Defining hotspots for toothed cetaceans involved in pelagic longline fishery depredation in the western Indian Ocean: a preliminary approach
False killer whale (Pseudorca crassidens), short-finned pilot whale (Globicephala macrorhynchus) and Risso’s dolphin (Grampus griseus) are the known cetacean species involved in pelagic longline depredation in the tropical and subtropical waters of the western Indian Ocean. In order to better understand interactions between these cetaceans and fisheries, it is crucial to investigate the spatial distribution, density and habitat preferences of these species. A review of the literature (published from 1973 to 2011) noted 500 presence sightings (P.crassidens 219, Globicephala macrorhynchus 108, Grampus griseus 173), resulting from ~1,991,112 kilometres of survey effort. Data were compiled for the western Indian Ocean region (IUCN region 12) using two approaches, those being the presence-only IUCN recommended α-hull and widely used density kernel. The study observed that although both methods utilised fundamentally different approaches there was observed a significant correlation between increasing mean regional density and the increasing mean-ranked occurrence of species presence within the region. The main hotspots for the three species investigated were located around the Seychelles. For false killer whales, highest densities were identified throughout the Mozambique Channel and off NE Madagascar. Risso’s dolphin highest densities were identified off the Seychelles, in the central and southern Mozambique Channel.