07 March 2006

Grey Nurse Shark species at risk of extinction, says genetic survey

Grey nurse sharks are in danger of becoming extinct, according to a genetic survey of the fish in their natural habitats.

Scientists analysed DNA from small populations of the sharks off the western and eastern coasts of Australia and South Africa to see how much the groups mixed and how diverse their genes were.

They found that grey nurse sharks living around the Australian coast are isolated from other small groups of sharks, suggesting that their dwindling numbers will not be boosted by sharks migrating from other waters.

The survey also discovered that each individual population of sharks had a very small range of genetic diversity, making the group vulnerable to dangerous infections and changes in habitat. "Low genetic variation lowers the potential for a species to adapt to environmental change such as global warming and also increases the risk of disease," said Adam Gow, of Macquarie University in Sydney.

Grey nurse sharks have been in rapid decline, particularly during the 60s and 70s, when they were hunted for sport using explosives. The species, Carcharias taurus, is now listed as critically endangered. "Although it has rather menacing needle-like teeth on display, it has become clear that they really are not people eaters. Their tooth structure is suited to grasping slippery fish rather than large mammals," said Dr Gow.

The decline of the grey nurse shark is exacerbated by its slow ability to bounce back from overfishing. The sharks take up to 10 years to reach full maturity and each female has a maximum of only two live young because the firstborn attempts to eat all of its siblings in the uterus, leaving only a single "pup" in each of the two fallopian tubes. "The population is so low now that with their slow recovery rate, they really are particularly vulnerable to extinction unless all mortality due to fishing and other human impacts is removed," said Dr Gow, whose study appears in the Royal Society journal Biology Letters.

"Along with the negligible migration into the critically endangered east Australian population, our finding of low genetic variation shows that the risk of extinction is higher than previously thought," he said.

The finding reinforces efforts to conserve the sharks' habitats and protect their current populations, which in Australia amount to no more than 400 in the wild. But more will need to be done, according to Dr Gow. To broaden the shark population's genetic diversity, sharks from distant waters will have to be introduced to those elsewhere in the hope that they breed young.

Mixing sharks from different regions should produce a more sturdy population, although there are risks to the welfare and migration behaviour of those chosen for transferring.

Source: www.sharklife.co.za


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