21 November 2005

South Africa: Green groups keeping tabs on False Bay Great White shark

A small fault with an acoustic tag that researchers attached to a Great White shark on Saturday is causing difficulty in tracking the shark when the water is rough.

But researchers still hope the tag will, for the first time, give them some clues as to the detailed movements of the shark around False Bay.

The bay, with beaches popular with both locals and tourists, has been the scene of sporadic shark attacks over the past few years, including a fatal one on Fish Hoek resident Tyna Webb a year ago.

Department of environment affairs marine scientist Herman Oosthuizen said the four-metre shark was tagged shortly before 3pm off Bailey's Cottage near Muizenberg and researchers were following it in a skiboat to monitor and map its movements.

"We haven't had much time to get anything out of this," said a Marine and Coastal management spokesperson who was in the boat on Sunday, adding that in the 11 hours they had managed to track the shark by mid-afternoon on Sunday, it had moved between Bailey's Cottage and Sunrise beach.

They were not sure what they were going to do about the fault. Although the tag's batteries would last about six months, the transmitter had a range of only 300m, so once the shark moved out of range, the researchers would lose the signal.

They hoped to be able to stay with the shark for at least 24 hours, he said. On a similar Great White tagging project in Mossel Bay, a team had managed to stay with one shark for 100 hours.

"Here at False Bay we've got no idea how long we'll be able to track it. This is the first time we've done it here: it's trial and error." A larger research vessel was on standby for crew changeovers, he said.

Oosthuizen said satellite tracking of the sharks, which hit the headlines recently after a Great White swam from Gansbaai on the Southern Cape coast to Australia and back in nine months, recorded only their broad movements.

The acoustic tag, which is jabbed into a shark's dorsal fin using a long pole, was aimed at discovering more about their fine movements and whether, for example, they had home ranges or moved freely around the whole bay.

"You sit on the tail of a shark and you can see what it does," he said.

More acoustic tagging of other Great Whites in the bay is to follow.

"We're trying to see if there are any patterns that make sense in why sharks attack humans," he said.

Twenty-three False Bay Great Whites have already been tagged with a different sort of acoustic transmitter, one that sends a signal when the fish comes within a few 100m of one of the 23 listening stations placed at the bottom of the bay.

That project does not record the movement of the sharks between stations.

Organisations co-operating in the Great White research include the department of environment affairs' marine and coastal management division, Cape Town's Iziko museums, the universities of Pretoria and Cape Town and the Natal Sharks Board.

Source: www.iol.co.za


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