Movement and diving patterns of crabeater seals during the Antarctic winter

J.M. Burns, D.P. Costa, M. Fedak, S. Trumble, M. Hindell, E. Chittick, N. Gales, and D. Crocker

As part of the US Southern Ocean GLOBEC program, we are studying the foraging ecology of crabeater seals (Lobodon carcinophagus) in Marguerite Bay (~68°S, 69°W) to determine how seals select foraging locations and optimize foraging success during the Antarctic winter when light and productivity levels are low. Crabeater seals tagged with high-resolution satellite relay data loggers (mfg. by SMRU) are demonstrating fundamentally different behaviors than those reported during summer months. Seals captured were much larger than those handled during the summer APIS cruises, had thick blubber reserves, and were clearly foraging successfully. Tracking data demonstrated that seals tagged within Marguerite Bay showed extremely directed movements to other regions, and concentrated their activities in regions of bathymetric discontinuity. The diving behavior indicated that the seals are expending considerable effort foraging: when in the water, seals spent more than half their time at depths > 6 m. The majority of dives were deeper than 100 m, and many of the dives approached the seafloor. In general, seals dove deepest during mid-day and shallowest at night. Seals did not haul out daily, but often went for several days without emerging onto the ice. One tag continues to transmit, and has recorded the first information on crabeater seal lactation strategies. Based on seal diving patterns, and trawl and acoustic prey surveys conducted by other SO GLOBEC participants, it seems likely that crabeater seals foraging in the winter months are supplementing their krill diet with fish. Diet samples, stable isotopes and fatty acid signatures will be used to resolve this issue. These findings emphasize the need to understand the year-round behaviors of seals when modeling trophic interactions and ecosystem dynamics.