Offshore distribution of bottlenosed dolphins in Natal coastal waters and Algoa Bay, eastern Cape

Abstract

The offshore distribution of the bottlenosed dolphin Tursiops aduncus was investigated on a steeply sloped section of the Natal coast and in the shallower waters of Algoa Bay, eastem Cape by aerial and surface surveys respectively, as part of the development of appropriate census methods. The offshore limit of the observed distribution was 7, 5 km in Natal and 10, 7 km in Algoa Bay. In Natal 92% of 24 sightings on survey were within 1, 2 km and 50% within 0, 2 km of the shore. In Algoa Bay numbers of sightings declined markedly> 6 km offshore. In both regions T. aduncus showed a significant preference for shallow water of < 15 to 20 m in depth, and occurrence in water > 30 m deep was infrequent, suggesting that water depth is an important determinant and indicator of offshore distribution. These results indicate that an aerial strip transect census along the beach coupled with appropriate correction factors for undercounting biases would be effective for future surveys in Natal.