A mystery disease almost wiped out seals entirely from the UK and scientists at the time had no idea why.
The tiny island of Anholt in the Baltic Sea was where the outbreak began in April 1988
But it soon started to spread to the Wadden Sea around Denmark, Germany and The Netherlands, where it killed 10,000 of them.
By August it was found off the coast of Norfolk where it claimed 6,000 victims.
Scientists were left baffled by the cause
They started by trying to establish whether it was the same type of disease that was killing seals in both the Baltic states and off the North Sea.
Greenpeace was forced to call an emergency conference in London to discuss ways of fighting the outbreak.
Fears the entire seal population would die out were very real.
The deaths of so many seals – 20% of the population in the Baltic and North Sea – left many pups vulnerable to predators and so more of them were killed.
Following the conference, scientists identified two viruses – one was a herpes virus and the other similar to a virus that causes polio in humans.
By the time of 1989 scientists named the disease phocine distemper virus (PDV).
Even before this deadly outbreak, it was predicted harbor seals were the most at risk.
Even after the outbreak, 80% of female seals in the Baltic Sea became sterile from the chemical pollution.
Seal hunts also resulted in drastically reduced numbers in the North Sea.
Eventually an agreement was signed in 1990 between countries including Denmark, Germany and the Netherlands to protect seals in the Wadden Sea.
But by May 2002 the disease came back with a vengeance.
Healthy seals from Hunstanton Sea Life Sanctuary were moved to Scarborough Marine Sanctuary which acted as a safe haven for pups and resident seals.
But despite this the overall number of dead seals reached 18,300 by the time the disease eventually stopped spreading.
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