558 



ECOLOGY AND BEHAVIOR 



20 



20- 



Number of 

 specimens ° J 



40 



20 



20 



J 



100 



200 



300 



Length ( cm) 



Figure 6 Size distribution of Carcharhinus leucas taken in different 

 parts of the St. Lucia system and in the sea off central and southern 

 Natal. (After Figure 15 of Bass et al. 1973.) 



It seems that adults usually avoid entering the system but pregnant 

 females may make short forays into the estuary, probably to give birth. 

 Many of the juveniles caught in the estuary and Narrows had open or 

 newly healed umbilical slits, showing that they had been born not long 

 before. (Bass et al. 1973) 



Somewhat larger sharks, between 110 and 200 cm in length, were prepon- 

 derant in the main lake. In addition, there was a predominance of females 

 in the main lake and of males in the Narrows. 



It appears that young C. leucas are born in the estuary where they tend to 

 stay for the early part of their lives. As they grow they tend to move into the 

 main lake and then out to sea as they mature. At all ages some movement 

 between the sea and the lake system probably takes place and, in particular, 

 sharks of 90 to 120 cm are rare in the Narrows. Further work might show 

 seasonal changes in the distribution of C. leucas in the St. Lucia system. 



As noted by Bass et al. (1973), 



The size distribution of C. leucas in systems such as the Zambezi river 

 and its tributaries may differ from that observed in St. Lucia. Adult 

 sharks are found in Lake Nicaragua where Thorsonet al. (1966) recorded 

 adult males. However, Thorson (1971) has recently proved that these 



