Cetaceans in the southwest Indian Ocean: a review of diversity, distribution and conservation issues

The coastal waters of the southwest Indian Ocean (including Mozambique, Tanzania, Kenya, the Seychelles, Comoros, Mayotte, Madagascar and the Mascarenes and their EEZ) countries are characterized by a high marine biodiversity. This paper review information on the diversity, distribution and conservation issues to cetaceans in this region. To date, up to 33 species of cetaceans have been recorded in the southwest Indian Ocean: 16 delphinids (Stenella longirostris, Stenella attenuata, Stenella coeruleoalba, Delphinus spp., Steno bredanensis, Grampus griseus, Sousa chinensis, Tursiops truncatus, Tursiops aduncus, Globicephala melas, Globicephala macrorhynchus, Pseudorca crassidens, Orcinus orca, Lagenodelphis hosei, Feresa attenuata, Peponocephala electra), 8 large toothed whales (Physeter macrocephalus, Kogia sima, Kogia breviceps, Mesoplodon pacificus, Mesoplodon densirostris, Mesoplodon mirus, Ziphius cavirostris, Mesoplodon gingkodens) and 7 baleen whales (Megaptera novaeangliae, Balaenoptera acutorostrata/ Balaenoptera bonaerensis, Balaenoptera physalus, Balaenoptera edeni, Balaenoptera musculus/ B. m. brevicauda, Balaenoptera borealis and Eubalaena australis). However, very little is known on the ecology, spatial distribution, stock identity and abundance of these species. Nonetheless, these data are of particular relevance for management and conservation purposes. Cetaceans are exposed to many anthropogenic threats in the coastal waters of this region. The most significant are bycatch in fishing gears, direct exploitation, habitat degradation, chemical and acoustic pollution and disturbances caused by the whale/dolphin watching activity. Fisheries bycatch have been recorded throughout the region, although the impact on local populations is unknown for most areas. The most commonly bycaught species are Indo-Pacific humpback, bottlenose dolphins and spinner dolphins. Direct exploitation is important in the southwest region of Madagascar, targeting small delphinids. The increasing human population and the development of uncontrolled tourism may also impact negatively on marine mammal populations in the region. Nevertheless, the presence of permanent populations, such as inshore dolphins and migrating humpback whales, may provide opportunities to develop local commercial activities dedicated focusing on observations of cetaceans. Marine mammals may thereby help local communities to develop tourism, and reinforce their economies.