Examination of connectivity of hammerhead sharks in northern Australia
There is increasing concern about the conservation and sustainable use of hammerhead sharks nationally and globally, with documented declines in many parts of their range. Several hammerhead species have been recently added to international conventions such as the Convention on International Trade in Endangered Species (CITES), and the Convention on the Conservation of Migratory Species of Wild Animals (CMS). Within northern Australia, three species of hammerhead regularly occur: scalloped hammerhead (Sphyrna lewini), great hammerhead (Sphyrna mokarran) and winghead shark (Eusphyra blochii).
A key information gap in managing Australia’s hammerhead sharks is whether Australian stocks are shared with neighbouring countries, especially Indonesia and Papua New Guinea, where there is heavy fishing pressure. Hammerhead sharks are known to be highly migratory and the extent of connectivity between countries is relevant to management approaches.
Synthesis of findings from three methods for scalloped hammerheads showed there was some evidence to suggest stock structuring and limited movement between Australia, Indonesia, and PNG, but this finding is subject to uncertainty. Uncertainty is driven largely by the failure to encounter large (i.e. > 3 m) individuals which are most likely to move the greatest distances and create connectivity. Tracking and parasite fauna data suggest limited movement for individuals up to ~2.8 m, while genetic data indicate connectivity, but limited gene flow between some areas, specifically between Western Australia and other parts of Australia and Indonesia/PNG.