Research blames Nature for contamination. Maria Burke reports.

Mercury in sea-fish could originate from natural sources rather than industrial pollution, according to Princeton University researchers.

They found that mercury levels in tuna off Hawaii had remained constant over the past 26 years and not increased as expected in line with rising levels of atmospheric mercury.

'People have assumed that the high mercury content in fish must be from pollution', says team leader Francois Morel.

'We have about tripled the mercury in the atmosphere, and therefore it should be tripled in the ocean, right? But maybe mercury that occurs in fish is a natural thing, and it may have been there all along.'

Fish absorb mercury from water in the form of methylmercury (MeHg).

Morel's team measured MeHg levels in yellowfin tuna caught off the coast of Hawaii in 1998 and compared their results to a similar study in 1971. The team found that levels in the tuna had remained relatively steady over the years. Significantly, their computer model had predicted an increase in MeHg concentration in tuna (and surface waters) between 9 per cent and 26 per cent from 1971 to 1998.

If sea-fish were absorbing MeHg derived from mercury deposited from the air into the upper levels of the ocean, then the tuna caught recently should have more MeHg than those caught years ago, says Morel.

Morel's team thinks that the MeHg could be coming from sulphate-reducing bacteria living in hydrothermal vents in the deep ocean.

The findings suggest, says Morel, that decreasing mercury pollution would not reduce mercury levels in tuna living in the open ocean. It would, however, reduce mercury in lakes and coastal waters, and the organisms that live there, he adds.

But other researchers are sceptical about Morel's conclusions. 'There are other possible explanations for the source of methylmercury in the oceans,' says biogeochemist Cynthia Gilmour at the Academy of Natural Sciences' Estuarine Research Center in Maryland. 'It could be that the tuna, which can travel long distances, are getting their mercury from another source, such as run-off from land; or it could be that changes in fishing practices have affected the fishes' diets.'