Three-dimensional space use during the bottom phase of southern elephant seal dives

Mov Ecol. 2017 Aug 31;5:18. doi: 10.1186/s40462-017-0108-y. eCollection 2017. Bras YL1, Jouma'a J1, Guinet C1. Author information 1 Centre d'Études Biologiques de Chizé, UMR 7372, CNRS-ULR, Villiers-en-bois, 79360 France. Abstract BACKGROUND: In marine pelagic ecosystems, the spatial distribution of biomass is heterogeneous and dynamic. At large scales, physical processes are the main driving forces of biomass distribution. At fine scales, both biotic and abiotic parameters are likely to be key determinants in the horizontal and vertical distribution of biomass, with direct consequences on the foraging behaviour of diving predators. However, fine scale three-dimensional (3D) spatial interactions between diving predators and their prey are still poorly known. RESULTS: We reconstructed and examined the patterns of southern elephant seals 3D path during the bottom phase of their dives, and related them to estimated prey encounter density. We found that southern elephant seal tracks at bottom are strongly dominated by a single horizontal direction. In high prey density areas, seals travelled shorter distances but their track remained strongly orientated according to a main linear direction. Horizontal, and more importantly, vertical deviations from this main direction, were related negatively to the estimated prey density. We found that prey encounter density decreased with diving depth but tended to be more predictable. CONCLUSION: Southern elephant seal behaviour during the bottom phase of their dives suggest that the prey are dispersed and distributed into layers in which their density relates to the vertical spread of the layer. The linear trajectories performed by the elephant seals would allow to explore the largest volume of water, maximizing the opportunities of prey encounter, while travelling great horizontal distances. KEYWORDS: Diving behaviour; Prey density; Prey distribution; Prey patch; Three-dimensional path reconstruction