Unexpected Changes in Reproductive Rates and Mean Age at First Birth during the Decline of the Pribilof Northern Fur Seal (Callorhinus ursinus)

From 1956 to 1968, female northern fur seals (Callorhinus ursinus) were harvested on the Pribilof Islands, Alaska, in an effort to increase the productivity of the herd. In theory, pregnancy rates should have increased and the age at first birth should have declined as population density was reduced. Instead the opposite happened: pregnancy rates dropped and age of first birth increased. It is unlikely that these changes were caused by shortages of food or poor physical condition of the females, given that body size increased over this period. The most likely explanations for the changes observed between 1958 and 1974 are related to altered age and sex ratios of breeding animals caused by the depletion of females and/or the harvesting of young males. Changes in pregnancy rates and age at first birth are inconsistent with the density-dependence paradigm and suggest that relative densities of mature age and sex classes on the breeding beaches (a product of social interactions and territory size) may be more consequential than absolute population densities in affecting the reproductive biology of northern fur seals.