The Distribution of cetaceans off the coast of South Africa and South West Africa / Namibia

Abstract

Thirty - seven species of cetacean are recorded from southern African waters. Four thousand, five hundred and thirty - seven records of approximately 60 000 cetaceans were analysed to defme distribution patterns of these species, including dedicated sightings (both scientific and commercial), incidental sightings, commercial catches and specimens. Records of dedicated scientific sightings and incidental sightings were analysed by water depth, sea surface temperature and salinity and season, while commercial sightings and catches were analysed by month and water depth. Of the thirty - seven species recorded within the region, four mysticetes and one odontocele could be described as migratory, having a marked seasonal occurrence within the region, three species were defmed as possible migrators as they were not recorded in the dedicated data and showed seasonality in the incidental sighting and specimen databases, 20 species (19 odontocetes and one mysticete) could be described as resident (in that they were found throughout the year) and five species were termed "semi - resident" (certain components or forms of which showed strong seasonal occurrence, while other individuals had a year- round occurrence). Records of a further four species were too few to defme distribution patterns or seasonality. Analysis of the distribution of the large species showed three species (Eubalaena australis, Megaptera novaeangliae and Caperea marginata) to occur in nearshore waters and four species (Balaenoptera musculus, B. physalus, B. borealis and P.macrocepha/us) to occur in deep waters only. B. acutorostrata was recorded in both nearshore and offshore waters, while the two forms of B. edeni were found in nearshore and offshore waters respectively. The distribution analyses of the smaller odontocele species showed a number of component patterns including cosmopolitan, pelagic cosmopolitan, tropical, sub tropical and warm temperate components of the Agulhas Current, warm temperate component of the south coast, a component of the Agulhas Bank, a south and east coast inshore component and west coast inshore and offshore components. The high diversity of species within the region results from the complex variety of zoogeographic components found within the relatively small study area. These arise from the complex oceanographic conditions, brought about by the upwelling of cold Central Atlantic water inshore on the west coast, the movement of subtropical water into the east coast region by the Agulhas Current and the mixing of Benguela and Agulhas Current waters to the south of the sub - continent.