They found no change in methylmercury levels in the tuna over that 27-year period.
The researchers predicted that mercury in the surface waters should have increased by up to 26 percent during this time, according to a computer model. The model took account of the change in atmospheric mercury, the sub-equatorial Pacific waters and the potential for mixing in the "thermocline" a transition layer in the ocean where temperature changes rapidly.
The findings imply that the high levels of methylmercury in these fish are not coming from increased pollution, but from a natural source. The specific source is not yet clear, Morel says, but he suggests it could be hydrothermal vents and deep ocean sediments.
The research should also extend to other ocean-going predatory fish, like swordfish and sharks, according to Morel, which could mean that whatever is passing the mercury up to the tuna is probably doing the same to these other fish.
Morel is more cautious, however, about extending the findings to coastal fish. Bluefish, for example, run up and down along the eastern coast of the United States feeding on the continental shelf, and they may be taking up human pollution there. Lake fish are also a different situation, Morel says, since scientists have established a strong link between pollution and mercury levels in lakes.