THE EFFECT OF STOCHASTICITY IN BIRTH AND SURVIVAL ON SMALL POPULATIONS OF THE HARBOR SEAL PHOCA-VITULINA L

Small groups of harbour seals are scattered along the entire Norwegian coastline and in some fjords. Some of these groups are separated by l ong distances from larger populations, and these groups may run a risk extinction due to demographic and environmental stochasticity. Here w e studied the viability of small groups of harbour seals assuming no m igration between the smaller, isolated groups and larger populations. We applied a stochastic population model and population parameters rec orded for harbour seals at the Norwegian coast. The model was divided into twenty-one different age categories (0-20 years of age), each wit h potential different survival and reproduction probabilities. Surviva l and reproduction were modelled at the level of the individual. The M inimum Viable Population (MVP) was defined as the minimum population s ize that ensures greater-than-or-equal-to 95 % probability that the po pulation will survive for a period of 100 years. For a hypothetical po pulation that was stable using a deterministic population model, the M VP was about 50 seals. In a population with annual growth of 1% the MV P was estimated at 30 seals. If hunting mortality of one seal per year was added to the latter population and the hunting mortality was rand om among all age groups of males and females, the MVP increased to abo ut 120 seals.