Spatial utilisation of fast‐ice by Weddell seals Leptonychotes weddelli during winter

Abstract

This study describes the distribution of Weddell seals Leptonychotes weddelli in winter (May–September 1999) at the Vestfold Hills, in Prydz Bay, East Antarctica. Specifically, we describe the spatial extent of haul‐out sites in shore–fast sea‐ice, commonly referred to as fast‐ice. As winter progressed, and the fast‐ice grew thick (ca 2 m), most of the inshore holes closed over, and the seals’ distribution became restricted to ocean areas beyond land and islands. Using observations from the end of winter only, we fitted Generalised Additive Models (GAMs) to generate resource selection functions, which are models that yield values proportional to the probability of use. The models showed that seal distribution was defined mainly by distance to ice‐edge and distance to land. Distance to ice‐bergs was also selected for models of some regions. We present the results as maps of the fitted probability of seal presence, predicted by the binomial GAM for offshore regions, both with and without autocorrelation terms. The maps illustrate the expected distribution encompassing most of the observed distribution. On this basis, we hypothesise that propensity for the fast‐ice to crack is the major determinant of Weddell seal distribution in winter. Proximity to open water and pack‐ice habitats could also influence the distribution of haul‐out sites in fast‐ice areas. This is the first quantitative study of Weddell seal distribution in winter. Potential for regional variation is discussed.