530 ECOLOGY AND BEHAVIOR 



Much of the information on shark populations is derived from catch records, 

 mainly of commercial species outside Australian waters (Ford 1921, Hickling 

 1930, Templeman 1954, Strasburg 1958, Springer 1960, 1963, 1967, 

 Holden 1965, Bullis 1967, Davies and Joubert 1967, Kato and Carvallo 1967) 

 but little is known of natural shark populations in Australian waters (Olsen 

 1954, McLaughlin and O'Gower 1971). While many interesting studies of 

 shark behavior have been made using scuba diving equipment (Eibl-Eibesfeldt 

 1965, Limbaugh 1963, Nelson 1969, Johnson and Nelson 1973, and others) 

 few attempts have been made to apply the advantages of using scuba equip- 

 ment in the organized study of natural, field populations of sharks (Nelson 

 and Johnson 1970, McLaughlin and O'Gower 1971). These latter, detailed, 

 underwater studies on heterodontids have added greatly to our understanding 

 of the activities of these sharks, especially Heterodontus portusjacksoni in 

 Australian waters. Together with the studies reported here, they give a very 

 complete picture of the ecology of this shark. 



Tagging of H. portusjacksoni (McLaughlin and O'Gower 1970) in con- 

 junction with studies on shark movements (McLaughlin and O'Gower 1971) 

 or on shark populations (Nash and O'Gower, unpublished data) has yielded 

 much information on the dispersal behavior of H. portusjacksoni, both on 

 inshore reefs and along Australia's eastern seabord. 



INSHORE POPULATIONS 



Aggregates of H. portusjacksoni have been observed by the authors and 

 others, mainly during August and September, as far north as Yagen (near 

 Seal Rocks) and Port Stephens, and as far south as Jervis Bay. In the 

 Sydney area aggregates have been sampled at 25 generalized localities, 

 with specific caves and trenches used year after year (Figure 1), but the 

 most detailed' study has been of the populations frequenting the South 

 Bondi reef (Figure 2). McLaughlin and O'Gower studied these from 1962 

 to 1964. 



During the 95 surveys of the South Bondi reef over the 3 -year study 

 period, the numbers of H. portusjacksoni seen were correlated inversely 

 with water temperature and directly with the reproductive cycle. Numbers 

 were generally low from November to June, with an early, seasonal peak in 

 July or early August, followed by a drop and a subsequent rise to a maximum 

 peak of gravid females in September to early October (Figure 3). Of the 

 sharks seen on the reef 85% were females, the bulk of which were sighted in 

 October, whereas of the sharks caught on long lines in deep water by fisher- 

 men 58% were females. It may therefore be concluded that the mature males 

 in the population spend most of their time in deeper water, only moving in- 

 shore in small numbers from March to July. On the South Bondi reef, horn 

 sharks were sighted at only six locations in spite of frequent, widespread 

 surveys (a-f in Figure 2). A general preference for sites a and b was observed, 

 the other sites being used mainly during the breeding season to accommodate 

 the large influx of visitors. Site e, a very large cave close to shore, on the 



