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.