Study authors Karin Harding and Tero Hrknen of Gteborg University and Caswell of WHOI have begun to explore how the frequency and severity of these seal plague outbreaks affect population dynamics. They note that while it is generally believed that survivors develop a life-long immunity to PDV, they estimate that no more than seven percent of the current European harbor seal population is made up of survivors of the 1988 outbreak and therefore believe immunity plays almost no role in the 2002 outbreak. The authors predict that 10,000 seals will die from PDV in 2002 in Sweden and Denmark alone.
"This is, to our knowledge, the first study where the consequences of PDV outbreaks have been evaluated in terms of quasi-extinction," the authors say. "The risk of a catastrophic decline of the population is small, but other factors, like hunting, will increase that risk, as will more frequent PDV outbreaks."
The authors note that PDV is spread by coughing, with the level of exposure to the virus dependent on how much contact there is between individuals. For example, colonies infected in late autumn, when seals spend less time on land, experienced low mortality rates while those infected in the summer when seals spent more than on land, had much higher mortality rates.
Noting that the 2002 PDV outbreak started more than a month later than the 1988 event, the authors predict that mortality rates will be more than 50 percent in the Dutch Wadden Sea and the southern Baltic region, with lower mortality rates in the German and Danish parts of the Wadden Sea, and substantially lower mortality rates in England, Scotland and Ireland. They predict the outbreak will subside in late November.
"PDV is now a recurring risk to harbor seal populations, they note. "It is important to begin to develop more elaborate population models that include age and sex structure, seasonal behavior, and other factors to help us monitor this situation."<
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Contact: Shelley Dawicki
Media@whoi.edu
508-289-3340
Woods Hole Oceanographic Institution
5-Nov-2002