554 ECOLOGY AND BEHAVIOR 



Table 3. Recaptures of C. obscurus tagged and released at Durban.* 



Recapture Locality 



Percentage of 

 Number Males Females Males 



More than 50 km north of Durban 

 50 km or less north of Durban 

 Tagged at Durban 

 Natal, south of Durban 

 East and south Cape coasts 



*After Table 47 of Bass et al. (1973). 



same area during December of the same year, and then recaptured in the 

 eastern Cape one month later. There is also evidence (see Bass et al. 1973) 

 that the number of recaptures from the eastern Cape is fewer than would be 

 expected of a gradual movement down the coast, and the conclusion is that 

 these sharks do not usually feed inshore while migrating and that the migra- 

 tions tend to be quick, nonstop movements rather than a gradual drifting 

 along the coast. 



The sharks migrating long distances tend to be larger than those staying 

 in the Durban area for some time— a few sharks have been recovered in the 

 Durban area 1 and even 2 years after tagging. Apart from this sex -related dis- 

 persal there is a north-south movement that can be correlated with seasonal 

 temperature changes, the population as a whole moving to the north as the 

 water cools in winter and then moving southward again in summer. During 

 the warmest months an offshore movement to deeper and presumably cooler 

 waters seems to take place. The seasonal distribution of young dusky sharks 

 from inshore waters has a marked correlation with temperature, as shown in 

 Figure 4; the preferred temperature range is 19° to 23° C. 



One certain effect of the size segregation in C. obscurus is that the young 

 sharks are kept away from the adults until they are large enough not to be 

 tempting prey for their elders. Sexual segregation, as occurring in the adults, 

 is easily explained: there is no need for the adult males to come anywhere 

 near the nursery areas. With immature sharks the phenomenon of sexual 

 segregation is not so easy to explain. Bass et al. (1973) suggest that 



. . . the young C. obscurus are genetically dimorphic with regard to the 

 direction they migrate in as they grow. This would result in a spreading 

 out of these sharks over a wide section of the coast (the secondary 

 nursery area) despite a comparatively small primary nursery area (the 

 southern Natal coast), although why the latter area should be so small is 

 not apparent at present. In this way population pressure would be re- 

 duced and unsuitable conditions in any part of the range would only 

 affect part of the population. The sexual segregation could have evolved 

 in response to the northern Natal coast being, on the average, the most 

 suitable area for the survival of young C. obscurus. Little is known about 



