ECOLOGY: INDIAN OCEAN SHARKS 577 



"The availability of nursery areas, not only suitable to each species but also 

 comparatively free of predation by larger sharks, is very important in inter- 

 species competition." 



The data gathered on shark distribution off the east coast of southern 

 Africa corroborate this view. It is apparent that not only the large species 

 exhibit marked segregation (see also, for instance, the study of the small 

 Galeus arae by Bullis (1967). In many cases the apparent homogeneity of 

 the populations of small species is due to sampling on too large a scale. 

 Springer concludes that sharks "usually have little difficulty in finding 

 enough food, despite annual or seasonal fluctuations in the supply. . . . The 

 implication must follow that some factors other than food supply hold 

 populations of sharks at some point below the level of the maximum popula- 

 tion density that could be sustained by the available food supply." He also 

 notes that the availability of suitable nursery grounds might be a factor 

 limiting the total population of a system. I fully agree with this reasoning 

 but would like to approach the subject from a different viewpoint— that not 

 only the numbers of individuals in a given population are affected in this 

 way but also the geographic range inhabited by the species. 



A first example is the sharks of the family Hexanchidae as represented in 

 the southwest Indian Ocean. The only inshore species, Notorynchus 

 cepedianus, will not be discussed here. Three further species are found on 

 the continental shelf. The small shark Heptranchias perlo is found chiefly 

 in tropical areas. In much the same habitat (as far as we know) lives Hexanch us 

 uitulus, a slightly larger shark growing to a length of about 180 cm. 

 Hexanchus griseus, by contrast, grows to more than 4 m. The adults of this 

 large species occur chiefly in tropical waters, but young specimens of a size 

 equivalent to that of mature H. uitulus appear to be restricted to the cooler 

 areas of Natal and the eastern Cape. It does not seem unreasonable to suggest 

 that H. griseus has a nursery area in a temperate region because of the com- 

 petition from adult H. uitulus, which inhabits the same latitudes as the adult 

 H. griseus. It could be an adaptation against predation by large H. griseus on 

 their own young, but, if so, it is difficult to understand how H. uitulus 

 manages to survive. Predation by large sharks may be less important in this 

 case than competition among equivalent-sized animals of different species. 



In the case of Hexanchus griseus, it seems that the nursery areas are in 

 temperate regions fringing the tropical habitat of the adults. The range of 

 any self-perpetuating population of H. griseus adults must therefore be 

 restricted to migratory distance from a suitable temperate nursery area. The 

 northward spread of H. griseus in the Indian Ocean may well be restrained 

 (at least in the western regions) by the lack of subtropical continental shelf 

 nursery areas for the young except off the southeast coasts of southern Africa. 

 An analogous situation occurs in Odontaspis taurus, in which the adults of 

 both sexes are found in clear, tropical inshore water off southern Mozambi- 

 que and Tongaland. Pregnant females migrate south during winter to drop 

 their young in the nursery grounds off the eastern and southern Cape. The 

 northern limits of O. taurus along the east African coast have not been 

 defined; it is unknown from Kenya, Tanzania, and Madagascar. Madagascar 



