› Did You Know?

Tuna Time Bomb

A new study says the mercury levels in tuna are high—and getting even higher.

Back in the early 1970s, the Smithsonian Institution loaned a bunch of preserved fish to a team of chemists at the University of California, Irvine. The animals had already been dead for 80 or so years, but that’s why George Miller and his colleagues needed them—to settle a debate about mercury. Some scientists worried that pollution from coal-fired power plants and gold mining was concentrating mercury in the oceans. Others argued that humans couldn’t possibly alter the chemistry of the unfathomably vast seas. Miller wanted to compare mercury levels in the Smithsonian’s tuna, caught between 1878 and 1909, to those of modern fish.

Miller reported good news: Although both the fin de siècle and 1970s fish were high in mercury, the levels had remained steady. The mercury in fish was mostly naturally occurring, according to Miller’s 1972 article in Science, and human activity was not to blame.

Marine biologists and hydrologists have long questioned Miller’s conclusions. But even if he were right, the oceans are not infinite. Eventually, humans would burn enough coal and mine enough gold to increase the seas’ mercury levels, and that extra mercury would find its way into the fish we eat. A study published yesterday in the journal Environmental Toxicology and Chemistry suggests that day has come. Mercury levels in fish are indeed rising, and it’s probably going to get worse.

Over the past 17 years, the researchers conclude, the concentration of mercury in yellowfin tuna has increased by approximately 3.8 percent annually. At the current rate of mercury emission, they could become a very dangerous thing to eat in a decade or two.

From Mine to Plate

When we dig a lump of coal out of the ground and burn it for energy, significant amounts of mercury are released into the atmosphere. Coal-fired power plants emit more than 500 tons of mercury per year worldwide, according to United Nations data.

There is, however, an even larger source of atmospheric mercury: artisanal gold mining. No, artisanal mining doesn’t involve small-batch gold, lovingly extracted from the ground by a skilled craftsman. It’s a crude form of mineral extraction, typically undertaken by poor miners in marginalized communities in Asia, Africa, and South America. Many of the 10 million artisanal miners worldwide are women and children, who mix mercury with a gold-containing compound to draw out the precious metal. They then burn the mixture, which isolates the gold and sends mercury vapor into the air. This process contributes 800 tons of mercury to the atmosphere annually (in addition to poisoning the miners).

Rain, snow, and dust bring the mercury vapor into the ocean, where bacteria consume the mercury and convert it into methylmercury. Plankton eat the bacteria, small fish eat the plankton, large fish eat the small fish…and the mercury accumulates in higher concentrations as it travels up the food chain. Eventually, it winds up in tuna, swordfish, and other of our seafood favorites.

CSI: Ocean Floor

Mercury doesn’t just come down from the sky. Lots of naturally occurring mercury also comes up from the seabed, and there have been long-running arguments over how much the atmosphere’s contribution even matters. Recent research, including yesterday’s study, seems to have settled the debate: It really, really matters.

Like many elements, mercury comes in a variety of configurations. While we can’t yet pinpoint the exact source of a particular bit of mercury—is it from a coal-fired power plant, gold mining, or the ocean floor?— we can determine the path it has taken to arrive in the oceans. If it has passed through the atmosphere or diffused into the water from the seabed, the mercury’s isotopic signature will let us know.

A 2013 paper in the journal Nature used this strategy to prove that between 20 percent and 40 percent of the ocean's mercury comes from the surface. According to another paper published last year, the concentration of mercury in the ocean has tripled since the start of the Industrial Revolution.

Enter the Yellowfin

Mercury in the ocean is measured in nanograms per liter, but those tiny units are misleading. Methylmercury is a potent toxin. Even at low concentrations, it can damage a fetus’ neurological development. In adults, ingestion can affect speech, hearing, coordination, and the sensation of pain.

More importantly, each time methylmercury takes a step up the food chain, it becomes concentrated by a factor of 10. That’s why health authorities typically counsel against eating shark and other top predators. A fish guide authored by NRDC (which publishes Earthwire) advises against eating more than three servings of yellowfin per month. Although the Food and Drug Administration considers the average yellowfin safe to eat, some of the fish tested in the agency’s random samplings have contained mercury far in excess of the recommended limit.

“The FDA publishes data on mercury in tuna, but that’s it,” says aquatic ecologist Paul Drevnick, the lead author of yesterday’s study. “Without knowing the size of the fish, it’s impossible to compare fish across time periods.”

Drevnick and his colleagues at the University of Michigan are helping us follow the trend of this toxin. They took existing data on mercury levels in yellowfin tuna caught near Hawaii over time and matched the fish by size and age. What they found was that although mercury content was stable for many years, it has been steadily rising since at least 1998.

The research comes with the usual caveats. It’s just one study, and the sample size of around 14 fish in the most recent dataset is small (although the mercury increase was still statistically significant). We need stronger data, and governments can do more to help researchers.

Spare a Thought for the Fish

Studying the health effects of mercury in humans is hard. You have to estimate their consumption, then look for small differences in a large sample over long periods of time. Studying the effects on tuna is much easier—all you have to do is deliberately feed them mercury and watch. (Sorry, tuna, our research ethics are strongly slanted toward human interests.) Drevnick has done this research, too, and he found significant effects on reproductive behavior.

“I had an undergrad who filmed pairs of fish in the lab,” he says. “He called it fish porn.”

Mercury contamination, however, makes for really boring porn. It suppresses testosterone production in males, so they’re not interested in mating. Females produce less estrogen, so they have nothing to fertilize. Mercury makes for some G-rated cinema, and reproduction in the contaminated lab fish is lower than in control groups. That’s especially bad news if those effects extend to wild tuna, whose numbers are already low

Now What?

Governments worldwide are aware of the mercury problem. In 2013, 140 countries ratified the Minimata Convention on Mercury, which requires countries to install the best available emission-control technologies on new coal-fired power plants. Mining for mercury itself will soon be banned, and companies will have to sharply reduce or eliminate the toxin in their products. The treaty has also helped bring funding for new research and remediation work.

As with most conventions, however, the legacy of Minimata will depend in large part on the implementation, which brings me back to gold mining. The delegates couldn’t agree on a single solution to artisanal gold mining, because the situation varies by region, says Susan Egan Keane, deputy director of the health program at NRDC. Some countries have a few thousands miners, who can be shifted into other economic pursuits. Others, like Ghana, have hundreds of thousands miners, whose ancestors have been mining gold for millennia, according to Keane, and there aren’t many other opportunities for them to earn money. Each country committed to developing its own plan to deal with artisanal gold mining, but fixing the problem could take decades. In the meantime, mercury will continue to accumulate in tuna.

“If I were a pregnant woman or a young child, I would avoid tuna,” says Keane.

If Drevnick’s study is accurate, that suggestion might apply to the rest of us, too.

onEarth provides reporting and analysis about environmental science, policy, and culture. All opinions expressed are those of the authors and do not necessarily reflect the policies or positions of NRDC. Learn more or follow us on Facebook and Twitter.

Join Us