Description: Satellite-relay data loggers (SRDLs) were deployed on southern elephant seals hauled out at Marion Island as part of the International Polar Year project, Marine Mammal Exploration of the Oceans Pole to Pole (MEOP). We describe the dive behaviour responses of one male elephant seal to changes in oceanographic conditions during two post-moult migrations. The seal concentrated his diving activity in a small area (〈200 square km) in close proximity to the Prince Edward Islands during both migrations. The restricted area utilised by the animal allowed for temporal comparisons in dive behaviour and excluded potential effects associated with changes in geographic position. The SRDLs transmitted a total of 16806 temperature points (1265 temperature profiles). Mean monthly in situ surface temperatures were significantly correlated with remotely sensed (MODIS) SST data for the study area. Surface and subsurface temperatures in the study area remained stable throughout both migrations, but were colder in October 2006 due to an intrusion of a cold, anti-cyclonic feature. Dive depths obtained by the seal were similar between years, though dives were longer during 2007. The differences in dive durations between years were a result of the seal spending more time at depths between 300m and 500m during 2007, when compared to 2006. During the colder period of October 2006, the instrumented seal performed shallower and shorter dives when compared to the rest of the migration period. The seal spent relatively more time near the maximum depths of dives during this period, indicating a likely increase in foraging effort. Our results highlight a link between oceanographic data obtained from seal-borne instrumentation and dive behaviour of elephant seals. This is even more pronounced by deploying tags to the same individual over several consecutive years, which excludes interspecific effects in multiyear comparisons. We furthermore suggest that elephant seals alter their dive behaviour on a fine scale to maximise foraging benefits associated with variations in oceanographic conditions, particularly in proximity to mesoscale features.

Description: 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.

Description: In order to gain insights into species-level behavioural responses to the physical environment, it is necessary to obtain information from various populations and at all times of year. We analysed the influences of physical environmental parameters on the mid-summer dive behaviour of Weddell seals (Leptonychotes weddellii) from a little-known population at Atka Bay, Antarctica. Dive depth distributions followed a typical bimodal pattern also exhibited by seals from other populations and seals targeted both shallow water layers of less than 50 m and depths near the seafloor. Increased stratification of temperature layers within the water column resulted in increased forage efforts by the seals through relatively high numbers of dives to the seafloor, as well as forage effort associated with shallow dives. We interpret these behavioural responses to be due to increased water temperature stratification resulting in the concentration of prey species in particular depth layers.

Description: Over the last decade, several hundred seals have been equipped with conductivity-temperature-depth sensors in the Southern Ocean for both biological and physical oceanographic studies. A calibrated collection of seal-derived hydrographic data is now available, consisting of more than 165,000 profiles. The value of these hydrographic data within the existing Southern Ocean observing system is demonstrated herein by conducting two state estimation experiments, differing only in the use or not of seal data to constrain the system. Including seal-derived data substantially modifies the estimated surface mixedlayer properties and circulation patterns within and south of the Antarctic Circumpolar Current. Agreement with independent satellite observations of sea ice concentration is improved, especially along the East Antarctic shelf. Instrumented animals efficiently reduce a critical observational gap, and their contribution to monitoring polar climate variability will continue to grow as data accuracy and spatial coverage increase.

Description: McIntyre et al. (2011a; Mar Ecol Prog Ser 441:257-272) illustrated a number of relationships between environmental variables and the dive behaviour of satellite-tracked southern elephant seals Mirounga leonina. One of these associations was that seals tended to increase their dive depths and spend less time at targeted dive depths when swimming in warmer waters. Boersch-Supan et al.’s (2012; Mar Ecol Prog Ser 461:293-298) comment on this study suggests that the link described between dive depths and in situ temperature is actually a link between dive depths and prey distribution. We do not dispute this assertion, having discussed this likelihood in McIntyre et al. (2011a). Boersch-Supan et al. (2012) further provide a number of criticisms, based partly on their observations of potential prey distributions within a comparatively small geographic area. We argue that their results are not directly comparable to those presented in McIntyre et al. (2011a) given the limited spatial overlap of the study areas and sparse, small-scale dataset presented. We further provide replies to technical comments by Boersch-Supan et al. (2012) pertaining to our data analyses.

Description: We describe the habitat use of 22 male southern elephant seals (Mirounga leonina) satellite tagged at Marion Island between 2004 and 2008. While a few areas of increased utilization appeared to be associated with areas of shallower bathymetry (such as sea-floor ridges and fracture zones), seals in our study did not target other areas of shallow bathymetry within close proximity to Marion Island. Rather, most elephant seals foraged pelagically over very deep water where much variation was evident in diel vertical migration strategies. These strategies resulted in generally deeper and longer dives than what has been reported for male elephant seals from other colonies. No significant differences were recorded for dive durations or dive depths between adults and sub-adults. However, younger animals displayed a positive relationship between dive durations and age, as well as between dive depths and age, while these relationships became negative for older animals. Mixed model outputs suggested that seals increased their aerobic fitness as migrations progressed, enabling them to undertake longer dives. We conclude that Marion Island male elephant seals exhibit much variability in dive strategy and are seemingly capable of exploiting a range of different prey types occurring in various depth layers.

Description: The potential effects of ocean warming on top 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 (46° 54’S; 37° 45’E) 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 Tmax below 100m, 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 influenced dive depth, our results also 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.