Winter habitat use and foraging behavior of crabeater seals along the Western Antarctic Peninsula

J.M. Burns, D.P. Costa, M. Fedak, C.J.A. Bradshaw, M.A. Hindell, G. McDonald, S.J. Trumble, E. Chittick, M. Gray, N. Gales, J. Barnes, S. Shaffer, K. Kuhn, P. Lovell, and D. Crocker

As part of the U.S. GLOBEC research program, this study quantified winter and spring distribution and foraging behavior of adult crabeater seals (Lobodon carcinophagus), and the influence that oceanographic features and prey aggregations have on the foraging strategies employed. To address these questions, we outfitted 34 seals (16 M, 18F) with Satellite Relay Data Loggers (SRDLs, mfg. by SMRU) in the Marguerite Bay Region of the Antarctic Peninsula (~67ºS, 67ºW) during the austral winters of 2001 and 2002. Tags transmitted position and dive information for between 4 to 145 days. From these data we determined the activity and diving patterns of individual seals, and examined spatial patterns of habitat use. Overall, winter activity patterns differed significantly from prior studies undertaken during summer months: seals in this study dove deeper (91.7 m ± 0.2 m, range 6-712.5 m) and longer (5.26 min ± 0.6, range 0.2-23.6 min), hauled out during the night rather than the day, and showed seasonal shifts in foraging patterns that suggested seals were selectively foraging on vertically migrating prey resources. However, these patterns were more pronounced in 2001 than in 2002, which suggest that seals in 2001 were relying more on non-krill prey resources. This corresponds with a lower abundance of krill in 2001 than in 2002. In contrast to annual variation in diving patterns, there were no strong differences in patterns of habitat use despite much greater ice cover in 2002 than in 2001. While some animals made long distance movements (furthest movements 664 km to northeast, 1147 km to southwest), most seals remained within 300 km of their tagging location. Within the Marguerite Bay/Crystal Sound region, seals appeared to select foraging location based on seafloor depth. Individuals were found in regions were bottom depth was between 50 and 450 m depth more often than expected given the distribution of seafloor depths in the available habitat. In both years, seals remained deep within the pack ice throughout the winter, and did not appear to select regions with less ice cover. General Linear modeling showed a trend for seal use to be related to shallow depths, higher bathymetric gradients, and higher sea ice concentrations. In combination, these findings suggest that crabeater seals alter their behavior to accommodate seasonal or annual fluctuations in oceanographic features and the distribution of prey, and that studying their behavior will shed light on how marine predators optimize behavioral patterns in the face of temporal and spatial variability in prey availability.

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