Population development of harbour seals Phoca vitulina in the Wadden Sea after the 1988 virus epizootic

The mortality as a result of the 1988 virus epizootic amongst harbour seals Phoca vitulina in the North and Baltic Seas is estimated at 60% in the entire Wadden Sea. In the years 1989–1994, a prosperous recovery of the population has been observed which is reflected in a high post-epizootic rate of increase. The average annual rate of increase for the entire area was 16%, highest in The Netherlands (average 21%) and lowest in Denmark (average 10%). This rate of increase is significantly higher than in the pre-epizootic period 1976–1987, when the population increased at a rate of around 9% per year. The difference is partly attributed to a considerably lower initial juvenile mortality after the epizootic. The present first-year mortality is statistically significantly lower than in the 1970s. It is about equal in all regions and estimated at approximately 40%, whereas it was approximately 65% in the 1970s. The post-epizootic reproductive rate in The Netherlands is significantly higher: 21% on average, against 13% before 1988; it is also higher in Niedersachsen (21 vs. 16%), but lower in Schleswig-Holstein (19 vs. 23%), while in Denmark it has not changed (both 17%). It is hypothesised that the improved reproductive rate in The Netherlands might be a result of selective mortality during the epidemic, which within the adult female segment would have predominantly affected those not reproducing. It is emphasised that though the population is recovering well, its size of almost 8800 animals in 1994 is still only one quarter of an estimated reference figure of 37 000 seals around 1900. How long the recovery will continue at its present rate depends on environmental conditions in the area, such as pollution, disturbance and food resources; nor can a recurrent flickering of the epidemic be excluded.