Water column usage and environmental determinants in southern elephant seals from Marion Island

• Main Author: McIntyre, Trevor
• Other Authors: Bester, Marthan Nieuwoudt, Ansorge, Isabelle J.
• Format: Thesis
• Language: unknown
• Published: University of Pretoria 2013
• Subjects: UCTD; Southern Ocean; Elephant Seals; Marion Island; Mirounga leonina; Southern Elephant Seals
• Online Access: http://hdl.handle.net/2263/30850; http://upetd.up.ac.za/thesis/available/etd-05242012-182303/

Southern elephant seals (Mirounga leonina) are relatively common top predators and major consumers within the Southern Ocean. This study aimed to describe the at-sea behaviour of a small population of southern elephant seals at Marion Island and to place this behaviour into an ecological and evolutionary context. Calculations of life-time habitat use for animals from this population revealed that seals spent an average of 77.59% of their lives diving at sea, 7.06% at the sea surface, and 15.35% hauled out on land. Animals from this population evidently tended to dive deeper than reported for other populations. Their extreme dive behaviour, and apparent shorter reproductive lifespans than animals from some other populations led to a ‘deeper diving – shorter life’ hypothesis, suggesting that Marion Island elephant seals may carry substantial physiological costs associated with deeper diving. Mean dive depths (± SD) recorded for female seals were 560 ± 170 m during the day and 394 ± 153 m at night. Male seals dived to a mean depth of 618 ± 259 m during the day and 480 ± 272 m at night. Female seals mostly foraged pelagically on vertically migrating prey, displaying positive diel vertical migration in their dive depths. Individual variation existed though, and some females tended to display a reverse pattern of diving deeper at night, compared to daytime dives. Adult male seals displayed more individual variation in forage strategies, though the majority still favoured foraging pelagically, and not benthically as described for other populations. Subadult males tended to use dive strategies that always resulted in dive patterns that exhibited diel variation in dive depths. By implementing a refined method that combines dive type analyses with relative amounts of time spent at the bottom of forage dives, descriptions are provided of the spatial areas of increased forage effort for male and female seals. Female seals tended to concentrate their forage efforts in areas further away from the island, rarely displaying ...