Variations in behavior and condition of a Southern Ocean top predator in relation to in situ oceanographic conditions

Abstract

Responses by marine top predators to environmental variability have previously been almost impossible to observe directly. By using animal-mounted instruments simultaneously recording movements, diving behavior, and in situ oceanographic properties, we studied the behavioral and physiological responses of southern elephant seals to spatial environmental variability throughout their circumpolar range. Improved body condition of seals in the Atlantic sector was associated with Circumpolar Deep Water upwelling regions within the Antarctic Circumpolar Current, whereas High-Salinity Shelf Waters or temperature/salinity gradients under winter pack ice were important in the Indian and Pacific sectors. Energetic consequences of these variations could help explain recently observed population trends, showing the usefulness of this approach in examining the sensitivity of top predators to global and regional-scale climate variability.

Fig. 1.

Orthographic view of the SO. (Upper) The four sites of instrument deployments are indicated by the filled black circles (SG, South Georgia; KE, Kerguelen Islands; MI, Macquarie Island; LI, Livingston Island), whereas the black lines represent mean locations of the major ACC fronts. From north to south, these include the Subtropical Front (dotted-dashed line) and SAF (dotted line) taken from Orsi et al. (36), followed by the PF (solid line) and SACCF (dashed line), taken from Moore and Abbott (51), the latter modified in the Scotia Sea region by data from Argo floats and CTD-SRDLs deployed on seals in this study (L.B., S.T., M.M., M.B., and M.A.F., unpublished work). The Weddell Sea (WS), East Antarctica (EA), Ross Sea (RS), and Bellingshausen Sea (BS) are also indicated. (Lower) The circumpolar movements of 85 southern elephant seals between January 2004 and April 2006. Colors represent tracks from South Georgia (dark blue), Kerguelen (green), Macquarie (light blue), and the South Shetlands, Antarctica (red). Note the contrast between seals in the Atlantic sector showing a preference for ACC waters compared with the rapid southerly migrations by most Kerguelen and Macquarie seals across ACC waters toward the continental margin of East Antarctica or into the Ross Sea. The longest track (326 days) is shown in black.

Fig. 2.

Circumpolar interpolated surface map of weighted mean nighttime dive depths of southern elephant seals.

Fig. 3.

Circumpolar map of physiological changes during winter migrations of elephant seals. Daily change in drift rate was calculated for 36 individuals during their winter migrations in 2004 and 2005. Blue shading represents a decrease in vertical change in depth during passive drifts, indicating reduced relative lipid content, whereas green–red shading indicates increased vertical depth change and increasing relative lipid content. Interpolated surfaces were created by using the same mapping as that used for Fig. 2. Differences in coverage between here and Fig. 2 are a result of the fact that drift dives (for which vertical change of depth during passive drifts can be calculated) represent only ≈8–10% of all dives. Thus, this surface is calculated based on a smaller data set than those in Fig. 2.

Fig. 4.

In situ θ-S measurements collected by instruments deployed on southern elephant seals at three of the main locations (South Georgia, Kerguelen, and Macquarie Island). The curved dotted lines indicate the water density corresponding to these θ-S properties. The red surfaces represent kernel densities of θ-S properties at the bottom of dives. Initially, two density surfaces were created for each location: one using only those dives occurring during periods of positive change in drift rate (i.e., periods of increasing relative lipid content) and the other based on dives during periods of negative change. The displayed surfaces represent the positive minus negative density surfaces, and the color intensity therefore highlights areas of predominantly increasing lipid content. Kernel surfaces were created by using a 50 × 50 grid over the range of θ and S, yielding a resolution of 0.056 × 0.199 for θ and S, respectively.

Fig. 5.

Generalized section of the SO, highlighting areas where southern elephant seals are predicted to change their relative body fat stores. We used typical sections of temperature and salinity from the Levitus 1° data set (52). The derived potential density values were matched to those in table 2 in Heywood and King (37) to highlight the main water mass boundaries between AAIW, Upper Circumpolar Deep Water (UCDW), and Lower Circumpolar Deep Water (LCDW). Colored contours represent the accumulated number of matches between temperature, salinity, and derived density values obtained from seals, and corresponding values in the schematic hydrographic section. We used only values at the deepest point of each profile, and only those profiles obtained during periods of positive change in drift rate. The arrows indicate the main circulation pathways as summarized by Toggweiler et al. (53). Note the preference for upwelling regions of Circumpolar Deep Water and water mass transformation regions adjacent to the Antarctic continent, and the avoidance of regions of AAIW subduction. The gradual deepening of predicted regions to the north agrees well with the patterns of daytime and nighttime dive depths shown in Fig. 2.