POPULATION DEVELOPMENT OF HARBOR 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 recover y of the population has been observed which is reflected in a high pos t-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 significantl y higher than in the pre-epizootic period 1976-1987, when the populati on increased at a rate of around 9% per year. The difference is partly attributed to a considerably lower initial juvenile mortality after t he epizootic, The present first-year mortality is statistically signif icantly lower than in the 1970s. It is about equal in all regions and estimated at approximately 40%, whereas it was approximately 65% In th e 1970s. The post-epizootic reproductive rate in The Netherlands is si gnificantly higher: 21% on average, against 13% before 1988; it is als o higher in Niedersachsen (21 vs, 16%), but lower in Schleswig-Holstei n (19 vs, 23%), while in Denmark it has not changed (both 17%). It is hypothesised that the improved reproductive rate in The Netherlands mi ght be a result of selective mortality during the epidemic, which with in the adult female segment would have predominantly affected those no t reproducing, It is emphasised that though the population is recoveri ng well, its size of almost 8800 animals in 1994 is still only one qua rter of an estimated reference figure of 37 000 seals around 1900. How long the recovery will continue at its present rate depends on enviro nmental conditions in the area, such as pollution, disturbance and foo d resources; nor can a recurrent flickering of the epidemic be exclude d.