Consumers wishing to avoid high mercury levels in tuna would do well to buy their sushi at the grocery store rather than at restaurants, according to new research combining DNA barcoding and mercury analysis.

A chef slices high-grade fatty Atlantic bluefin tuna at a sushi restaurant in Tokyo. (Itsuo Inouye/Associated Press) Scientists with the American Museum of Natural History and New Jersey's Rutgers University found grocery store sushi tends to be yellowfin tuna, which has lower mercury levels than other species of tuna.

"We found that mercury levels are linked to specific species," Jacob Lowenstein, a graduate student affiliated with the museum, said in a release.

Restaurants and fish merchants are not required to identify species.

"But species names and clearer labeling would allow consumers to exercise greater control over the level of mercury they imbibe," said Lowenstein.

Their research was published on Wednesday in Biology Letters, a peer-reviewed journal.

The study is based on sushi samples from 54 restaurants and 15 supermarkets in New York, New Jersey, and Colorado.

Concerned about all levels

Despite their findings about grocery store tuna, the researchers say their study shows that all species exceed or approach levels permissible by Canada, the EU, Japan, the U.S., and the World Health Organization.

Mercury is a naturally occurring element and a serious health hazard. Chronic exposure can damage the brain, spinal cord, kidneys, liver and developing fetus. Exposure in the womb can lead to neuro-developmental problems in children.

In general, mercury levels are significantly higher in lean fish because it has an affinity for muscle and not fatty tissue. That means higher levels in bluefin akami (sushi from lean, dark red tuna) and all bigeye tuna than in bluefin toro (sushi from fatty tuna) and yellowfin tuna akami.

The researchers caution that there seem to be other factors involved. Although yellowfin tuna is very lean, it tends to have less mercury, likely because the fish are typically smaller than other tuna and are harvested at a younger age.

In addition, yellowfin are tropical and don't need to eat as much as warm-blooded bigeye tuna and bluefin tuna to maintain their energy level. That could mean yellowfin tuna don't increase their level of toxins as quickly as other species.

The study results are based on 100 samples, all of which were identified with DNA barcoding and tested for relative mercury content.

"This is one of first applications of DNA barcodes in a non-academic setting-using this method in any human health context and not just for determining whether barcodes can quickly and accurately identify a species," Sergios-Orestis Kolokotronis, a geneticist at the museum, said in a release.