Why the population of the northern fur seals (Callorhinus ursinus) of Tyuleniy Island does not recover following the harvest ban: Analysis of 56 years of observation data

Following many years of managed harvest, the population size of Northern fur seal on Tyuleniy Island appeared to be depressed. In order to preserve the population, the harvest was first significantly limited, and then banned altogether. However, the birth rates have never recovered. We perform the estimation and analysis of survival rates of different groups of males in the population with the goal of elucidating the underlying changes in intra-population dynamics. We use the estimated numbers of adult males and pups in the rookery along with the exact numbers and ages of animals, harvested over the years. Our investigation shows that the unexpectedly slow growth of the population is most likely not caused by an abrupt decline of survival rates of any age groups. There is no substantial change in juvenile survival, and the survival rates of the older animals are actually increasing. However, the analysis of density dependent factors of juvenile survival demonstrates significant increase in intra-species competition. This may be caused by change in environmental conditions such as quality and availability of food. Also, the dynamics of pups' viability is telling: it was the highest at the beginning of the period of observations, and then it sharply decreased following intensification of harvest, and finally started growing again in response to strong conservation measures, but never achieved its original level. These facts may be consequence of changes in the population gene pool, which shows the importance of research of the evolutionary consequences of harvest. (C) 2017 Elsevier B.V. All rights reserved.