Beneath the sea ice : exploring elephant seal foraging strategy in Earth's extreme Antarctic polar environment

Understanding how physical properties of the environment underpin habitat selection of large marine vertebrates is crucial in identifying how and where animals acquire resources necessary for locomotion, growth and reproduction and ultimately their fitness. The Southern Ocean harbors one of the largest and most dynamic marine ecosystems on our planet which arises from the presence of two majors physical features dominating the ecosystem dynamics, (i) the Antarctic Circumpolar Current and (ii) the seasonal sea ice cover region. In the Antarctic, marine predators are exposed to climate-induced shifts in atmospheric circulation and sea ice. However, because these shifts vary regionally, and because much remains to be understood about how individual animals use their environment, it has been difficult to make predictions on how animals may respond to climate variability. Spatio-temporal variability in ecosystem structure and function are likely to translate into modification of top predator population dynamics, however, there is currently a long-standing gap in our understanding of ecosystem functioning under winter Antarctic sea ice. Southern elephant seals (Mirounga leonina) are a major consumer of Southern Ocean resources and use two main large scale foraging strategies, (i) feeding in the frontal zone of the Southern Ocean, or (ii) feeding in the seasonal sea ice region. In the present thesis I examined the winter post-moulting foraging strategies of 46 male and female Kerguelen southern elephant seals which utilized the second strategy. Using an eleven year time-series of tracking, diving, and seal-collected hydrographic data (from 2004-2014) we assessed their movements and foraging performance in relation to in situ hydrographic and sea ice conditions. The influence of both the spatio-temporal and inter-annual variability of sea ice around seal locations was investigated, and an investigation on the role of polynya for male southern elephant seal during winter conducted. Two general sex specific patterns of sea ice usage were observed: females tended to move with the ice edge as it extended northward, and the males remained on the continental shelf despite increasing sea ice. Female foraging activity was greater over shallower seabed depths and at the boundary between the overlying Antarctic SurfaceWater (AASW) and the underlyingModified Circumpolar DeepWater (MCDW). They also foraged more in the outer part of the pack ice, 150 - 370 km south of the ice edge in late autumn. Within persistent regions of compact sea ice, females foraged most intensively (i) in the highest sea ice concentration at their position, but (ii) their foraging activity was longer when there were more patches of low concentration sea ice around their position (either in time or in space; 30 days and 50 km). The high spatio-temporal variability of sea ice around female positions probably allowed them to exploit concentrated sea ice patches while still having regular access to open water. Despite the lack of information on prey availability, females may exploit the ice algal autumn bloom that sustains meso-pelagic prey in the under-ice ecosystem. They may also take advantage of fishes overwintering at the upper boundary of the MCDW. In marked contrast, the males' foraging effort increased when they were deep within sea ice over the shelf (420 - 960 km from the ice edge). Male foraged most intensively (i) in the lowest sea ice concentration at their position, and (ii) their foraging activity was longer when there weremore patches of low concentration sea ice around their position (either in time or in space; 30 days and 50 km). Two distinct foraging strategies were observed for males: (i) Pelagic dives within the Antarctic Slope Front on the continental shelf break where upwelling of nutrient rich Circumpolar Deep Water onto surface water may enhance and concentrate resources; (ii) shallower pelagic and benthic dives within coastal polynyas sustaining biologically rich ecosystems throughout the ice season. For the first time, we demonstrated that coastal polynyas in East Antarctica are winter oases‚ÄövÑvp for male southern elephant seals spending up to 75% of their total foraging trip in them. The autumn transition is a key moment in the winter foraging ecology of seals with the highest foraging activity, dive duration and deepest dives observed inside polynyas. The seals may take advantage of the secondary production resulting from the deepening of the seasonalmixed-layer, entraining the remnant deep chlorophyll maximuminto the surface layer and presumably stimulating an autumn bloom. Finally, I demonstrate the role of inter-annual variability of near-surface meridional winds, incorporating large-scale climatic variability, in influencing foraging activity of female southern elephant seals through their effects on the timing of sea ice advance. Years of stronger meridional winds may lead to earlier sea ice advance, and both associated with increased foraging activity presumably through trophic cascading effect from enhanced ice algal autumn bloom to southern elephant seal prey. This work contributes to better understand the ecological mechanisms taking place in the under sampled under-ice ecosystem, while elucidating a crucial part of the annual cycle of a major top predator of the Southern Ocean. It also proposes mechanisms by which climate forcing affects both abiotic and biotic components of the Antarctic marine ecosystem. In the context of the Kerguelen southern elephant seal population, the capacity of seals to employ two foraging strategies (frontal or Antarctic zone strategies) may confer on the population the ability to adapt to short-term (decadal) variations on resource availability, presumably explaining the current stable demographic trend of the population.