The Survival Dynamics of Male Northern Fur Seals (Callorhinus ursinus) on Tyulenii Island, Sea of Okhotsk, Based on Long-Term Observations

Abstract

We discuss and improve the methods proposed previously for estimating the survival rate of male northern fur seals representing different age groups. Estimates of the number of pups and bulls per rookery are given, as well as information on the age structure of animals hunted during the coastal harvest. The data were obtained by researchers from the Pacific Research Fisheries Center over 56 years of observation of the fur seal (Callorhinus ursinus) herd on Tyulenii Island, located in the southwestern part of the Sea of Okhotsk, south of Cape Terpenia, 15 km from Sakhalin Island. Lander’s method and its modifications were used for estimating the juvenile survival rate of male fur seals. This methodology was found to have not been working properly for the past several decades (since the end of the 1980s) due to changes in population harvesting. Satisfactory estimates for all characteristics of the male life cycle were obtained. Structural changes in survival ability were revealed as having occurred at the end of the 1980s, i.e., the survival of subadult and adult males increased slightly. New estimates of survival rates allow us to create a model of the dynamics of bull numbers that is in good agreement with that observed, its mean error of approximation equaling 3.2%.