Burning Less Coal Means Less Mercury in Your Tuna

Coal emissions reductions have started to help make this fish slightly healthier.
Japanese fishers catch tuna. (Photo: Stringer/AFP/Getty Images)
Nov 16, 2016· 2 MIN READ
John R. Platt covers the environment, wildlife, and technology and for TakePart, Scientific American, Audubon, and other publications.

Want proof that reductions in coal burning are starting to make a difference? Look no further than the Atlantic bluefin tuna. A new study finds that the levels of mercury in these popular fish declined by about 19 percent between 2004 and 2012.

The toxic heavy metal is carried into the ocean by way of emissions from coal-burning power plants. Tuna sit at the top of the food chain, so their levels of mercury contamination build up to very high concentrations as they eat smaller fish over their long life spans. For decades, health officials have warned that tuna contained enough mercury that their consumption could cause health problems in humans, especially children and pregnant women.

Tuna still contains pretty high mercury levels, the study found, so those health risks have not vanished. They have, however, slightly lessened.

The study’s senior author, Nicholas Fisher of the State University of New York at Stony Brook, said tuna is the “largest pathway for mercury exposure for people” but added that until now there hasn’t been a thorough study of mercury in bluefin tuna, especially in the Western Atlantic population.

“We had the opportunity to analyze about 1,300 fish,” said the study’s lead author, Cheng-Shiuan Lee, a Ph.D. student in Fisher’s laboratory, “thus creating the largest such database for mercury contamination in these tuna.” The tuna were caught over a nine-year period, allowing them to assess whether mercury concentrations changed over that time, which they did.

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“We found it somewhat surprising that for all aged fish, there was a consistent decline in mercury concentrations over time,” Fisher said. “We also found it striking that this decline in tissue concentrations of mercury was nearly identical to declines reported for mercury emissions from North America over the same time period as well as declines in mercury concentrations in North Atlantic air and in seawater.”

That, the authors said, suggested that efforts to reduce mercury emissions can have a demonstrable effect in a relatively short period.

The study came out at about the same time as news that worldwide carbon emissions have stayed relatively flat for the past three years, accounted for by both a decrease in coal use in North America and Europe and an increase in other parts of the world. That means that tuna species in other oceans may not have enjoyed this same level of mercury reduction.

“It is likely that mercury depositions in the Indian and Western Pacific waters have not declined,” said Abel Valdivia, an ocean scientist with the Center for Biological Diversity, who was not affiliated with the study.

Fisher and Lee said this is an important time to study tuna, in the wake of the 2013 Minamata Convention on Mercury in which 140 nations agreed to reduce their mercury emissions. To figure out if those steps will be effective, there needs to be an effort to establish baseline levels of mercury in fish today.

“We believe that further baseline evaluations of major fish populations need to be determined in various ocean regions,” Fisher said.

Valdivia expressed concern about how North American coal emissions could change under the incoming Trump administration, which has pledged to end the “war on coal.”

“It is very likely that a potential increase in coal production and use in the U.S. with the incoming administration will increase mercury deposition into the North Atlantic and thus reduce or reverse this declining trend of mercury levels,” he said.