Three-dimensional assessment of hunting strategies in a deep diving predator, southern elephant seal Mirounga leonina

Abstract

Few studies have examined the foraging ecology of top marine predators by taking into account the vertical and horizontal dimensions simultaneously. In this study, using 3-dimensional reconstruction of underwater animal paths, we investigated the relationship between area restricted search (ARS) and foraging behaviors in the southern elephant seal Mirounga leonina. We equipped 6 post-breeding females on the Kerguelen Islands with an Acousonde™ recording simultaneously sounds, pressure, acceleration and magnetic field-strength in 3-dimensions (n = 879 dives). Using the spherical first passage time analysis, we identified ARS in 46.4% of the dives. They represented 36.9% of the bottom time, but due to a higher prey encounter rate, 67.6% of catch attempts took place in ARS. These areas, occurring within a sphere of 48.2 ± 25.7 m radius, were characterized by an increase in energy expenditure and a strong variability in the roll and heading compared to non-ARS regions. While within an ARS, we identified 3 hunting modes from unsupervised classification based on acceleration properties previous to captures. We found that 32.8% of attempts matched a passive-gliding approach from above the target. The other 2 modes corresponded to an actively swimming approach associated with increases of variance in heading and/or rolling movements for the first one (56.6% of captures), and to a much larger swimming effort for the second one (10.6% of captures). These data revealed a strong link between seals’ fine-scale movements and feeding activity, suggesting that seals fed mainly on patchily distributed resources, with different hunting strategies that may be indicative of several prey species targeted.