Some Fish Are Still Full of Mercury, for a Worrying Reason

Emissions of mercury have declined, but levels in fish could still increase thanks to overfishing and a changing climate.

Bluefin tuna steaks
Because of their diet, tuna tend to accumulate mercury in their bodies. (Kim Kyung Hoon, Reuters)

Environmental success stories are seemingly in short supply, but the fall of mercury is one of them. Released by coal-burning power plants and other industries, mercury—a toxic metal—circulates in the atmosphere, enters the ocean, worms up the food web and, via the seafood we eat, ends up in our bodies. For decades mercury in seafood has been a health scourge, because it inflicts long-term harm on the brain and increases the risk of heart disease. It’s especially risky for developing fetuses, and mothers-to-be have long been warned away from mercury-rich tuna and swordfish.

But from 1995 to 2010, mercury concentrations in the Northern Hemisphere fell by 30 percent, thanks to aggressive regulations, falling coal use, and phaseouts of mercury in commercial goods. And in 2017, the first global treaty on reducing mercury emissions came into force.

You’d expect, then, that mercury levels in fish would have also fallen, and would continue to fall. But Amina Schartup and Elsie Sunderland of Harvard University have found that in some cases, tomorrow’s seafood will contain more mercury, not less.

That’s thanks to two unlikely culprits—overfishing and climate change, both of which could nudge fish toward pursuing more heavily contaminated prey. Although there’s less mercury in the environment, our actions mean that fish like tuna are more likely to concentrate what’s already in their bodies. The carbon we pump into the atmosphere ends up affecting the amount of neurotoxin on our dinner plate.

Climate change “is not just about what the weather is like in 10 years,” Schartup says. “It’s also about what’s on your plate in the next five.”

Once mercury enters the ocean, microbes convert it to a compound called methylmercury, which then enters the food web. Each animal accumulates all the methylmercury in all of its prey, and all of its prey’s prey, and so on. So predatory fish, such as tuna, cod, and swordfish, accumulate the highest levels of the toxin, which they then bequeath to humans who eat them. In the U.S., 80 percent of methylmercury exposure comes from seafood, and 40 percent is from tuna alone.

This simple pattern hides a more complex one. Researchers have noted that trends in mercury levels can vary considerably between different species of fish, even those that live in similar environments. “And when people looked at trends, some would go up, some would go down, and some would be flat,” Schartup says. “Why, if they’re all experiencing the same declining mercury levels in seawater?” That’s especially confusing for regulators, who reasonably expect to see their emissions-curbing work lead to consistent benefits.

To find out, Schartup and her colleagues collated three decades of data on fish stocks and mercury levels from the Gulf of Maine. They plugged that info into a model that simulated the region’s food webs, in which virtual fish grow up in virtual seas, eating virtual plankton, and accumulating virtual mercury. And by tweaking the model to account for changing environments over the past half century, they showed how human activities radically shaped the amount of mercury that gets into different fish.

In the 1970s, the gross overfishing of herring, the favored prey of Atlantic cod and spiny dogfish, forced these predators to switch to different targets. Cod moved on to other small fish, such as shad and sardines, which contain less mercury. Dogfish, however, moved to squid, which scavenge the bodies of animals further out on the food web, and so contain more mercury than expected for creatures of their size. As the herring recovered, both cod and dogfish returned to eating them. So since the ’70s, mercury levels have increased in cod, and decreased in dogfish. “Everyone who’s looking at those different trends in fish: You’re not all crazy,” Schartup says.

Temperature matters, too. The water in the Gulf of Maine has warmed considerably since the 1960s, and more so than most other parts of the world’s oceans. Because most fish are cold-blooded, their physiology is yoked to the warmth of their surroundings. As oceans get hotter, they become faster and more active, they eat more prey, and they consume more mercury. (Even tuna, which can partly control their body temperatures, experience this effect, because everything else they eat has already built up more of the toxin.)

What will happen in the future if mercury emissions stay low but temperatures continue rising and herring are overfished again? It depends on the fish. Schartup and Sunderland’s model reveals that mercury levels will likely go down in cod, and go up in dogfish. And for Atlantic bluefin tuna, among the most significant current sources of mercury exposure, the outlook is poor. While mercury levels in this species have indeed fallen thanks to reduced emissions, warming temperatures will almost entirely reverse those gains by 2030. Recent data from actual tuna show that the team’s model is correct, and that this reversal is already under way. “Even if we maintain mercury emissions at a constant rate, we’ll see an increase in mercury levels in tuna just due to seawater temperatures,” Schartup says.

“It’s important for regulators to know that if they’re not seeing as fast a decline [in mercury within food] as they expected, that’s not because regulation isn’t working,” she adds. “It would be worse if they hadn’t reduced emissions in the first place.”

While other studies have focused on mercury levels in predatory fish, this one is unique in considering the entire ecosystem. “It’s an important study, showing how the quality of our seafood is intimately connected to a healthy, balanced ocean and that human behaviors—fishing and climate change—directly affect the contamination profiles of that seafood,” says Anela Choy of the Scripps Institution of Oceanography.

Of course, the dose makes the poison. Are mercury levels in fish, current or future, relevant to human health? The answer, Sunderland says, is yes—and always yes. Epidemiological studies have shown that mercury exposures are linked to impaired brain development and cognitive abilities, especially when people are exposed in the womb. (Mercury can cross from a pregnant mother’s bloodstream into her fetus.) “It doesn’t look like there’s a threshold,” Sunderland says. “Everyone would like to see less methylmercury in their seafood. It’s never beneficial for human health.”

Eating less fish might seem like an obvious solution, but many populations around the world subsist on seafood. “We’re not saying people shouldn’t eat fish, because when people substitute it, it tends to be for a less nutritious choice,” Sunderland says. “We’re saying that we can make that food even healthier. But it also requires action on climate.”

Even the existing guards against mercury pollution might be weakening, though. Under the Trump administration, the Environmental Protection Agency has been trying to loosen Obama-era regulations that protect the environment from mercury. Arguing that such protections are too costly for the coal industry, the EPA (now under the direction of the former coal lobbyist Andrew Wheeler) proposed a rule that would change how regulators evaluate the benefits and costs of mercury restrictions. The rule would take several environmental and health benefits out of consideration, skewing the calculus in favor of industry. “It would dramatically weaken the regulations and open the door to lax regulation in the future,” Sunderland says. “And that’s to no one’s benefit.”

A rollback of existing regulations would be enormously wasteful, because it has cost $18 million to fully implement them—a process that’s now complete. It would also be a counterproductive way of snatching defeat from the jaws of victory. As Schartup and Sunderland have shown, mercury regulations need to be strengthened, not weakened.