ECOLOGY: PORT JACKSON SHARKS 535 



to deposit them in rock crevices, and as male sharks have been observed to 

 spit out crushed eggs presumably having swallowed their contents, it seems 

 that egg mortality is mainly the result of adult shark predation, either 

 accidental or deliberate, rather than drilling by carnivorous gastropods 

 (Grover 1972).) 



Several localities in sheltered bays and inlets appear to be traditional 

 oviposition sites, as eggs have been seen in particular crevices over several 

 years. Fresh eggs may be deposited over old eggs containing fully developed 

 embryos, with empty, brittle egg cases deeper in the crevice. The location 

 of such sites in sheltered bays and inlets is fortuitous and selective in that 

 the juveniles develop and grow in the estuaries. 



MIGRATORY BEHAVIOUR 



Almost all shark species studied have been found to migrate seasonally 

 (Olsen 1954, Kauffman 1950, Backus et al. 1956, Holland 1957, Strasburg 

 1958, Springer 1960, 1967, Holden 1967). McLaughlin and O'Gower 

 (1971), from their limited tag returns (four recaptures, or 1.5% recap- 

 ture rate), postulated a north -south axis for the migratory route of the 

 H. portusjacksoni on the eastern seabord of Australia. 



Continued underwater studies by the authors on H. portusjacksoni has 

 given a 6% recapture rate, with 14 tags recovered from 230 sharks tagged. 

 This rate compares favourably with other recapture rates for sharks. Olsen 

 (1954) reported a 4% tag recovery for Galeorhinus australis, and Holden 

 (1967) and Aasen (1960) recorded 5% and 6% recoveries respectively in their 

 studies of Squalus acanthias. As the probability of recapturing a tagged shark 

 is obviously much greater for commercial species than for harmless, noncom- 

 mercial species such as H. portusjacksoni, our 6% return was commendable. 



The sites of tagging and recapture for the 14 recently recovered tags, 

 plus McLaughlin and O'Gower's (1971) four previous tags, are shown in 

 Figure 4. Three animals (1, 2, and 14) were recaptured over 700 km from 

 the tagging site, the longest journey being about 850 km. The highest 

 minimum rate of movement away from the tagging site was achieved by 

 shark no. 9, which covered approximately 200 km in 31 days, a speed of 

 6.5 km /day. This rate greatly exceeds that recorded by McLaughlin and 

 O'Gower (1971) with shark 16, which was caught 223 days after tagging 

 and some 400 km from the point of tagging, a minimum rate of 1.8 km 

 per day. Sharks 2, 9, 13, and 18 were captured in shallow water close to 

 the coast. Sharks 4, 5, 6, 11, 12, 15, and 16, on the other hand, were 

 captured in deep water during the summer and autumn. 



From the data in Table 1 it appears that the sharks migrate southward 

 in early Australian summer, staying close inshore. They then spend the 

 summer and autumn months in the deep waters of Bass Strait, and begin 

 their northward migration in winter. The evidence from our studies, dis- 

 cussions with professional fishermen, and personal experience on trawlers 

 indicates that this northward movement occurs in the deeper offshore 



