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Assessment of scale-dependent foraging behaviour in southern elephant seals incorporating the vertical dimension: a development of the First Passage Time method

Authors

  • Frederic Bailleul,

    Corresponding author
    1. Centre d’Etudes Biologiques de Chizé, Centre National de la Recherche Scientifique, 79360 Villiers en Bois, France;
      *Correspondence author. E-mail: bailleul@cebc.cnrs.fr
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  • David Pinaud,

    1. Centre d’Etudes Biologiques de Chizé, Centre National de la Recherche Scientifique, 79360 Villiers en Bois, France;
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  • Mark Hindell,

    1. Antarctic Wildlife Research Unit, School of Zoology, University of Tasmania, PO Box 252-05, Hobart Tasmania 7001, Australia; and
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  • Jean-BenoÎt Charrassin,

    1. Equipe Physique de l’Océan Austral, DMPA, Muséum National d’Histoire Naturelle, Paris, France
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  • Christophe Guinet

    1. Centre d’Etudes Biologiques de Chizé, Centre National de la Recherche Scientifique, 79360 Villiers en Bois, France;
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*Correspondence author. E-mail: bailleul@cebc.cnrs.fr

Summary

  • 1Identifying the spatial scales at which top marine predators forage is important for understanding oceanic ecosystems. Several methods quantify how individuals concentrate their search effort along a given path. Among these, First-Passage Time (FPT) analysis is particularly useful to identify transitions in movement patterns (e.g. between searching and feeding). This method has mainly been applied to terrestrial animals or flying seabirds that have little or no vertical component to their foraging, so we examined the differences between classic FPT and a modification of this approach using the time spent at the bottom of a dive for characterizing the foraging activity of a diving predator: the southern elephant seal.
  • 2Satellite relayed data loggers were deployed on 20 individuals during three successive summers at the Kerguelen Islands, providing a total of 72 978 dives from eight juvenile males and nine adult females.
  • 3Spatial scales identified using the time spent at the bottom of a dive (inline image = 68·2 ± 42·1 km) were smaller than those obtained by the classic FPT analysis (inline image = 104·7 ± 67·3 km). Moreover, foraging areas identified using the new approach clearly overlapped areas where individuals increased their body condition, indicating that it accurately reflected the foraging activity of the seals.
  • 4These results suggest that incorporating the vertical dimension into FPT provides a different result to the surface path alone. Close to the Antarctic continent, within the pack-ice, sinuosity of the path could be explained by a high sea-ice concentration (restricting elephant seal movements), and was not necessarily related to foraging activity.
  • 5Our approach distinguished between actual foraging activity and changes in behaviour induced by the physical environment like sea ice, and could be applied to other diving predators. Inclusion of diving parameters appears to be essential to identify the spatial scale of foraging areas of diving animals.

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