The winter movements of Weddell seals in the sea ice zone of eastern Antarctica

Winter sea ice plays a major role in the Antarctic ecosystem due to its influence on the abundance and recruitment of keystone species and the secondary production associated with the ice edge on which many Antarctic predators rely. For one of these predators, the Weddell seal, long term data sets indicate that the population dynamics of this species is closely tied up with the winter ice environment. However, very little information exists on the winter movements of Weddell seals or their interactions with the winter ice environment. Over 3 years, adult female Weddell seals were equipped with satellite relay data loggers at Dumont d'Urville and the Vestfold Hills to determine winter behaviour and the role of the local environment, including ice concentration and weather, on this behaviour. Satellite relay data loggers transmit information via the Argos satellite system on seal location and behaviour. However, the ability to answer spatial questions of this data and the role of the environment in animal movement is hampered by aspects of the animal's behaviour and Argos location error. For Weddell seals, it was found that haulout locations were over represented in the data set. As such, any spatial analyses that include haulout locations of Weddell seals run the risk of overestimating the importance of haulout sites. In addition, when extracting environmental information at highly uncertain Argos locations, there is a high probability of extracting the wrong information leading to inaccurate assessments of the role of the environment in animal behaviour. A state-space modelling approach was applied to the Weddell seal tracks to address this. The resulting locations were a vast improvement on Argos locations when compared to GPS tracks of the same individuals and haulout locations were fixed to avoid over estimation of their importance. The error distribution associated with each location was incorporated into an approach whereby the extraction of a location specific environmental variable was weighted by the location's error distribution. This method of extracting environmental variables was then applied to assess the role of the local environment on the winter behaviour of Weddell seals. The local environment influenced both winter haulout and foraging behaviour with Weddell seals tending to haulout more under conditions of low wind speed and higher temperatures and remain submerged when the opposite was true. Seals were also more likely to terminate diving bouts under conditions of very heavy ice concentration when compared to lighter ice concentrations in the fast ice environment. Whilst winter foraging could be classed as either predominantly benthic or pelagic with a high level of individual variability, all Weddell seals employed both foraging strategies at some stage throughout the tracking period. These results reveal that aspects of the local environment have the ability to drive the winter behaviour of Weddell seals and must be considered in conjunction with the influence of prey availability and larger scale climatic phenomena. Individual foraging strategies imply that Weddell seals within the one population may display varied responses to climate-mediated changes in prey availability. As such, Weddell seals are vulnerable to changes in the ice environment that are long term and large scale as well as short term and local.