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UBC Theses and Dissertations

Population dynamics of the Pribilof Islands North Pacific fur seal (Callorhinus ursinus) Trites, Andrew W.


A conceptual model is proposed that describes the dynamics of the Pribilof Islands North Pacific fur seal (Callorhinus ursinus). It is suggested that fur seals are regulated most strongly at population levels close to the limits set by the availabilities of food and breeding space. Population growth appears to be primarily regulated by changes in the rates of survival, reproduction, and dispersal of young animals. The proposed conceptual model is used to shape a mathematical formulation that incorporates basic fur seal life history features. The simulation analysis is able to explain the decline of the Pribilof Islands population by reconstructing pup estimates and counts of adult males over the period 1950 to 1980. Simulation results show that commercial female harvesting and a series of poor juvenile survival rates were responsible for the observed decline in pup production on Saint Paul Island. The lower number of bulls counted during the last two decades can be accounted for if adult male survival was higher during the 1950s. The decline in adult male survival, possibly due to debris entanglement, does not appear to be equally true of females. The major driving variable of the fur seal system and the key factor that accounts for the overall decline of the Pribilof herd is the survival of juveniles. In this regard, the continued population decline through the 1980s appears to be maintained by exogenous factors that are independent of current management practices. Model results are considered reliable if simulation parameters (particularly adult survival) contain little error. A detailed procedure for analysing the sensitivity of model output to errors in simulation parameters is described in an adjoining appendix as is a revised procedure for estimating the survival of juvenile fur seals. The thesis concludes with recommendations for future research.

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