Seasonal diving behaviour in lactating subantarctic fur seals an AmsterdamIsland

Diving behaviour was investigated in female subantarctic fur seals (Arctoce phalus tropicalis) breeding on Amsterdam Island, Indian Ocean. Data were co llected using electronic Time Depth Recorders on 19 seals during their firs t foraging trip after parturition in December, foraging trips later in summ er, and during winter. Subantarctic fur seals at Amsterdam Island are noctu rnal, shallow divers. Ninety-nine percent of recorded dives occurred at nig ht. The diel dive pattern and changes in dive parameters throughout the nig ht suggest that fur seals follow the nycthemeral migrations of their main p rey. Seasonal changes in diving behaviour amounted to the fur seals perform ing progressively deeper and longer dives from their first foraging trip th rough winter. Dive depth and dive duration increased from the first trip af ter parturition (16.6 +/- 0.5 m and 62.1 +/- 1.6 s respectively, n = 1000) to summer (19.0 +/- 0.4 m and 65 +/- 1 s, respectively, n = 2000) through w inter (29.0 +/- 1.0 m and 91.2 +/- 2.2 s, respectively, n = 800). In summer , subantarctic fur seals increased the proportion of time spent at the bott om during dives of between 10 and 20 m, apparently searching for prey when descending to these depths, which corresponded to the oceanic mixed layer. In winter, fur seals behaved similarly when diving between 20 and 50 m, sug gesting that the most profitable depths for feeding moved down during the s tudy period. Most of the dives did not exceed the physiological limits of i ndividuals. Although dive frequency did not vary (10 dives/h of night), the vertical travel distance and the time spent diving increased throughout th e study period, while the post-dive interval decreased, indicating that sub antarctic fur seals showed a greater diving effort in winter, compared to e arlier seasons.