The study, reported by top Dutch scientists in the journal of the Royal Swedish Academy of Science, is the first to show that pollution seriously threatens the sea's mammals.
The death of the seals is only one of a chain of disasters to have hit the shallow, semi- enclosed sea. Others include the starvation of 100,000 seabirds in February and the gradual extinction of some shellfish because pollution is causing them to change sex.
Top scientists from all eight North Sea countries met in Copenhagen last week to assess the state of the waters that divide and unite them.
In the late Eighties, public opinion blamed the deaths of 6,000 seals on pollution. The new Dutch research backs this up. It shows that pollution depresses seals' immune systems, which makes them vulnerable to disease and enables epidemics to take hold.
The scientists' conclusions were reached after they took 22 seals and split them into two groups, with one being fed reasonably uncontaminated fish from the Atlantic, the other (after at first refusing to touch the stuff) having to eat fish from a polluted part of the Baltic.
The second group's resistance to disease fell dramatically and stayed low throughout the two-year experiment.
The research adds to mounting evidence that sinister changes are taking place in the North Sea. About 164 million people live in its catchment area; 300 oil and gas platforms dot its waters; and 420,000 ships pass through it each year. It is surrounded by some of the most industrialised countries, and its rich fisheries are among the most over-exploited.
Last week's Copenhagen gathering examined a new 'North East Quality Status Report' drawn up by the coastal governments. It is a closely negotiated document which takes great care not to offend any of the North Sea nations. But it does highlight that the North Sea is running out of fish, due to over-exploitation.
Every year more than half of the cod and haddock in the sea are caught: 70 per cent of young cod are netted even before they begin to mature. As these fish have declined, the boats have moved on to exploit smaller species for meal.
Meanwhile, every year the sea receives more than 30 tons of mercury, more than 125 tons of cadmium, and some 3,000 tons of lead, among other toxic metals. Pesticides and polychlorinated biphenyl (PCB), the chief suspects in seal mortality, add to the witch's brew, which spins round in the giant oceanic cauldron carried by currents. It takes about two years for a drop of water to escape from the land-bound sea, so the pollution has plenty of time to mix, concentrate, stick to the seabed and do its damage.
Much of it accumulates on the Dogger Bank, where about a third of the fish are born deformed and more than half have abnormalities in their chromosomes. Unsurprisingly, fish are worst affected. In areas where there is most pollution, more than 80 per cent of the fish embryos in the southern part of the sea are deformed and diseased.
Comparison of today's terns with museum specimens stuffed at the beginning of the century shows that they have nearly four times as much mercury in their feathers.
Tributyltin, used as anti-fouling paint on ships' hulls, is driving the common dog whelk to a bizarre extinction. It makes the females grow penises and sperm ducts, putting an end to reproduction. In many areas there are now no females to be found, and the shellfish are dying out. Chemicals from fertilisers sweep off the land and down the rivers, nourishing plagues of algae. In 1988 a slick of toxic algae six miles wide and 30ft deep spread through the eastern North Sea, blighting 120 miles of coast and killing millions of fish.
Four years ago, the North Sea countries, reeling from the mass seal deaths, agreed drastic cuts in the pollutants they released to the sea. But some of these targets now seem unlikely to be met, and others are increasingly looking over-modest. Next year they meet again, but there is, as yet, little sign that they plan more rigorous action.
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