Regional Variation in Winter Foraging Strategies by Weddell Seals in Eastern Antarctica and the Ross Sea

Abstract

The relative importance of intrinsic and extrinsic determinants of animal foraging is often difficult to quantify. The most southerly breeding mammal, the Weddell seal, remains in the Antarctic pack-ice year-round. We compared Weddell seals tagged at three geographically and hydrographically distinct locations in East Antarctica (Prydz Bay, Terre Adélie, and the Ross Sea) to quantify the role of individual variability and habitat structure in winter foraging behaviour. Most Weddell seals remained in relatively small areas close to the coast throughout the winter, but some dispersed widely. Individual utilisation distributions (UDi, a measure of the total area used by an individual seal) ranged from 125 to 20,825 km2. This variability was not due to size or sex but may be due to other intrinsic states for example reproductive condition or personality. The type of foraging (benthic vs. pelagic) varied from 56.6 ± 14.9% benthic dives in Prydz Bay through 42.1 ± 9.4% Terre Adélie to only 25.1 ± 8.7% in the Ross Sea reflecting regional hydrographic structure. The probability of benthic diving was less likely the deeper the ocean. Ocean topography was also influential at the population level; seals from Terre Adélie, with its relatively narrow continental shelf, had a core (50%) UD of only 200 km2, considerably smaller than the Ross Sea (1650 km2) and Prydz Bay (1700 km2). Sea ice concentration had little influence on the time the seals spent in shallow coastal waters, but in deeper offshore water they used areas of higher ice concentration. Marine Protected Areas (MPAs) in the Ross Sea encompass all the observed Weddell seal habitat, and future MPAs that include the Antarctic continental shelf are likely to effectively protect key Weddell seal habitat.

Assessing the potential for resource competition between the Kerguelen Plateau fisheries and southern elephant seals

“…However, the nature of the interaction in the fishing ground is unclear and is likely to remain so until a more comprehensive understanding of elephant seal diet is available. Nonetheless, it is likely that toothfish are a minor component of their diet in the fishing grounds because toothfish are not a commonly reported prey item overall (Slip, 1995) and the bottom waters deeper than 500 m tend to be home to larger and older fish (Farmer et al, 2019), which may be too hard for the seals to catch (but similar sized Weddell seals, Leptonychotes weddellii, do capture some large fish Harcourt et al (2021)). Ice fish are also patchily distributed throughout the fishing grounds and are not targeted in the French EEZ).…”

“…We focused on the post-molt trips of adult females and subadult males as these were age and sex classes for which we had sufficient data to reliably characterize their use of the Kerguelen Plateau. The analysis was raster-based, using a grid of 5 x 5 km cells (Harcourt et al, 2021). This grid size was slightly coarser than the location uncertainty of elephant seal positions after state-space modelling .…”

Elephant seal foraging success is enhanced in Antarctic coastal polynyas

“…Following other recent ecological studies [16,55], we used the East Antarctic coastal polynyas areas identified by Arrigo & van Dijken [13] (as shown in their fig. 1b).…”

Sea Ice Elevation in the Western Weddell Sea, Antarctica: Observations From Field Campaign

“…The discrepancy can also be caused by uncertainty in the ACF estimation of α , or by biases from the regression fitting of grid bathymetry data (Smith & Sandwell, 2004). Note that our knowledge of bathymetry in the Southern Ocean is very limited (Millan et al., 2020), and the GEBCO bathymetry may have some uncertainty as some recorded seal dive data show deeper sea floor than that provided by GEBCO (Harcourt et al., 2021). The results highlight the requirements of in‐situ measurements (such as CTD, ocean current, wind speed and direction, high‐resolution sonar scans, etc.)…”