ECOLOGY: INDIAN OCEAN SHARKS 559 



sharks travel between the sea and Lake Nicaragua and this population 

 is not landlocked. There are other parallels between the sharks in Lake 

 Nicaragua and in St. Lucia. Thorson (personal communication) notes 

 that the population of C. leucas near the mouth of the Rio San Juan 

 (which connects the lake to the sea) includes a large number of juveniles 

 about 50 to 80 cm in length. The 90- to 100-cm size class is scarce but 

 above that size they become common again. 



A further peculiarity of the St. Lucia system is that it may become hyper- 

 saline during times of drought when evaporation exceeds the inflow of fresh- 

 water so that the level is maintained by seawater entering by the Narrows. 

 Salinities approaching 1007 oo have been recorded in the past. The sharks 

 appeared to be partly at home in moderately hypersaline waters, evidence 

 showing that only salinities greater than 50°/oo were avoided. During this 

 time most of the sharks in the lake were in noticeably poor condition, 

 despite the facts that food (teleost fish) was abundant and the sharks were 

 not confined to the lake but regularly traveled between it and the sea. Possi- 

 bly the extra stress of coping with the high salinities was the cause of the 

 poor condition. 



Odontaspis taurus 



Commonly known as the ragged-tooth or sand shark, Odontaspis taurus 

 (Figure 7) is another large, wide-ranging shark with a tropical distribution. 

 Born at a length of about 100 cm, males mature at about 210 cm and females 

 at 230 to 240 cm. Maximum length is about 260 cm in males and about 290 

 cm in females from the southwest Indian Ocean. Armed with a conspicuous 

 set of lanceolate teeth in upper and lower jaws, O. taurus feeds mainly on 

 fish and small sharks, most of which are swallowed whole. The several attacks 

 on humans for which it has been blamed were usually based on flimsy 

 evidence (see Baldridge 1973 and Davies 1960, 1964). As Bass et al (1975c) 

 point out, ". . . the habits and dentition of O. taurus makes it an unlikely 

 maneater except possibly at times when factors such as shot fish may cause 

 unusual excitement." 



In the southwest Indian Ocean O. taurus has been recorded from shallow 

 inshore waters ranging from the southern Cape to southern Mozambique. 

 Rather sluggish, it is usually found near reefs. It exhibits a remarkable 

 method of buoyancy control by means of air swallowed at the surface and 

 held in the stomach (Bass and Ballard 1972). Unlike the faster swimming 

 species of the genus Carcharhinus, O. taurus is able to pump sufficient water 

 over its gills to satisfy its respiratory requirements while at rest. This method 

 is also used while swimming slowly, but when above a certain speed this 

 shark relies solely on its movement through the water to ventilate the gills. 



O. taurus has distinct segregation and migration patterns. As with many 

 other tropical species, mating and the early part of pregnancy takes place in 

 warm waters, in this region in southern Mozambique and off the Tongaland 

 coast. Each winter the pregnant females migrate southwards along the coast, 

 the peak of this movement passing Durban in July and the southern Natal 

 coast during August and September. The young are born in the eastern Cape, 

 after which a slower movement back to the north takes place, passing the 



