Mammal zyxwvutsrqpon Rev. zyxwvutsrqpo 1993, Volume 23. Nos 314. zyxwvut 149-154. Printedin Great Britain. Seals and epizootics zyxw - what factors might affect the severity of mass mortalities? zyxw PAUL M. THOMPSON & AILSA J. HALL* Aberdeen University, Department zyxwvu of Zoology, Lighthouse Field Station, Cromarty, Ross-shire IVIl 8YJ and *Sea Mammal Research Unit, clo British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 OET, UK INTRODUCTION When over 18 OOONorth Sea Common Seals Phoca vitulinadied in 1988,the event was widely reported as a man-made disaster. Subsequently, it was shown that the primary cause of death was morbillivirus infection (Kennedy ef al.. 1988; Osterhaus el al.. 1988). and that the same or similar viruses were widespreadamongst marine mammals from other regions (Heide-Jergensen etal., 1992).Furthermore, reviews of the historical literature suggested that several mass mortalities had occurred previously in UK waters (Harwood &Hall, 1990)and severalpopulations have shown signs of arapid recovery since 1988 (ICES, 1992). Was the suggestion that human activities had caused the disease just another example of a media industry hungry for disaster, or are there genuine reasons to zyxwv he concerned that anthropogenic factors such as pollution may have exacerbated the outbreak? This paper provides a brief history of the 1988 epizootic. We then explore the potential role of natural and man-made factors in causing variations in the severity of the outbreak and describe some of the consequences of the mass mortality on North Sea seal populations. Finally, using the experience gained from the 1988 epizootic, we highlight areas where further research is required in order to determine the relative importance of various factors affecting the dynamics of UK seal populations. HISTORY OF THE 1988 EPIZOOTIC The disease was first observed in April 1988 when an unusual number of abortions occurred amongst Common Seals in the central Kattegat. Within a few weeks, older seals were also found dead as the disease spread throughout the Kattegat and into the Danish and Dutch Waddensea. By June and July, seals from the Skaggerak, German Waddensea, southern Baltic and Oslo fiord were also affected and in August deaths were reported from United Kingdom waters. In most areas, the majority of deaths were reported over a period of only 30-1 15 days (Dietz, Heide-Jergensen & HSrkonen, 1989) and only in the Dutch and German Waddensea and Northern Norway was there any evidence of further small scale outbreaks in 1989 (Krogsrud et al., 1990). By late August 1988,the primary cause of the deaths had been identified as a morbillivirus infection (Kennedy efal., 1988;Osterhaus etal., 1988).Further studiesproved this to be anewly identifiedvirus, similar to but distinct from canine distemper virus (Cosby et al., 1988; Mahy ef al., 1988). It was subsequently termed Phocine Distemper Virus (PDV). Although PDV was the primary cause of the disease throughout the North Sea, the secondary pathogens which were often the final cause of death differed between areas. Studies were carried out on the nature of the disease in many affected areas; reviews of the now extensive literature on this aspect of the epizootic can be found in Kennedy (1990) and Heide-J~rgensen et al. (1992). Although the overall death rate amongstthe North Sea Common Seal population was high,theextent of mortality varied between areas. In Dutch and Danish waters, mortality approached 60%, whereas in the UK it varied from 50% in the Wash to 15% or less around the Scottish coast. There were also