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Marine Scientists Fear for Future as European Seal Plague Subsides

John Pickrell
for National Geographic News
December 31, 2002
 
As many as 30,000 harbor seals in Europe may have died this summer from
a devastating epidemic of distemper. Though the current outbreak is now
subsiding, a new report suggests that if the disease strikes again
within the next few years, it could reduce European harbor seal
populations to near extinction levels.

The risk of catastrophic population collapses are increased by the scale of the mortality and the close proximity to the previous distemper epidemic in 1988, said Karin Harding, a biologist at Gothenburg University in Sweden.

Harding, along with colleagues at Gothenburg and the Woods Hole Oceanographic Institution in Massachusetts, are behind a report in the November issue of the journal Ecology Letters, which predicts that frequent recurrences of epidemics could threaten the long term survival of European colonies of harbor seals, (Phoca vitulina).



Researchers were alerted to this year's epidemic of phocine distemper virus (PDV) in May when dead seals washed up on Denmark's Anholt Island. By mid-September the disease had spread along the coastlines of Scandinavia and the Wadden Sea and as far as the eastern coast of England and Scotland. PDV is a virus related to both distemper in dogs and measles in people.

Shocking Death Toll

Current data suggests that around 22,000 animals have been found dead, but the real death toll may be up to 30,000 said Harding. The total harbor seal population in Europe was thought to number around 88,000 prior to the epidemic.

Scientists know of no simple way to stop the spread of the disease, which attacks both the respiratory and immune systems of seals.

"Harbor seals develop pneumonia that gets so bad that air is pushed out in the lung tissue," said Harding. "They get more and more air in the tissue and finally become inflated and can't dive anymore."

Reduced immunity leads to secondary viral and bacterial infections, she said.

Harding and her team were concerned about the relatively recent recurrence of the disease—just 14 years after the catastrophic 1988 epidemic in harbor seals. Using mathematical models and data from Swedish seal populations affected in both epidemics, the researchers plotted the course of this year's epidemic.

They also used the data to explore how the frequency and severity of PDV epidemics might impact future populations in Sweden and other areas.

"Little is known about epidemics in the wild, and how they affect the long-term survival of populations," said Harding.

The 1988 epidemic took the lives of at least 18,000 seals. Mortality this year has been slightly lower in some regions because the infection began to strike populations five weeks later in the year than in 1988.

The risk of infection is highest during the summer months when harbor seals gather in groups on land; therefore, the earlier an epidemic begins, the more damage it can cause.

However, the total seal population in 2002 was larger than in 1988, so more seals have died this year. Computer models indicate that in some seal colonies more than 50 percent of the seals have succumbed to the infection.

The findings also suggest that if epidemic outbreaks occur at 14-year intervals and fewer than 80 percent of the animals perish, then each seal population has a chance to recover. However, if an epidemic was to recur in the next seven years, it could potentially push European populations to the brink of extinction.

Most harbor seal populations will probably continue to grow if epidemics come only every 14 years or so, agreed Paul Jepson of London's Institute of Zoology in England. "However," he added, "we'd have reason for concern if the frequency of future epidemics increases."

Prior to the 1988 epidemic, harbor seal populations were still recovering after over-hunting for fur, meat, and oil in the 19th and early 20th centuries had drastically reduced their numbers.

English Harbor Seals Threatened

Though the disease has now subsided in much of Europe, dead seals continue to be found at a high rate in England and Scotland, said Ian Robinson, veterinary manager at the Wildlife Hospital of the Royal Society for the Prevention of Cruelty to Animals located in Norfolk, England.

Robinson worries that the consequences could be even more severe for the small population of seals found on the eastern coast of England.

"The population growth rate [here] is one hell of a lot lower than the one used in the model," said Robinson.

Though Swedish seals recovered rapidly from the 1988 epidemic, English harbor seals had barely returned to pre-epidemic levels 12 or 13 years later. Another epidemic anytime soon might put the "extremely threatened" colony at a very high risk of extinction, said Robinson.

However, most of the United Kingdom's harbor seals are found in Scotland, said Robinson, and these colonies have experienced much lower death rates.

Other species of seal, such as the gray seal, Halichoerus grypus, which travels extensively, and the arctic harp seal, Phoca groenlandica, may be carrying PDV between different populations of harbor seals. Both species are far less susceptible to the infection than harbor seals.

One of the most intriguing aspects of this year's epidemic is that it was first discovered in Anholt, said Harding. The Danish island was home to the first colony infected in 1988.

"We don't know why," she said, "but it can't be a coincidence."
 

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