Ringed seal post-moulting movement tactics and habitat selection

Abstract

Intra-specific and intra-population variation in movement tactics have been observed in many species, sometimes in association with alternative foraging techniques or large-scale habitat selection. However, whether animals adjust their small-scale habitat selection according to their large-scale tactics has rarely been studied. This study identified two large-scale movement tactics in ringed seals (Phoca hispida) during their non-breeding, post-moulting period. First-passage times (FPT) were used to explore these large-scale patterns. Subsequently, habitat selection was quantified by modelling the FPTs as a function of habitat attributes using Cox proportional hazards models. Some seals moved far offshore into areas preferentially containing 40-80% ice coverage, while other individuals spread along the coasts of Svalbard concentrating their time near glacier fronts. Both tactics resulted in ringed seals being in highly productive areas where they had access to ice-platforms to rest. When offshore, habitat selection was influenced mainly by sea ice concentration and season. Late in the season (autumn), increased risk of leaving an area was identified, even when ice conditions were still favourable, reflecting their need to return to over-wintering/breeding areas before the fjords of the archipelago freeze. For ringed seals that remained inshore, habitat use intensities were influenced mainly by the distance to glacier fronts and season. These animals were already close to their over-wintering habitat and hence their risk of leaving an area decreased as winter approached. This study of ringed seals habitat selection reveals how they fulfil their biological requirements in this dynamic, heterogeneous habitat. Individuals within the same population employed two distinct large-scale movement tactics, adjusting their decisions for small-scale habitat selection accordingly. This flexibility in ringed seal spatial ecology during summer and fall is expected to result in increased population viability in this high Arctic environment.