ECOLOGY: INDIAN OCEAN SHARKS 553 



In the southwest Indian Ocean C. obscurus has a tropical distribution, the 

 home range of the adults being centered in the Mozambique channel where 

 they occur chiefly on the deeper parts of the continental shelves and to a 

 lesser extent close inshore. A fair number of adults, mostly females, range 

 south to Natal and occasionally as far to the southwest as Cape Point. All 

 mating activity takes place in Mozambique waters, as do the early stages of 

 pregnancy. Most of the females found in Natal waters are in the late stages of 

 pregnancy or have recently given birth. There is no marked peak in the 

 numbers born at any one time, but there may be an increase in births from 

 April until June. 



As the young sharks grow they tend to move out of this area, most of the 

 young males moving to the southern Cape coast and most of the females to 

 the northern Natal coast and, to a lesser extent, the Tongaland and southern 

 Mozambique coasts. As they grow, young dusky sharks seem to move into 

 deeper water and then northward. C. obscurus up to 220 cm long are con- 

 centrated off the southern Natal coast but definitely further offshore than 

 the areas where very young (80 to 130 cm) sharks are caught. Above this size 

 range, most of the sharks seem to move northward into Mozambique waters. 



The movements of the young (less than 120 cm) dusky sharks are worth 

 considering in greater detail. Much of the information on these movements 

 came from a tagging program based in Durban (see Davies and Joubert 1966; 

 Bass et al. 1973). Quite significant sexual segregation is apparent in these 

 immature sharks, varying according to locality as shown in Table 2. 



The numbers of sharks examined from southern Natal and the Cape 

 coasts were rather small, but further evidence of segregation is seen in Table 

 3, which documents the recaptures of C. obscurus tagged and released at 

 Durban. 



The biased dispersal of the tagged sharks (females tending to move north- 

 ward and males southward) is marked. This dispersal is primarily a migration 

 rather than a gradual movement, particularly in the case of the sharks re- 

 covered in the southern and eastern Cape. Ten of those recovered in this 

 region had moved at speeds of at least 16 km (10 mi) per day (calculated 

 from the day of release to that of recapture). Proof that the sharks may stay 

 in the Durban area for some time before migrating is shown by one tagged 

 and released in Durban during September, recaught and rereleased in the 



Table 2. Sexual segregation of immature C. obscurus.* 



Percentage of 

 Locality Males Females Males 



Central and northern Natal 940 1599 37.0 



Southern Natal 25 21 54.4 



Eastern and southern Cape 31 21 59.6 



♦After Table 46 of Bass et al. (1973). 



