Mercury levels in supermarket sushi may be lower than at those fancy places

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Mercury levels in tuna seem to vary by species, according to a new study that used DNA bar coding to see which fish had the most metal. Sushi aficionados might want to note that the stuff in the supermarket may contain less mercury.

One hundred samples of sushi tuna were taken over a two-year period from 54 restaurants and 15 supermarkets in New York, New Jersey and Colorado and analyzed for mercury content. Tuna species were determined using DNA bar coding, which identified species with a short DNA sequence from a standardized position in the genome. The species studied were bigeye tuna, yellowfin tuna and bluefin tuna.


Although mercury concentrations varied, average concentrations for all species tested exceeded concentrations allowed by Japan as well as the maximum daily consumption deemed safe by the U.S. Environmental Protection Agency. Average mercury levels for bluefin akami tuna were greater than what’s permitted by the FDA, Health Canada and the European Commission.

Overall, researchers found concentrations of mercury lower in supermarket samples than restaurant samples. Higher mercury concentrations were found in bluefin akami tuna and all bigeye tuna samples than in bluefin toro (fatty tuna) and yellowfin. Supermarket sushi and sashimi usually feature yellowfin tuna.

As for why various species of tuna contain different mercury concentrations, size could be one factor. Yellowfin tend to be smaller and younger when caught, allowing for smaller amounts of the metal to accumulate. Bluefin and bigeye tunas also eat considerably more than the yellowfin, paving the way for mercury levels to increase over time.

The authors suggest that health agencies consider putting bigeye and bluefin tunas on the mercury advisory list, since mercury levels are similar to those in fish the FDA and EPA advise pregnant or nursing women and children not to eat.

The study was published online in the journal Biology Letters.

-- Jeannine Stein