ECOLOGY: INDIAN OCEAN SHARKS 579 



An interesting feature of the distribution of nursery areas along the east 

 coast of southern Africa is that secondary nursery areas of tropical species 

 often extend further into subtropical regions than do the primary nursery 

 areas. By primary nursery areas is meant those where the young sharks are 

 actually born and spend the first part of their lives. Secondary areas are 

 those inhabited by the slightly older but not yet adolescent or mature sharks. 



An example of this phenomenon is shown by Carcharhinus plumbeus. 

 Immature C. plumbeus of 90 to 160 cm are fairly common in Natal waters, 

 but newborn specimens (60 to 65 cm in length) are completely absent. The 

 only record in this size range comes from a depth of 280 m off southern 

 Mozambique. The few records of adult C. plumbeus from Natal are mostly 

 of pregnant females with full-term embryos (Bass et al. 1973). It would seem 

 that the primary nursery area is off southern Mozambique and that the 

 young sharks move southward into Natal waters when they attain a length of 

 about 90 cm. Carcharhinus obscurus shows a similar pattern with the primary 

 nursery area on the southern Natal coast and a secondary area ranging from 

 northern Natal to the southern Cape. This general pattern of having second- 

 ary nursery areas in cooler areas than primary nurseries is also seen in 

 Carcharhinus amboinensis, Carcharhinus limbatus, Carcharhinus breuipinna, 

 Carcharhinus altimus, and Sphyrna lewini. One reason for the evolution of 

 such a pattern could be that the adults do not then have to make extensive 

 migrations into unfamiliar and possibly unsuitable environments which can 

 still be used as nursery areas by their young. 



Reproductive migrations in small sharks (maximum length less than 150 

 cm) generally seem to involve movements from one depth to another with- 

 out any large-scale geographic movements. The nursery areas of such species 

 are usually close to the adult habitats but at markedly different depths. Ex- 

 amples include Galeus arae— the adults live in greater depths than the juveniles 

 (Bullis 1967)— and Holohalaelurus regani— the nursery areas are in deeper 

 water than that normally occupied by the adults (Bass et al. 1975a). In con- 

 trast to the restraints on the range of a species such as Odontaspis taurus 

 caused by the use of distant nursery areas, small sharks are able to use suit- 

 able habitats to the full. 



This is particularly noticeable in the case of oceanic islands, of which 

 Tromelin Island in the western Indian Ocean is a good example. An isolated 

 oceanic island to the east of Madagascar (Figure 1), Tromelin is over 200 

 n.mi. from any other piece of land or continental shelf. Despite this the 

 island has a flourishing population of Triaenodon obesus, a sluggish shark 

 normally found in shallow water in the vicinity of reefs but nevertheless 

 widely distributed over most of the Indian and western Pacific oceans. Only 

 a species having a concise distribution during its life cycle would be able to 

 survive around most small oceanic islands. 



Recognition Among Sharks 



Segregation by size and sex is virtually universal among sharks as is the use 

 of distinct nursery areas, usually at different depths from those inhabited by 



