Supermarkets tend to sell sushi made from yellowfin tuna, which contains less mercury than other tuna species, according to a new study that used DNA bar coding to see which fish had the most metal.

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.

"We found that mercury levels are linked to specific species," Jacob Lowenstein, a graduate student working with the American Museum of Natural History in New York City, said in a news release from the museum.

"So far, the U.S. does not require restaurants and merchants to clarify what species they are selling or trading, but species names and clearer labeling would allow consumers to exercise greater control over the level of mercury they (consume)."

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.

Higher mercury levels were found in bigeye tuna and bluefin akami, which is a lean, dark red tuna, than in bluefin toro, a fatty tuna, and yellowfin tuna akami, the researchers said. Mercury tends to accumulate in muscle rather than fat, so mercury content is usually — but not always — higher in leaner fish. Yellowfin tuna, for example, is lean, but may accumulate less mercury because it is smaller and harvested earlier than other species, they said.

The seafood industry took a critical view of the report.

"This is a study that tests mercury levels in fish, but stops short of any work exploring what — if anything — those levels mean for health," Gavin Gibbons, director of media relations at the National Fisheries Institute, said in an institute statement.

He added that research has shown that "eating fish as a whole food — omega-3s, selenium, lean protein, traces of mercury and all — is a boost to heart and brain health."

Average mercury levels for bluefin akami tuna were greater than what's permitted by the FDA, Health Canada and the European Commission.

The report was published online in the journal Biology Letters.

"People who eat fish frequently have a particular need to know which species may be high in contaminants," said Michael Gochfeld, professor at Robert Wood Johnson Medical School in New Jersey. "Some agencies have been afraid that any mention of contaminants will discourage people from eating any fish."

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.