Defenders of Wildlife found the highest levels of mercury in tuna from Ecuador and Mexico — countries known for setting nets where they see dolphins to catch large tuna swimming below.
"They tend to catch larger, more mature fish, which tend to have higher levels, being at the top of the food chain," said Bob Irvin, the group's senior vice president for conservation.
The group is a longtime advocate of dolphin-safe tuna.
The group had a laboratory test 164 cans of tuna labeled as being from Ecuador, Mexico, Costa Rica, Thailand, Malaysia, the Philippines and the United States. Tests were done by New Age/Landmark laboratory, a Benton Harbor, Mich., company that has been used by the federal government.
Analysis of the samples found:
--Average mercury content of U.S. tuna was generally lower than imported tuna.
--Tuna from Asia had the lowest average levels of mercury.
--Tuna from Latin America had the highest mercury levels, with some exceeding the government limit of 1.0 parts per million.
The lab found higher levels of mercury even in light tuna, which the Food and Drug Administration considers to be low in mercury.
FDA says it's safe to eat two meals a week of fish and shellfish that are lower in mercury, such as canned light tuna, shrimp, salmon, pollock and catfish.
But the agency says to limit albacore, or "white," tuna to one meal per week because it contains higher levels of mercury.
Defenders of Wildlife said people should limit light tuna to one meal each week, instead of two, and avoid canned tuna that says it is imported on the label.
"The occasional tuna sandwich is not going to cause any problems, but we are saying the government needs to do a better job of looking at mercury content in light canned tuna, which up to now has been touted as a low-mercury source of protein," Irvin said.
The federal government advises pregnant women, nursing mothers and young children to avoid fish with high levels of mercury — shark, swordfish, king mackerel or tilefish. Elevated mercury levels have been linked to learning disabilities and developmental delays in children and to heart, nervous system and kidney damage in adults.
Traces of mercury are found in nearly all fish and shellfish. Released through industrial pollution, mercury falls and accumulates in streams and oceans as methylmercury. Methylmercury builds up in fish and shellfish as they feed, in some types more than others.
However, eating fish also has widely acknowledged health benefits. The American Heart Association advises people to eat fish at least twice a week.