Saturday, November 22, 2008

Great White shark set a deep diving record!

A great white shark tagged off Stewart Island has set a world diving record as it crossed the Tasman Sea to the Great Barrier Reef.

Three four-metre-plus great whites tagged off the Chathams have also surfaced in Tonga for a midwinter feast of humpback whale calves.

News of the sharks migrating to the tropics surprised Malcolm Francis, of the National Institute of Water and Atmospheric Research in Wellington.

"We used to think great white sharks were shallow-water coastal species that lived in cold areas, where there were lots of seals to eat," he said.

"Now we have changed our impression of what they do."

Niwa and the Conservation Department have been attaching satellite tags to great whites, to measure position, depth and water temperature. After several months, the tags pop off and float to the surface, where the data is transmitted to a satellite.

Dr Francis said that this year two Stewart Island whites had gone 4000 kilometres to Queensland's Great Barrier. Surprisingly, they go in a straight line.

"They seem to know where they are going," he said, noting that they moved through the water at between 4kmh and 5kmh, or an impressive 120km a day.

Up to 70 percent of the time they are near the surface but this winter one of the whites dived. "We've got what we think is a world record of 1000 metres for a white shark."

He believes it would have gone after a giant squid or phosphorescent fish. At that depth it would be pitch black and the white would have been guided in by the fish glow.

A tag on a white shark popped up off Mana Island but its data was partly corrupted. The shark had been north to the tropics for winter and had come back in summer.

Three of six sharks tagged at the Chathams were found to have swum 3000km north to Tongatapu, the main Tongan island.

"They head off to the tropics, and we're not quite sure what they are doing but we think they are showing interest in humpback whales," Dr Francis said.

The areas the tags were found are humpback whale calving areas.

Evidence exists of whites feeding on dead whales and dead calves.

Now Niwa thinks the whites follow the whales when numbers begin declining in seal colonies over winter.


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