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

Labrousse, SA 2017 , 'Beneath the sea ice : exploring elephant seal foraging strategy in Earth’s extreme Antarctic polar environment', PhD thesis, University of Tasmania.

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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” 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.

Item Type: Thesis - PhD
Authors/Creators:Labrousse, SA
Keywords: elephant seals, foraging behaviour, Antarctic sea ice, oceanography, polynya, marine ecology, telemetry
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Copyright 2016 the Author

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