How Elephant Seals (Mirounga leonina) Adjust Their Fine Scale Horizontal Movement and Diving Behaviour in Relation to Prey Encounter Rate.

Yves Le Bras,Baptiste Picard,Christophe Guinet,Joffrey Jouma’A,Jan M Hemmi

doi:10.1371/journal.pone.0167226

Yves Le Bras, Baptiste Picard + Show 3 more

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https://doi.org/10.1371/journal.pone.0167226

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Abstract

Understanding the diving behaviour of diving predators in relation to concomitant prey distribution could have major practical applications in conservation biology by allowing the assessment of how changes in fine scale prey distribution impact foraging efficiency and ultimately population dynamics. The southern elephant seal (Mirounga leonina, hereafter SES), the largest phocid, is a major predator of the southern ocean feeding on myctophids and cephalopods. Because of its large size it can carry bio-loggers with minimal disturbance. Moreover, it has great diving abilities and a wide foraging habitat. Thus, the SES is a well suited model species to study predator diving behaviour and the distribution of ecologically important prey species in the Southern Ocean. In this study, we examined how SESs adjust their diving behaviour and horizontal movements in response to fine scale prey encounter densities using high resolution accelerometers, magnetometers, pressure sensors and GPS loggers. When high prey encounter rates were encountered, animals responded by (1) diving and returning to the surface with steeper angles, reducing the duration of transit dive phases (thus improving dive efficiency), and (2) exhibiting more horizontally and vertically sinuous bottom phases. In these cases, the distance travelled horizontally at the surface was reduced. This behaviour is likely to counteract horizontal displacement from water currents, as they try to remain within favourable prey patches. The prey encounter rate at the bottom of dives decreased with increasing diving depth, suggesting a combined effect of decreased accessibility and prey density with increasing depth. Prey encounter rate also decreased when the bottom phases of dives were spread across larger vertical extents of the water column. This result suggests that the vertical aggregation of prey can regulate prey density, and as a consequence impact the foraging success of SESs. To our knowledge, this is one of only a handful of studies showing how the vertical distributions and structure of prey fields influence the prey encounter rates of a diving predator.

Highlights

Foraging behaviour and foraging success, is critical to the growth, reproduction and survival of animals and is subject to natural selection [1]
As the objectives of SESs are different in transit and in bottom phase, we focused on the bottom and selected the Prey Encounter Events (PEE) rate at bottom as an index of the prey encounter density which is independent from the duration of the transit phases
Additional descriptive statistics of diving behaviours are available in the Table C in S1 Appendix

Summary

Introduction

Foraging behaviour and foraging success, is critical to the growth, reproduction and survival of animals and is subject to natural selection [1]. While foraging at sea, diving predators are central place foragers from the ocean’s surface, where they need to come back to breathe between dives required to reach their prey at depth [5,6]. Under such constraints, the efficient diving behaviour of a predator is presumably the key to the optimization of their energy balance. Pitch angle, derived from three-dimensional acceleration data, provides spatial information on the vertical movements which cannot be obtained from time-depth dive profiles. The spatial information obtained from pressure, accelerometer and magnetometer data are used to quantitatively assess how changes in vertical and horizontal diving behaviours relate to prey encounter rate

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Conclusion