EPIZOOTIOLOGY OF THE SEAL DISEASE IN THE EASTERN NORTH-SEA

1. The intensity of the 1988 epizootic among harbour seals, Phoca vitulina, present in the vicinity of haul-out sites in the Kattegat-Skagerrak was estimated as more than 95%. 2. More males than females died during the first half compared with the last half of the epizootic period at six localities. In the year-classes of seals more than 8 months old, 54% of the dead seals examined were males. 3. The epizootiology was analyzed by use of a discrete stochastic model. The results suggested that, in contrast to what is predicted by most standard models, no fixed threshold size of seal herd (within a size range of 80-1500 animals) is required for an epizootic to expand. 4. The disease developed in a similar way in herds of different size, indicating that herd size as such had little effect on the course of the epizootic. Widely separated seal herds showed similar dynamics of the epizootic. The most probable reason for this development is that harbour seals usually aggregate densely, and rates of contact between individuals are high. 5. The model was tested by simulation. Random events in the development of the disease in 95% of the cases affected the probability of infection by a maximum of (+/-) 6.3%, and the intensity of the epizootic exceeded 95% in most cases. 6. Distance between infected seal herds was inversely proportional to dispersal rate of the disease.