Department of Marine Ecology, University Pierre and Marie Curie, 75005 Paris, FRANCE
Departments of Marine Ecology, University of Tasmania, Hobart TAS 7005, AUSTRALIA.
The Southern Ocean and more specifically the sea-ice zone supports globally significant ecosystems including abundant populations of marine mammals and seabirds. In the marine environment, resources are heterogeneously distributed and structured in patches driven by physical features of the environment at different spatio-temporal scales. Among the activities included in the habitat use concept, foraging is one of the most important because obtaining adequate food supply is a basic requirement of all other life-history traits. The optimal foraging theory predicts that predators should adjust their movements and behaviour in relation to prey density (in both horizontal and vertical dimensions in the case of marine predators). Thus, studying the movement patterns and diving behaviour of top predators in relation with biotic and abiotic environmental features can provide valuable insights in the behavioural tactics they have evolved and/or learned to maximize prey acquisition in a given environment. This is even more relevant in polar regions where animals face particularly harsh conditions (e.g. darkness and associated reduced productivity for most of the year, sea-ice cover, cold water and air, strong winds). The Weddell seal is the only marine mammal inhabiting the coastal fast-ice area year-round. While its behaviour has been well studied in summer when individuals are breeding or moulting on the sea-ice, virtually nothing is known about their winter ecology. However, winter is a crucial period in Weddell seals life cycle during which they spend 80% of their time diving under the ice to store the energy needed for the following breeding season. Using telemetric data, the main aim of this thesis was to improve our understanding of the foraging strategies adopted by Weddell seals during winter in two locations of East Antarctica (Dumont D’Urville and Davis). First, we developed two methods to identify and quantify within dive foraging effort from both high and low-resolution dive datasets. Then, these foraging metrics were used to investigate the influence of several key abiotic parameters of the Antarctic environment on Weddell seals’ foraging behaviour. Although Weddell seals from Davis travelled more during winter, overall Weddell seals from both locations essentially remained and foraged in areas close to the coast associated with highly concentrated ice. Our results showed sea-ice concentration did not influence Weddell seals’ behaviour. However, the pluri-annual residency of focal seals to similar areas suggested they relied on smaller features within the fast-ice, such as perennial tide cracks close to land. At both locations, seals increased their foraging effort during winter likely responding to the approach of the pup birth (individuals were mainly females). The seals foraged essentially in shallow waters in areas where the topography is likely inducing upwelling of the nutrient enriched water masses, such as the modified circumpolar deep water in which Weddell seals from DDU increased their foraging effort throughout winter. At both locations, Weddell seals exhibited complex diving behaviour and used both pelagic and benthic strategies, reflecting the opportunistic nature of their feeding. They also adapted their diving behaviour to light intensity suggesting they follow the vertical migration of their prey, such as P. antarcticum. Overall, Weddell seals seemed to optimize their foraging strategies during winter by adapting their foraging behaviour in response to physical parameters of their environment (e.g. features in the fast-ice, topography and hydrology) that are likely to be associated with better prey availability and accessibility, as well as regular access to breathing sites. At finer scale the foraging behaviour of Weddell seals appear to respond to the distribution and availability of prey in the water column (i.e. switching from pelagic to benthic foraging, exhibiting diurnal behaviour, and the complexity of the dives). Our study revealed some key foraging strategies adopted by the Weddell seals demonstrating that they actively optimize their spatial use of the fast-ice environment in both the temporal, horizontal and vertical dimensions during winter.