Mercury Levels in Tuna Just as High as in 1971

Despite decades of pollution controls put in place to reduce mercury in the atmosphere, researchers have learned that tuna contains the same high levels of the toxic substance that it did 50 years ago. This likely comes from old accumulations of mercury, which is being stirred up to the shallower areas where the fish eat, per a new study published in Environmental Science & Technology Letters. Lead author Anais Medieu told the BBC that a "huge amount" of the chemical found in the deeper subsurface ocean "mixes with the surface ocean, where the tuna swim when they feed." This "continuous supply" of what the researchers call legacy mercury may have polluted the ocean decades, or even centuries, ago.

Per the New York Times, mercury pollution derives largely from human activities, like burning coal and mining, with most of it falling from the atmosphere into the ocean. As it enters marine ecosystems, microorganisms convert it into its most toxic form, methylmercury. The researchers analyzed 3,000 tropical tuna muscle samples caught from 1971 to 2022. The types of tuna (skipjack, bigeye, and yellowfin from the Pacific, Atlantic, and Indian oceans) represent nearly all of the tuna we eat—94% of what's fished globally.

Though there's been a worldwide decrease in mercury emissions since the '70s—with a 90% drop since 1990—mercury levels in the most recent samples were consistent with those taken 50 years ago. In parts of Asia, where mercury emissions are higher, levels in skipjack caught in parts of the Pacific even rose. The researchers say it will take decades for the older accumulations to show a decrease in contamination, and only if even stricter measures are taken. "Our study suggests that we will need massive mercury emissions reductions to see a decrease in tuna mercury levels," said co-author Anne Lorrain. While the samples were within regulatory standards, young children, pregnant women, and developing fetuses are most vulnerable to consistent exposure, which can later cause issues in nervous, digestive, and immune systems. (The ocean is changing color.)