RECENT moves by the NSW Government to protect hammerhead sharks (by listing the great hammerhead shark as a vulnerable species and the scalloped hammerhead shark as an endangered species in NSW), have once again thrown this group of fishes into the limelight. Hammerhead sharks (family Sphyrinidae) are one of the most distinctive and well known ocean dwellers, due to the unique structure of their heads, which are flattened and laterally extended into a “hammer” shape scientists call a “cephalofoil”.
Hammerheads inhabit tropical to temperate oceans worldwide, with juvenile sharks in estuaries and shallow inshore reefs, with adults patrolling the deeper waters off continental shelves. There are nine species recognised within the hammerhead family, of which four occur in Australian waters, including Sphyrna mokarran (the great hammerhead, which grows to 6 metres long), the common or smooth hammerhead (Sphyrna zygaena, growing to around 5 metres long), the scalloped hammerhead (Sphyrna lewini, grows to around 4.2 metres long) and the Winghead shark (Eusphyra blochii, grows to around two metres long).
Shark biologists consider that hammerheads have a relatively slow growth rate. For example, male scalloped hammerheads take between six and nine years to mature (at a length of 1.5 – 2 metres), while male great hammerheads take at least nine years to mature (at a length of around 2.2 metres). Most of the larger hammerhead sharks are females, which mature even later and at larger sizes than the males. Maximum age of the largest female scalloped hammerheads exceeds 40 years old, while great hammerheads probably reach a similar age.
The reproductive potential of hammerhead sharks is relatively high compared to other large sharks (such as white sharks), but still remains low compared to bony fish. All hammerhead sharks are viviparous, which means they give birth to live, fully developed young (also called pups). The number of pups varies between species, but all have moderately long gestation periods. For example, great hammerheads are born at about 65cm long after an 11 month gestation period, with litter sizes of between six and 33 pups. Gestation in common hammerheads takes between 10 and 11 months, producing between 20 and 50 pups per litter, while scalloped hammerheads are born at 45-50cm long after nine to ten months gestation, usually with 30 to 41 pups per litter.
Because the gestation period approaches a full year, it appears likely that most female hammerheads breed every second year, rather than annually. Because of these reasons, combined with their late maturity, hammerheads have a low resilience to sustained fishing pressure.
NSW fisheries have published some of the information they used to declare scalloped and great hammerhead sharks as protected at the following link: http://www.dpi.nsw.gov.au/__data/assets/pdf_file/0009/432792/Scalloped-hammerhead.pdf
It is interesting to note that the report found that marine protected areas did not protect these species, probably because they are highly migratory, which means that fishing activities in places such as SE Asia affects stocks of hammerhead sharks in other areas, including along Australia’s east coast. Sources of mortality included foreign fishing fleets, local shark meshing programs, commercial fishing (mainly for shark fins) and recreational fishing, although NSW fisheries data published in other pubications noted that over 95 per cent of hammerheads caught by recreational fishers in NSW are released alive, with good survival. In contrast, they noted that mortality rates on long-lines, in gill nets and in the shark meshing programs was very high, often approaching 100 per cent.
It appears from the information presented by NSW Fisheries that the precautionary principle was invoked to permit listing of great and scalloped hammerheads as vulnerable and endangered, respectively. This is slightly different to the current situation in Queensland where it is now illegal to take any shark over 1.5 metres, but the result remains the same for recreational fishers in both states, this being that large sharks are fast becoming a release-only proposition, based on changing community expectations as well as likely significant reductions in their population sizes.