Marine Mammals Chapter 8

Information on diversity, spatial and temporal distribution, abundance, population structure of marine mammals in the Southwest Indian Ocean (SWIO) is relatively limited, despite evidence for exposure of these vulnerable species to a variety of threats. This chapter reviews existing information on the status and conservation issues of marine mammals in the SWIO, from 0 to 30°S, from eastern Africa to 60°E. Within the region, a total of 37 marine mammal species have been recorded (authenticated records, including sightings and/or strandings), including 32 cetaceans, 1 sirenian (the dugong Dugong dugon) and 4 pinnipeds (30% of global marine mammal biodiversity). Species diversity and distribution have been undertaken in many areas within the region. The existing literature does not provide sufficient information to identify cetacean hotpots in the SWIO, but it seems that oceanic islands and archipelagoes provide quality habitats for a diversity of toothed cetaceans. Among cetaceans, the humpback whale (Megaptera novaeangliae) is the most common and widely distributed large whale species during austral winter. The region constitutes a major breeding ground for this species in the southern hemisphere. The amount of knowledge on abundance and distribution of other large cetaceans is far more limited in the SWIO. The dugong is most likely the most endangered marine mammal species in the region. Dugongs have progressively declined in most countries of the region, and the only known viable population is located in the Bazaruto Archipelago, Mozambique. While bycatch is the most important threat to marine mammals, including the dugong, in the SWIO, other threats, including disturbance and noise pollution, have been identified. Overall, marine mammal knowledge has significantly increased over the last decade in the SWIO. However, many gaps remain on the location of hotspots of abundance and on the impact of major threats on their populations, especially through bycatch.