Distribution and diving behaviour of crabeater seals (Lobodon carcinophaga) in the Weddell Sea

dominik.nachtsheim [ at ] awi.de


The crabeater seal (Lobodon carcinophaga) is the most abundant seal worldwide and inhabits the circumpolar pack ice zone of the Southern Ocean. Around half of its population is found in the Weddell Sea. By now, information on important environmental factors which affect their distribution as well as on geographic and seasonal differences in diving and foraging behaviour is limited due to the inaccessibility of their habitat. In austral summer 1998, a heterogeneous group of 12 crabeater seals belonging to both sexes and different age classes was equipped with satellite-linked time-depth recorders (SDRs) at Drescher Inlet (72.85°S, 19.26°E) in the eastern Weddell Sea. The transmitters provided data for a duration between 7 and 117 days (mean = 54.9 d). During this time the tagged seals dispersed radially in the Weddell Sea and covered large distances (mean = 1,763 km). To identify environmental variables which influence the distribution of crabeater seals and to predict suitable habitats, a maximum entropy (Maxent) modelling approach was implemented. It revealed that sea ice concentration mattered most in modelling species distribution with increasing probabilities of presence towards the ice edge. However, seals spent an unusually high amount of 64.4% in open waters and were only occasionally found in ice-covered zones during the study period. This is likely to be related to the comparatively low sea ice cover of the Weddell Sea in summer 1998. Although crabeater seals are generally closely associated with pack ice, it seemed to be that they can deal better with open water conditions than previously thought. Further important factors identified by Maxent were surface temperature, water depth and distance to the shelf break. All these four environmental variables are known to influence and determine the distribution of Antarctic krill (Euphausia superba), the preferred prey of crabeater seals. In general, predicted suitable habitats were congruent with probable habitats of krill. Beside geographic locations, satellite-linked data logger record dive data. Diving behaviour in this thesis was characterized by short (>90% = 0 – 5 min) and shallow (>80% = 0 – 72 m) dives. This pattern reflects the typical summer and autumn diving and foraging behaviour of crabeater seals since krill is abundant in the upper 150 m of the water column during summer. Differences between age classes were not evident. In contrast, diving behaviour showed seasonal differences with dives becoming shorter and shallower in autumn. This behaviour corresponds to the biology and ecology of krill which inhabits the under ice habitat during autumn and winter. This shows that both the vertical and horizontal distribution of crabeater seals is closely related to its primary prey.

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Nachtsheim, D. (2014): Distribution and diving behaviour of crabeater seals (Lobodon carcinophaga) in the Weddell Sea , Bachelor thesis, Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, Bremerhaven, Germany, and University of Bremen, Bremen, Germany.

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