Counting sperm whales and visualising their dive profiles using two-hydrophone recordings and an automated click detector algorithm in a longline depredation context.

Odontocetes depredating fish caught on longlines is a serious socio-economic and conservation issue. A good understanding of the depredation behaviour by odontocetes is therefore required. Within this purpose, a method is described to follow diving behaviour of sperm whales, considered as proxy of their foraging activity. The study case took place around Kerguelen Islands from the Patagonian toothfish fishery. The method uses the coherence between direct-path sperm whale clicks, recorded by two synchronized hydrophones, to distinguish them from decoherent clicks that are reflected by the water surface or seefloor (due to surface roughness). Its low computational cost permits to process large dataset and bring new insights on sperm whales behaviour. Detection of sperm whale clicks permits to estimate the number of sperm whales and to assess their diving behaviour. Three diving behaviour were identified as "Water Column" (individual goes down and up), "Water Wander" (individual seems to go up and down multiple times in the water column), and "Seafloor" (individual spend time on the seabed). Results suggest that sperm whales have different diving behaviours with specific dives as they are either "interacting" or "not-interacting" with a hauling vessel.