Beschreibung: Fidelity to foraging sites and foraging strategies is potentially beneficial to individuals using habitats with patchily distributed resources, as in a number of marine mammal species. We assessed inter-annual and long term (up to eight year) patterns in the spatial distribution and diving strategies used by southern elephant seals (Mirounga leonina) from sub-Antarctic Marion Island, instrumented with satellite-relay data loggers over multiple foraging migrations. We calculated inter-annual three-dimensional (3D) kernel density (KD) range overlaps for seals tracked over at least two post-moult foraging migrations in order to assess overlap for both the water depths and temperatures targeted during foraging migrations. We used intra-class correlation coefficients from linear mixed effects models to calculate intra- and inter-individual variance for a series of track and dive metrics. A repeatability index was ultimately derived for individual metrics, where higher values (between 0.5 and 1) indicate individually unique consistent behaviours, since greater variance occurs between, and not within, individual behaviours. Individual seals displayed high levels of fidelity to vertical depth layers where mean overlap for 95% 3D KD homerange estimates were 52.6% (inter-annual) and 34.3% (multi-year). Similarly, high levels of fidelity to temperature zones were evident where mean overlap for 95% 3D KD homerange estimates were 48% (inter-annual) and 35% (multi-year). Repeatability index values ranged between 0.38 and 0.57 for all metrics tested, indicating that the variance between individuals tested was generally not substantially more than within individuals. This study is the first to show that southern elephant seals display high levels of foraging niche fidelity in terms of spatial areas used, vertical depth layers targeted, as well as preferred in situ thermal conditions. Such observed inter-migration stability in habitat use patterns likely confers long-term energetic advantages to individual seals, despite little evidence for individually unique behaviours between these seals. While previous studies suggested likely inter-sex and inter-age-class avoidance of intra-specific competition in southern elephant seals, the apparent lack of individually unique foraging strategies reported here indicates that there is little individual level avoidance of intra-specific competition.

Beschreibung: Long-term fidelity to foraging areas may have fitness benefits to individuals, particularly in unpredictable environments. However, such strategies may result in short-term energetic losses and delay responses to fast environmental changes. We used satellite tracking data and associated diving data to record the habitat use of nine individual southern elephant seals over 34 winter migrations. By assessing overlap in two- and three-dimensional home ranges we illustrate strong long-term (up to 7-year) fidelity to foraging habitat. Furthermore, a repeatability statistic and hierarchical clustering exercise provided evidence for individual specialization of foraging migration strategies.We discuss the possible influences of stable long-term foraging migration strategies on the adaptability of individual elephant seals to rapid environmental change. Our results further illustrate the need for more long-term longitudinal studies to quantify the influence of individual-level site familiarity, fidelity and specialization on population-level resource selection and population dynamics.

Beschreibung: The potential effects of ocean warming on marine predators are largely unknown, though the impact on the distribution of prey in vertical space may have far reaching impacts on diving predators such as southern elephant seals. We used data from satellite-tracked southern elephant seals from Marion Island to investigate the relationship between their dive characteristics (dive depths, dive durations and time-at-depth index values) and environmental variables (temperature at depth, depth of maximum temperature below 100 m, frontal zone and bathymetry) as well as other demographic and behavioural variables (migration stage, age-class, track day and vertical diel strategy). While other variables, such as bathymetry and vertical diel strategy also influenced dive depth, our results consistently indicated a significant influence of temperature at depth on dive depths. This relationship was positive for all groups of animals, indicating that seals dived to deeper depths when foraging in warmer waters. Female seals adjusted their dive depths proportionally more than males in warmer water. Dive durations were also influenced by temperature at depth, though to a lesser extent. Results from time-at-depth indices showed that both male and female seals spent less time at targeted dive depths in warmer water, and were presumably less successful foragers when diving in warmer water. Continued warming of the Southern Ocean may result in the distribution of prey for southern elephant seals shifting either poleward and/or to increasing depths. Marion Island elephant seals are expected to adapt their ranging and diving behaviour accordingly, though such changes may result in greater physiological costs associated with foraging.

Beschreibung: A novel classification system was applied to the sea level anomaly (SLA) environment around Marion Island. We classified the SLA seascape into habitat types and calculated percentage of habitat use of ten juvenile southern elephant seals (SES). Movements were compared to SLA and SLA slope values indicative of ocean eddy features. This classification provides a measure of habitat change due to seasonal fluctuations in SLA. Some of the seals made two migrations in different seasons, each of similar duration and proportions of potential foraging behaviour. The seals in this study did not use any intense eddy features, but their behaviours varied with SLA class. Potential foraging behaviour was positively influenced by negative SLA values (i.e. areas of below average sea surface height). Searching behaviour during the winter was more likely at eddy edges where high SLA slope values correlated with low SLA values. Though the seals did not forage within newly spawned eddies, they did forage near the sub-Antarctic front. Plankton and other biological resources transported by eddies formed at the subtropical convergence zone are evidently concentrated in this region and enhance the food chain there, forming a foraging ground for juvenile SES from Marion Island.

Beschreibung: Fidelity to foraging sites is potentially beneficial to individual animals utilising habitats characterised by patchily distributed resources, as in a number of marine mammal species. We assessed inter-annual and long term (up to eight year) patterns in spatial distribution and depth use for southern elephant seals (Mirounga leonina) instrumented with satellite-relay data loggers over multiple foraging migrations from Marion Island, southern Indian Ocean. Individual seals displayed very high levels of fidelity to spatial areas used during their at-sea migrations, with inter-annual home ranges overlapping on average by 72% (max. 〉90%). Patterns of spatial distribution remained similar for individuals tracked over longer time periods, where home ranges consistently overlapped by more than 50% between migrations, even after periods of up to seven years. Patterns of depth use were generally also consistent between migrations, with individuals mostly targeting similar depth layers between successive migrations. Furthermore, in situ water temperatures at the median dive depths of seals overlapped substantially between migrations, with overlap ranges between 49% and 70%. Our study is the first to show that elephant seals display high levels of foraging niche fidelity, as assessed by spatial areas used, vertical depth layers targeted, as well as in situ thermal conditions. While elephant seals are known to display substantial behavioural plasticity within migrations, inter-migration stability in habitat use patterns may confer long-term energetic advantages to individual seals. Such behavioural consistently likely also plays important roles in advancing patterns of resource partitioning and avoidance of intra-specific competition.