A University of North Carolina Lab found increased 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 potential health risks posed by mercury.
The federal government currently advises pregnant women, nursing mothers and young children to avoid fish with high levels of mercury, like shark, swordfish, king mackerel or tilefish.
Increased mercury levels have been linked to learning disabilities and developmental delays in children, as well as heart, nervous system and kidney damage in adults.
The average levels found were 1.1 parts per million, just over the government's limit of 1.0 ppm. If mercury levels exceed that limit, The Food and Drug Administration can take legal action to remove a product from the market.
Two samples from Maine and Rhode Island contained double the federal mercury limit.
Traces of mercury are found in nearly all fish and shellfish. As it's released through industrial pollution, mercury falls and accumulates in streams and oceans as methylmercury, which builds up in fish and shellfish as they feed; some types of fish prove more susceptible 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.
The Food and Drug Administration advises consumers to limit albacore, or "white," tuna to one meal per week because of higher levels of mercury found in it.
Of 31 tuna steaks sampled, mercury levels averaged 0.33 ppm, a level similar to that of canned albacore tuna.
