In 1984, FDA switched from enforcing the mercury action level based on total mercury to a methyl mercury basis. The change occurred for two reasons. An acceptable test for methyl mercury was available, and evidence indicated that methyl mercury was a small part of the total mercury in some fish. Only Some Fish Exceed Limits Swordfish and tuna are the only commercially popular fish that may have a mercury content above 1 ppm. These two species accumulate mercury as they grow larger because they consume large amounts of small fish.
Commercial fishermen capture tuna and swordfish at sea, far from any source of industrial pollution.
The mercury in their system must come from natural sources. For years, we have probably eaten tuna and swordfish with mercury levels above FDA's limit without harmful effects. Analysis of museum specimens of tuna caught from 1879 to 1909 reveal that they contain levels of mercury as high as those in fish being caught today. Scientists therefore conclude that mercury levels in tuna, and probably swordfish, have not changed in the past 100 years. Why We Have Not Suffered From Eating Fish Researchers found that some fish, including tuna, can block and reduce the toxicity of mercury in their tissues.