Meeting Abstract
Mammals undergo major dietary changes upon weaning, however their life histories can make it difficult to observe foraging behaviors in nature. Northern fur seals (Callorhinus ursinus; NFS) are primarily in the Bering Sea and North Pacific Ocean, and roughly half the world’s population breeds on the Pribilof Islands, Alaska. Pups are born in July, nurse intermittently for ~4 months, then wean and depart the island for an extended pelagic migration. The often cryptic, pelagic foraging of NFS makes them ideal candidates for stable isotope analysis (SIA) which has been used to describe the foraging behaviors and habitat use of adults and juveniles. Because pups rely on their mother’s milk before weaning, we assumed their stable isotope values would reflect a higher trophic level than that of their mothers throughout their nursing period. When pups begin consuming a likely diet of small fish and crustaceans, we expected to see lower stable nitrogen isotope values (δ15N) in pup tissues. The degree to which pups forage for prey to supplement milk consumption, however, or whether they abruptly switch when the migration begins, is unknown. To estimate this shift, we collected milk and prey contents from stomachs, vibrissae, muscle, liver, and serum samples from pups taken during subsistence harvests on St. George Island, Alaska, throughout the nursing period (Sept, n=5; Oct, n=31; Nov, n=21). NFS vibrissae grow throughout their lifetimes, so they provide a continuous record of assimilated diet. SIA of vibrissae segments allowed for a chronological measure of pup diet from in-utero to weaning, whereas SI values from liver and serum provided assimilated diet data from the week previous to sample collection. We compared stable isotope values from pup tissues to those from NFS milk and other prey collected from their stomachs to determine how well SIA captures the weaning process.
