Fine-scale habitat selection of crabeater seals as determined by diving behavior
J.M. Burns, M.A. Hindell, C.J.A. Bradshaw, and D.P. Costa
Previous studies within the Marguerite Bay Region of the Antarctic Peninsula (~67ºS, ~67ºW) demonstrated that crabeater seals (Lobodon carcinophagus) were not randomly distributed across available habitat, but instead were more likely to be located in nearshore waters where bathymetric gradients and ice concentrations were high. Here we investigate how the diving patterns of crabeater seals vary in response to these habitat characteristics, and interpret seal behaviors in light of information on the distribution of their primary prey, Antarctic krill (Euphausia superba). Diving and movement patterns were obtained from 34 seals (16 M, 18 F) fitted with Satellite-Relay Data Loggers (SRDLs) during the 2001 and 2002 Southern Ocean GLOBEC cruises. Tags transmitted position and dive information for 4 to 174 days, during which time we received an average of 21 positions/day, and information on a total of 124,681 dives. To account for diel variation in diving behavior, daytime and nighttime behaviors were analyzed separately. Overall, generalized linear mixed-effects models indicated that seals dived significantly deeper (158 vs. 73 m) and longer (7.2 vs. 6.0 min) during the day than at night. In addition, seals spent a greater proportion of the dive at the bottom (31 vs. 29 %), and dived with greater efficiency (24 vs. 21 %) during the day. However, seals spent relatively more time at a given depth during the night than during the day, suggesting that either prey were more difficult to catch under low-light conditions, or that seals were more successful at foraging during night. When dive patterns were examined with respect to bathymetry, generalized additive models indicated that seals were foraging closer to the bottom and over shallower waters during the day than at night (341 vs. 398 m). In combination, these findings suggest that crabeater seals foraging during the day used shallower locations where they could exploit zooplankton schools which become compressed close to the bottom. At night when zooplankton were dispersed and light levels low, foraging activity was less frequent and seals concentrated their diving closer to the surface over a broader range of habitat depths. As individual seals moved an average of only 4.1 ± 1.4 km between daytime and nighttime positions, these results suggest that crabeater seals diving along the Western Antarctic Peninsula selected areas of high bathymetric gradients so that they can maximize foraging success over a 24-hour cycle without the need to travel long distances.