2.4 Environmental data
Strong relationships exist between cetaceans distribution and dynamic environmental variables (Mannocci et al. 2014a, b), such as sea surface temperature (SST), sea surface height (SSH), ocean currents (U and V components) and ocean current velocity. These variables were therefore tested as potential drivers of sperm whales’ movements and to predict their potential distribution in the SWIO. In addition to surface variables, the mixed layer depth(MLD) was also considered as this variable is known to be closely related to primary productivity. However, the deep diving behaviour of sperm whales might also be influenced by temperatures at the bottom of the water column where they mainly forage. Consequently, bottom temperature was also considered a likely driver of sperm whales’ movements. Bathymetry was also extracted from GEBCO at a spatial resolution of 1 km and the slope was subsequently derived from the bathymetry and expressed in degrees to get a proxy of the seafloor roughness. The dynamic variables were extracted monthly from the products Global Ocean Physics Reanalysis Glorys S2V4 (PHYS 001-024) and theGlobal Ocean Physics Reanalysis Glorys12v1 (PHY-001-030) at a resolution of 0.08° (from E.U. Copernicus Marine Service Information). All variables were then set to the same spatial resolution of 0.08 decimal degree. Monthly grids of each predictor were then averaged for each season: between December and March for the wet season, and between April and November for the dry season.