556 ECOLOGY AND BEHAVIOR 



Carcharhinus leucas 



The Zambezi or bull shark, Carcharhinus leucas (Figure 5), is a well-known 

 species found in all three major oceans and recorded from many freshwater 

 localities. Specimens have been recorded along the east coast of Africa from 

 Mombasa to central Natal. C. leucas is slightly smaller than C. obscurus, and 

 maturity takes place at about 225 cm in both sexes, which reach a maximum 

 length of about 300 cm. The young are born about 60 to 70 cm long. These 

 figures refer to the southeast African population. In other parts of the world 

 C. leucas may be considerably smaller. For instance, males from Lake 

 Nicaragua mature at 160 to 170 cm (Thorson et al. 1966). The teeth of C. 

 leucas are similar to those of C. obscurus, being well adapted for cutting and 

 enabling this shark to feed on relatively large prey. In addition to feeding on 

 live prey, C. leucas is something of a scavenger and has been held responsible 

 for several attacks on human beings. Smaller sharks and other elasmobranchs 

 form a good part of its diet. 



C. leucas is best known for its ability to enter freshwater, which has given 

 rise to the common name of Zambezi shark in southern Africa where it has 

 been recorded more than 1000 km from the sea in the Zambezi river system. 

 The distribution is basically tropical, with adult sharks found mainly in the 

 shallow waters of northern Natal and the warmer regions to the north. The 

 population off southern and central Natal consists mainly of adolescent 

 sharks ranging from 150 to 230 cm plus a few adults and a small proportion 

 of juveniles of less than 150 cm. There are no records of this species south of 

 Natal, although it is quite likely that occasional specimens range down to the 

 eastern Cape during the summer months. No adult females with ripe ova or 

 with young embryos have been recorded from Natal, and it seems that (as in 

 the case of C. obscurus) all mating activities and the early stages of pregnancy 

 take place in tropical waters. 



The size distribution in freshwater systems is quite different from that in 

 the sea. C. leucas has been recorded from most of the river and lake systems 

 of the east African coast from the Zambezi to Durban Bay. The bulk of 

 these records are of juvenile and adolescent sharks, as shown in Figure 6, 

 which compares the size distributions of C. leucas taken in the sea off south- 

 ern and central Natal with those taken in different parts of the Lake St. 

 Lucia system in northern Natal. This system consists of an H-shaped lake 

 some 40 km long and about 20 km across at its widest point, connected with 

 the sea by a channel some 20 km in length (commonly known as the Nar- 

 rows). 



A tagging program on the sharks in this system showed them to move 

 freely between the sea and the main lake— one animal was tagged in the main 

 lake, recaught in the Narrows near the sea after four days, then caught again 

 in the main lake three weeks later. The only adult sharks caught in the St. 

 Lucia system were four large females taken close to the mouth of the estuary. 

 Only two were examined internally; one proved to be pregnant with full- 

 term embryos while the other had recently given birth. 



