Observations and simulations of interannual variability in the first migration of Bering Sea northern fur seal pups

Survival of young animals is an important ‘bottleneck’ or population regulatory mechanism in marine mammal species. In northern fur seals (NFS; Callorhinus ursinus), understanding and prediction of early survival is complicated by the species’ long winter migration across a wide range of habitats in the North Pacific Ocean. Further quantitative understanding of this migration for naïve, newly-weaned pups is critical for the development of environmental metrics that may explain their first winter survival and aid in NFS conservation. We describe efforts to simulate the migration of NFS pups from the Bering Sea using statistical models of animal movement, which are developed from satellite telemetry observations of five separate migrations. Observed pups show differing dispersal directions by year, with differences greatest between 10-50 days at sea. These differences are similar in character to the prevailing winds – prominently, strong westerly winds in 1997 and 2015 were associated with pup displacements to the east. This is consistent with previous evidence of wind-influenced dispersal and movement of pups within the Bering Sea. To facilitate simulation of pup migration, movements are modeled as a combination of background, wind-associated, and random components, where the background movements vary spatially; simulations based on these models have realistic spatial distributions and good skill in reproducing the observed differences by year. These results are consistent with the hypothesis that observed interannual differences in pup habitat use are largely due to the winds, or more specifically, the interaction between winds and the background mean movement. The exact mechanisms underlying either the background or wind-associated movements are not presently known. The simulations’ skill in reproducing observed migratory patterns indicates that they are a valid tool for hindcasting dispersal and habitat use in years without satellite tracking, which would allow for comparison to historical and ongoing time series of NFS pup survival estimates.