PERSISTENCE THRESHOLDS FOR PHOCINE DISTEMPER VIRUS-INFECTION IN HARBOR SEAL PHOCA-VITULINA METAPOPULATIONS

1. This paper explores the concept of the critical community size for persistence of infection in wildlife populations. We use as a case stu dy the 1988 epidemic of phocine distemper virus in the North Sea popul ation of harbour seals, Phoca vitulina. 2. We summarize the available data on this epidemic and use it to parameterize a stochastic compartm ental model for an infection spreading through a spatial array of patc hes coupled by nearest-neighbour mixing, with replacement of susceptib les occurring as a discrete annual event. 3. A combination of analytic al and simulation techniques is used to show that the high levels of t ransmission between different seal subpopulations, combined with the s mall annual birth cohort, act to make persistence of infection impossi ble in this harbour seal population at realistic population levels. Th e well known mechanisms by which metapopulation structures may act to promote persistence can be seen to have an effect only at weaker level s of spatial coupling, and higher levels of host recruitment, than tho se empirically observed.