Thursday, November 24, 2005

Tagged great white educational hope!

Researchers on Saturday managed to attach an acoustic tag to a Great White shark that they hope will for the first time give some clues to the detailed movements of the creatures around Cape Town's False Bay.

The bay, with beaches popular among 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 off Bailey's Cottage near Muizenberg, and researchers were now following it in a skiboat to monitor and map its movements.

Though the tag's batteries would last about six months, the transmitter had a range of only some 300m, so once the shark moved out of range, the researchers would lose the signal.
They hoped to be able to stay with it 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."

'We're trying to see if there are any patterns...'

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 the broad movements of the creatures.

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, the creatures 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 would follow, he said.

The researchers, who had been doing acoustic tagging for three or four years in Mossel Bay, had moved over to False Bay not only because it was relatively sheltered, but also because of the shark attacks which had taken place there.

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

The team had hoped to carry out the tagging on Friday, but a raging south-easter made it impracticable.

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 hundred metres 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.


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