546 ECOLOGY AND BEHAVIOR 



INTRODUCTION 



The background data for the ideas presented here are taken chiefly from the 

 results of a taxonomic and biological survey of the sharks of the east coast of 

 southern Africa undertaken by the Oceanographic Research Institute in 

 Durban (D'Aubrey 1964; Bass et al. 1973. 1975a, 19756, 1975c, 1975d, 

 1976). This survey was begun in 1959 by Jeanette D'Aubrey under 

 the guidance of the late David Davies and continued by the author from 

 early 1968. Continuity was ensured by the expert technical assistance of 

 Nadraj Kistnasamy, who also provided many of the original observations. 

 The scientific names used here follow those used in the final results of this 

 survey. It should be noted that the shark described under the name 

 Carcharhinus spallanzani will soon be described as a new species by Dr. 

 J. A. F. Garrick; the specific name spallanzani is incorrect. 



PHYSICAL ENVIRONMENT OF THE 

 SOUTHEAST AFRICAN COAST 



The east coast of southern Africa, arbitrarily defined here as ranging from 

 Beira in the northeast to Knysna in the southwest, is shown in Figure 1 to- 

 gether with the major current systems of the southwestern Indian Ocean. 

 The temperature regime of the waters bathing the east coast varies from 

 tropical in the north to warm temperate in the south. In the northeast, warm 

 water is supplied by the South Equatorial Current driven from the central 

 Indian Ocean towards the African coast by the southeast trade winds. In the 

 southwest part of the region is cool Atlantic water from the Benguela Cur- 

 rent, which flows northward along the west coast of southern Africa. Further 

 to the south, at a latitude of about 42°, lies the subtropical convergence and 

 the cold West Wind Drift. 



The South Equatorial Current divides near Madagascar, one stream flow- 

 ing westward to strike the African coast slightly south of the equator, the 

 other flowing along the east and south coast of Madagascar and eventually 

 reaching the African coast at about latitude 25° S to flow southwards as the 

 Agulhas Current. This follows the edge of the relatively narrow continental 

 shelf, indicated in Figure 1 by the 100-fathom depth contour. At about 

 34° S the current is deflected from the coast by the extensive area of con- 

 tinental shelf known as the Agulhas Bank and eventually turns southward 

 and eastward to become the Return Agulhas Current. The surface currents to 

 the south of the Agulhas Bank are confused by interactions between the 

 Agulhas and Benguela currents, while the Agulhas Bank is covered by a 

 mixture of warm Agulhas and cool Atlantic water. 



The flow of the Agulhas Current varies according to season. During the 

 southern summer the Somali Current of the northeast African coast flows 

 southward under the influence of the northeast monsoon and joins part of 

 the South Equatorial Current to form the Mozambique Current, which in 

 turn merges with the Agulhas Current. During the southern winter the Somali 

 Current flows northward because of the southwest monsoon and the 



