Fine-scale habitat selection of crabeater seals as determined by diving behavior

Abstract

Previous studies within the Marguerite Bay region of the Antarctic Peninsula (∼67°S, ∼67°W) demonstrated that during winter, 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, krill (Euphausia superba or Euphausia crystallorophias). Diving and movement patterns were obtained from 34 seals (16 male, 18 female) fitted with satellite-relayed data loggers (SRDLs) during the 2001 and 2002 Southern Ocean GLOBEC cruises. Tags transmitted position and dive information for 4-174 days, during which time we received an average of 21 positions/day, and information on a total of 124,681 dives. A series of generalized linear mixed-effect models (GLMM) were used to evaluate the relationship between diving behavior and temporal and physical features of the habitat, and models contrasted using AICc and BIC weights. Overall, we found that the most parsimonious models included year, month, and period (day, dusk, night). In general, seals dived deeper (158 vs. 73 m) and longer (432 vs. 360 s) during the day than at night. In addition, daytime dives included slightly more time at the foraging depths (142 vs. 102 s), and were slightly more efficient (24% vs. 21% of the dive cycle spent at the bottom). When dive patterns were examined with respect to bathymetry, models indicated that seals were foraging in shallower waters (366 vs. 410 m) and closer to the bottom (dives were 50.3% vs. 26.3% of bathymetric depth) during the day than at night. In combination, these findings suggest that crabeater seals foraging during the day exploited zooplankton schools compressed along 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 select areas of high bathymetric gradients so that they can maximize foraging success over a 24-h cycle without the need to travel long distances. However, annual differences in behavior and the generally low amount of deviation explained by models also suggests that seals vary their diving behavior in response to finer-scale biological, temporal, and/or physical features that were not monitored as part of this study. © 2008 Elsevier Ltd. All rights reserved.