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

Abstract

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 forage effort dives within a radius of ~ 250 km from Marion Island. Adult males concentrated their forage effort dives in areas in closer proximity to the island, while subadult males displayed more variation and often foraged at similar distances from the island and within similar areas as adult females. These results suggest that subadult males and adult females are more reliant on vertically migrating prey in pelagic environments than adult males from the same population. Nevertheless, competition for food resources between subadult males and adult females appears unlikely, since subadult males target deeper water layers than adult females. Due to the extreme sexual size dimorphism exhibited by southern elephant seals, it is unclear whether observed differences in dive behaviour are due to increased physiological capacity of males (when compared to females) or differences in activity budgets and foraging behaviour. By making use of mixed-effects models on dive results obtained from a sample of similarly-sized male and female elephant seals, I investigated the comparative influences of sex, body size and age on measured dive parameters. Model outputs indicated that, while individual variation accounted for substantial portions of total model variance for many response variables, differences in maximum- and targeted dive depths were always influenced by sex, and only partly by body length (used as a proxy for body size). Conversely, dive durations were always influenced by body length, while sex was not identified as a significant influence. These results support hypotheses that dive durations of elephant seals are limited by physiological capacity associated with body size. However, the influence of sex on the depths dived to indicate differences in forage selection between sexes in this species and possible avoidance of inter-sexual competition. Further investigations into the influences of various environmental variables (bathymetry, temperature at depth, Tmax below 100m) as well as demographic and behavioural variables (migration stage, age-class, track day and vertical diel strategy) on dive behaviour indicated a consistent association between dive depths and in situ water temperature. While much individual variation was apparent and other variables also played significant roles, animals consistently dived deeper, and spent less time at targeted depths, when diving in warmer water masses. This is most likely explained by differences in suitable prey distributions at different temperatures. Predicted climate change in the Southern Ocean suggests an overall continued warming, resulting in elephant seals from Marion Island likely having to dive to deeper depths in search of suitable prey and/or shift their migration routes poleward. This may have negative consequence for this population, since animals from Marion Island are presumably already operating closer to their physiological limit compared to other populations.