Weddell seals foraging ecology in relation to interannual variability in sea ice conditions between 2006 and 2024 in Terre Adélie, East Antarctica

Overview & Synopsis

In marine environments, where resources are distributed heterogeneously, predators rely on environmental cues to locate prey aggregations and optimise energy intake through foraging. Variability in local conditions thus influences prey distribution and availability, which are reflected in predators' foraging behaviour and spatial patterns. Studying this behaviour and its variability offers insights into environmental drivers, prey dynamics, and broader ecosystem processes. In polar regions, predators depend on sea ice not only as a platform for breeding, moulting, and resting but also as a crucial foraging habitat. Sea ice supports an entire ecosystem both at the ocean/ice interface and underneath. The structure, extent, and seasonal dynamics of Antarctic sea ice are central to Southern Ocean processes and ecology. Sea ice influences the timing of phytoplankton blooms, with cascading effects throughout the food web. Abrupt episodic events like glacier calvings can profoundly reshape the sea ice landscape, altering bloom timing and redistributing key habitats. These include predator haul-out platforms, ocean access points for land-based species, benthic habitats subject to iceberg scouring, under-ice feeding and nursery zones for fish, and sea ice algae growth areas. Weddell seals (Leptonychotes weddellii) are the only marine mammal fully dependent on landfast ice year-round. From 2006 to 2024, the at-sea movements and diving behaviour of female Weddell seals were studied using animal-borne devices in Terre Adélie, East Antarctica. Two types of devices were deployed: Satellite-Relayed Data Loggers (SRDLs, 2006-2024), which recorded low-resolution dive and location data, and microsonar tags (2019, 2022), which captured high-resolution dive data and preyscape. By integrating behavioural data with seafloor topography, high-resolution (1 km) landfast ice data from NASA MODIS, and ROV footage of local fish species, we established a baseline for understanding the foraging ecology of the Dumont d'Urville Weddell seal population in relation to its environment. Females foraged in relatively small areas located exclusively on the continental shelf and within 50 km of Dumont d'Urville Station. Most dive time occurred in regions with shallow bathymetry and steeper slopes, likely corresponding to canyon edges. Their distribution overlapped both horizontally and vertically with that of fish larvae, mainly Pleuragramma antarcticum, and adult Trematomus newnesi. The seals exhibited benthic, epibenthic, and pelagic dives, and their movements closely followed areas covered by landfast ice. We observed high interannual variability but a consistent seasonal pattern: deeper dives and increased foraging effort in autumn (March-May) compared to winter (June-August). After the Mertz Glacier Tongue calving in 2010, the seals began using a new landfast ice-covered area in Commonwealth Bay. There, they dived and foraged at greater depths, suggesting shifts in diving behaviour, foraging strategy, or diet. During energetically demanding periods such as lactation, females reduced their foraging range (1-3 km from the pup), with exclusively benthic dives. In addition to enhancing knowledge of the species' foraging ecology, this study demonstrated that animal-borne devices can yield valuable information about sea ice habitat characteristics, including ice thickness and the presence of a platelet layer—aggregated ice crystals beneath the sea ice. Overall, this work provides additional evidence of the tight link between Weddell seal's at-sea ecology and the dynamics of landfast sea ice, and of their high level of behavioural plasticity in a variable environment.