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Tuna trouble. Bluefin and bigeye tuna sushi contains mercury levels that sometimes surpass FDA limits.

Heather Gordon

No Silver Lining in Restaurant Sushi's Mercury Levels

Sushi lovers should think twice before ordering another helping of maguro rolls. Mercury levels in restaurant tuna sushi are higher than those of supermarket tuna sushi, a new study reveals.

Mercury tends to concentrate in species at the top of the food chain; the larger the species, the greater the threat. That’s a worrisome fact for sushi lovers, as the prized bluefin tuna can weigh more than 500 kilograms. Because of tuna’s levels of mercury, which can cause severe neurological problems in humans, the U.S. Food and Drug Administration (FDA) and the Environmental Protection Agency (EPA) warn that pregnant women and young children should limit the amount they consume. But that's just general advice; mercury levels may not be the same for all tuna species. To complicate matters, sushi restaurants often do not tell diners exactly which species of tuna they’re getting.

Michael Gochfeld, an environmental toxicologist at Robert Wood Johnson Medical School in Piscataway, New Jersey, wanted to find out which types of tuna pose the greatest risk. Using DNA bar coding, a technique that categorizes organisms based on specific genetic markers, Gochfeld and his team identified five species served in 100 sushi samples from 54 restaurants and 15 supermarkets. They also measured mercury levels in each sample, as the researchers report today in Biology Letters.

The team found that restaurants sold tuna sushi with higher levels of mercury than supermarkets. Bigeye tuna or lean bluefin tuna, which are more common in restaurants, had concentrations that approached or overshot by about 4% the FDA limit— of 1.0 parts per million. The study gives another reason to avoid eating bluefin, most populations of which are in danger of collapse, Gochfeld says. Regulatory agencies should specifically mention bluefin and bigeye tuna in mercury advisories, he says.

Yellowfin tuna, a cheaper and more plentiful species found in supermarket sushi, contained less mercury. Yet samples from all species exceeded the daily EPA limits (more conservative than those of FDA, which also incorporates the nutritional benefits of fish) and the concentrations permitted by Japan’s health ministry. What’s more, measures of mercury concentration in bigeye and yellowfin tuna samples surpassed previous FDA estimates.

“It’s absolutely a completely novel use of bar coding in the area of consumer protection,” says David Schindel, who leads the Consortium for the Barcode of Life in Washington, D.C. He noted one previous study that used DNA bar coding to identify poisonous puffer fish mislabeled as monkfish, but this is the first to use the technology to assess the relative risk of fish consumption.

Gochfeld hopes the findings will start to put pressure on supermarkets and restaurants to accurately label tuna species. In the meantime, consumers should beware what they eat. “If you’re going to eat sushi frequently, you should certainly stay away from tuna sushi,” Gochfeld says. “It should be only an occasional treat.”

* Correction: This story originally listed Michael Gochfeld's affiliation as Rutgers University, one of the affiliations listed on the paper. He has moved to Robert Wood Johnson Medical School. The story has also been modified to reflect the fact that not all populations of the three species of bluefin tuna are threatened with collapse.