Ranging and diving behaviour of adult female southern elephant seals from Marion Island

Abstract

The diving and ranging behaviour of thirteen adult southern elephant seal, Mirounga leonina, post breeding (n = 9) and post moulting (n = 4) females from sub-Antarctic Marion Island (46°54'S 37°45'E) were recorded during their pelagic annual cycling of 1990 - 1994, using Geolocation-Time-depth Recorders. A total of 66 807 dives were categorized into seven distinct types which were hypothesized to serve as functional transit, exploratory and foraging dives. No benthic foraging dives were recorded. Dive depth and duration were positively correlated and both unimodal and bimodal in their frequency distributions. Extended surface intervals were more common during long journeys at sea and were probably associated with successful foraging. It is postulated that the seasonal and individual variation in the diving behaviour of females are related to their geographic locations and the abundance and behaviour of their prey. Movements were categorized into outbound transit, distant foraging and inbound transit phases. The relative frequency of foraging, exploratory and transit dive types, as well as the duration and location of the different phases of movement suggest two clearly different seasonal foraging strategies used by post breeding and post moulting females. Bathymetrical features could possibly provide cues to females at least during part of the journey, to reach distant foraging grounds. Post breeding and post moulting females foraged largely within inter-frontal zones. Diel differences in the depth of hypothesized foraging dives appear to be associated with concentrated foraging on vertically migrating prey. A homogeneous temperature layer at foraging depth seems to cause shifts in the diel patterns of prey dispersal, whereas a heterogeneous temperature layer appears to promote little difference in the diel patterns of migrating prey, thereby causing differences in the diel diving depth of foraging elephant seals. Elephant seals could be using changes in water temperature to predict the presence of prey when foraging at inter-frontal zones, away from readily locatable oceanic features where primary production is known to be high.