Oceanographic signatures and foraging areas of southern elephant seals

Abstract

Marine mammals forage in dynamic environments. Marine habitats are characterized by dynamic variables that are continuously changing in relation to large-scale oceanographic processes. Sea-surface temperatures, chlorophyll concentrations and sea-surface height anomalies all influence elephant seal movements, although the degree to which these effects are observed differ amongst individuals. Movements vary between juvenile and adult elephant seals from Marion Island. Juveniles display more predictable behaviour and are dependant upon the interaction between the Antarctic Circumpolar Current (ACC) and the Andrew Bain Fracture Zone (ABFZ), that lies to the west of the island, to form eddies characterized by variable sea-surface height anomalies which can either have warm cores (waters from north of the Subantarctic Front) or cold cores (waters from south of the Antarctic Polar Front). The relative positions of these frontal zones may influence the interaction between the ACC and the ABFZ, either enhancing or restricting the formation of eddies, altering the available foraging areas for elephant seals from Marion Island. Sub-adult southern elephant seals display individual variation in movements, and present an interesting platform for ontogenic studies. Sub-adult males face impending resource limitation imposed by rapid growth spurts and associated increased energetic demands. Sub-adult females are smaller and less reliant upon individual strategies to find food, and thus display movements driven by surface variables, much like the juveniles. Adult southern elephant seals display sexual differences in foraging behaviour. Adult female seals were tracked during their post-breeding and post-moult migration. Individual variation was more pronounced during the post-moult migration which is longer in both duration and distance travelled. Individual variation during the post-breeding migration is a good proxy for resource availability in this study. Adult females, in times of low survival, display more individual variation during the post-breeding migration while movements are more homogenous in times of high survival. Adult male elephant seals from Marion Island and King George Island display a great amount of individual variation. Adult male elephant seals from Marion Island regularly move to the north of the Subantarctic Front while those from King George Island have adapted to their regional environment and make use of areas covered with high sea ice concentrations, significantly the Weddell Sea. This individual variation in the location of foraging areas has been proposed as a strategy to avoid intersexual competition. It could also be as a result of improved foraging capabilities and increased energetic requirements due to large body size, enabling male elephant seals to enhance chances of breeding success. The erratic survival of pubescent and adult male southern elephant seals in conjunction with the high level of individual specialization observed in their movements is evidence that this group of the population is often faced with limited food availability. The availability of resources during the austral summer, a period of supposed abundance, is likely to be a driving factor in the survival of smaller bodied juveniles and females.