Store-bought swordfish contained mercury levels (search) above the legal limit in a study released Thursday by environmental groups.

A University of North Carolina Lab found elevated mercury concentrations in 24 swordfish samples from supermarket chains including Safeway, Shaws, Albertsons and Whole Foods.

Groups that paid for the analysis want supermarkets to post signs warning shoppers of health risks from mercury, and they want the government to increase its testing.

"Americans have a right to know what's in their food, and posting warning signs in grocery stores where these fish are sold is a simple, commonsense solution that fulfills that right," said Jackie Savitz of the advocacy group Oceana (search).

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.

A supermarket industry group said it was not surprised by the survey, because swordfish and tuna are known to have higher levels of mercury. Many stores already offer brochures or have signs, the group said.

"The issue of asking for supermarkets to provide information at the store level is something companies are either engaged in doing or in the process of doing," said Karen Brown, senior vice president of the Food Marketing Institute. "Certainly, we would support that. We also would not be opposed to increased testing by FDA."

Average levels were 1.1 parts per million, just over the government's limit of 1.0 ppm. The Food and Drug Administration can take legal action to remove a product from the market if mercury levels exceed that limit. Two samples, from Maine and Rhode Island, contained double the federal limit for mercury.

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 (search). 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.

FDA and EPA advise even at-risk people to eat up to 12 ounces — about two meals a week — of fish and shellfish that are lower in mercury, such as shrimp, salmon, pollock, catfish and canned light tuna. FDA says to limit albacore, or "white," tuna to one meal per week because it contains higher levels of mercury.

Of 31 tuna steaks sampled, mercury levels averaged 0.33 ppm, a level comparable to that of canned albacore tuna.