How does ocean seasonality drive habitat preferences of highly mobile top predators? Part I: The north-western Mediterranean Sea

Contrasting to the overall oligotrophic Mediterranean Sea, the north-western basin is characterised by high productivity and marked by seasonality, which induces spatiotemporal heterogeneity of habitat. Cetaceans and seabirds are expected to perceive this repetition of the seasonal cycle and to anticipate the recurrent variability of their environment. Because phenology imposes strong constraints over marine predators, especially through reproduction, we expected them to exhibit variations in their habitat preferences over seasons. Indeed, during reproductive period, marine predators have to face their own needs and those of their young, while out of this period, they can focus on maximising their own survival only. We therefore hypothesised that some species would change their habitat preferences to exploit the most favourable habitat during each season, while other species might accommodate the same habitat all year-round, for example thanks to the use of an habitat favourable all the year. To explore these hypotheses, we used aerial surveys data conducted over north-western Mediterranean Sea during winter 2011-2012 and summer 2012. Generalised Additive Models were used to link the species density to a set of 12 physiographic and oceanographic predictors describing their environment. Habitat models resulted in deviances from 12 to 47%. Our results provided the first assessment of habitat preferences for the winter season for most of our studied species. Small-sized delphinids (mostly stripped dolphins), fin whales, Globicephalinae (long-finned pilot whales and Risso׳s dolphins) and small-sized shearwaters (Yelkouan and Balearic shearwaters) exhibited no habitat variations between seasons, although for the first two, abundances were lower in winter. On the contrary, bottlenose dolphins switched from coastal habitat in summer to pelagic habitat in winter, while Cory׳s shearwaters and storm petrels exhibited the largest habitat variations between seasons with a complete absence during winter.