposed to significant levels of methylmercury have shown no signs of adverse effects for years. With the Seychelles Child Development Study, which followed infants after fetal methylmercury exposure, McSorley says the mercury picture has gotten more complicated.

“Mothers in the Seychelles are exposed to methylmercury concentrations at least 10 to 100 times greater than fish consuming mothers in Western countries and yet we have consistently found no adverse associations of methylmercury with neurodevelopment in three mother-child cohorts, the first of which has been followed up to adulthood,” she writes.

“We interpreted these findings as indicating that the benefits of fish consumption during pregnancy outweighed any risks,” she adds.

So while conventional scientific wisdom has long held that methlymercury is a dangerous environmental toxin, it’s possible that its dangers have been overstated. 

The spiny dogfish, another top-level predator, accumulates mercury in its flesh over the course of a lifetime consuming prey.

Are Mercury Levels Really Rising in Seafood?

For now, this research is just a projection of possible future scenarios, based upon about 30 years of data collected on fish samples and ocean water samples.

The study’s authors outline how MeHg concentrations could change in the near future from rising ocean temperatures.

Both laboratory and field mesocosm data have demonstrated that rising temperatures lead to increases in MeHg concentrations in estuarine and freshwater fish, but the magnitudes of potential changes in wild species are poorly understood,” they write.

When it comes to the effects of overfishing, the logic is sound, but the future is similarly unclear. Due to overfishing of herring, they write, these three large predators switched to eating more squid and other invertebrates, which are higher in MeHg.

But the evidence for these conclusions is still somewhat limited.

“This paper looks like it is good quality, and the suggestion based on modelling that concentrations of methylmercury may have increased in some fish is probable. But these are predictions from a model, not actual measures,” wrote Sean Strain, Ph.D., an emeritus professor of human nutrition at the University of Ulster who wasn’t involved in the study, in an invited commentary.

As with any such prediction, future studies will be needed to confirm whether this predicted pattern plays out in the future.

Whether or not these predictions bear out, the data presented by the researchers highlights an ocean ecosystem that’s out of balance. These top level predators are surviving, but they have had to adapt to human interference in their food chains, and it’s not clear how much more disruption they can take.

Abstract: More than three billion people rely on seafood for nutrition. However, fish are the predominant source of human exposure to methylmercury (MeHg), a potent neurotoxic substance. In the United States, 82% of population-wide exposure to MeHg is from the consumption of marine seafood and almost 40% is from fresh and canned tuna alone. Around 80% of the inorganic mercury (Hg) that is emitted to the atmosphere from natural and human sources is deposited in the ocean, where some is converted by microorganisms to MeHg. In predatory fish, environmental MeHg concentrations are amplified by a million times or more. Human exposure to MeHg has been associated with long-term neurocognitive deficits in children that persist into adulthood, with global costs to society that exceed US$20 billion. The first global treaty on reductions in anthropogenic Hg emissions (the Minamata Convention on Mercury) entered into force in 2017. However, effects of ongoing changes in marine ecosystems on bioaccumulation of MeHg in marine predators that are frequently consumed by humans (for example, tuna, cod and swordfish) have not been considered when setting global policy targets. Here we use more than 30 years of data and ecosystem modelling to show that MeHg concentrations in Atlantic cod (Gadus morhua) increased by up to 23% between the 1970s and 2000s as a result of dietary shifts initiated by overfishing. Our model also predicts an estimated 56% increase in tissue MeHg concentrations in Atlantic bluefin tuna (Thunnus thynnus) due to increases in seawater temperature between a low point in 1969 and recent peak levels—which is consistent with 2017 observations. This estimated increase in tissue MeHg exceeds the modelled 22% reduction that was achieved in the late 1990s and 2000s as a result of decreased seawater MeHg concentrations. The recently reported plateau in global anthropogenic Hg emissions suggests that ocean warming and fisheries management programmes will be major drivers of future MeHg concentrations in marine predators.