540 



ECOLOGY AND BEHAVIOR 



NEWCASTLE 

 SYDNEY 



ERVIS BAY 



Figure 6 Ocean current systems and gene frequencies of H. portusjacksoni from vari- 

 ous places off the southern shores of Australia. 



known of the inshore current patterns, Rochford (1974) states that inshore 

 countercurrents accompany the major currents off the Continental Shelf 

 and that these countercurrents could act as the barrier between the two 

 shark populations. The migratory movements of H. portusjacksoni do indicate 

 the presence of some natural barrier across Bass Strait (See "Migratory 

 Behaviour"). 



McLaughlin and O'Gower (1971) have shown that the sharks return to 

 the same area year after year to breed. These studies, plus the present 

 study, have delineated two major breeding sites on the east coast, at 

 Sydney and Jervis Bay, and have failed to find any significant breeding 

 site between these two localities. Although Sydney and Jervis Bay are 

 only about 140 km apart, the differences in phenotypic distribution be- 

 tween the two populations taken from these areas (Table 3) are signifi- 

 cant (Table 4). Finally, sharks tagged in either the Sydney or the Jervis 

 Bay areas have not been recorded in the other area at breeding times. From 

 these arguments it seems reasonable to postulate that these two subpopu- 

 lations of H. portusjacksoni can be termed semi-isolated subpopulations. 



The distribution of phenotypes in the two major populations sampled, 

 Sydney and Jervis Bay, are in such good agreement with the Hardy- 

 Weinberg Law that the genetic hypothesis of three codominant alleles, 

 coding for the polymorphis protein is, theoretically, fully substantiated. 

 Practically, however, in all such studies, "where pedigree data can be ob- 

 tained from laboratory breeding, the genetic basis of the variation can be 

 unequivocally determined" (Koehn 1972). Although the Port Jackson 

 shark will breed readily in captivity, there' were (and are) no facilities avail- 

 able to the authors for testing the genetic hypothesis in this manner. 



