Diet and movement behaviour of Weddell seals in the deep southern Weddell Sea

Horst.Bornemann [ at ]


We used stable carbon (δ13C) and nitrogen (δ15N) isotope analysis of whiskers as well as tracking data to assess dietary and seasonal movements of five Weddell seals (Leptonychotes weddellii) in the deep Weddell Sea and compared results to populations in other areas of Antarctica. The greatest overall distances were travelled in autumn and winter during periods of intense foraging to build up energy reserves lost during summer and spring. Shorter distances were travelled in spring and summer during periods of annual moult and breeding where seals spent a greater proportion of time hauled out on the ice. Seasonal variation in total distance travelled per day between sexes was driven by different energetic demands of territory defence for males and reproductive gestation and lactation for females. Haul out behaviour switched from a primarily nocturnal behaviour (autumn and winter) to a diurnal pattern (summer and spring) in response to Pleurgramma antarcticum, Weddell seals primary prey source, diel movement in response to changing light and nutrient availability. Weddell seals in the deep Weddell Sea prioritised the use of polynyas in winter and autumn as productive foraging areas and breathing holes, usage declined during spring although the edges were still used as suitable haul out substrates. A high average δ13C value (-21.43 ‰) compared to populations in the Ross Sea(-24.3 ‰- -22.5‰) and similar δ13C compared to the Western Antarctic Peninsula(-22.4‰ - -20.1‰) highlights the seals preference for foraging near productive coastal zones and in enriched benthic communities and is influenced by the δ13C latitudinal gradient in the Southern Ocean. Weddell seals in the study area are top trophic predators and consume similar prey types as revealed by average δ15N (13.85 ‰) values similar to Weddell seals in other areas of Antarctica. Variation in SEAc overlap was apparent in four out of the five sampled seals. Differences in deployment location and sea ice extent influenced the overlap; AF1, deployed on the Ronne ice shelf in the Weddell Sea was limited by extensive sea ice cover and did not overlap with any of the other seals, all deployed off the Filchner ice shelf. This study is the first to assess Weddell seals behaviour in the southern deep Weddell Sea, a largely inaccessible area due to extensive year-round ice cover. Our results reveal foraging and movement behaviour similar to populations of Weddell seals in other areas of Antarctica, where variation occurred it was as a result of differences in hydrographic features, ocean chemistry, and individual life history traits such as age and sex.

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Webb, K. (2019): Diet and movement behaviour of Weddell seals in the deep southern Weddell Sea , Bachelor thesis, Mammal Research Institute, Department of Zoology and Entomology, University of Pretoria.

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