KEY POINTS:
Great white sharks living in New Zealand waters may be heading for Australia each spring in the hunt for baby humpback whales.
Research by the National Institute of Water and Atmospheric Research suggests some great whites leave the seal feeding grounds of New Zealand around the end of winter for Australia.
Niwa fisheries scientist Dr Malcolm Francis said new information on the secret lives of great white sharks was slowly emerging from the use of "pop-up" satellite tags, which detach after a set time, float to the surface and send information on their position to a satellite.
Recently the tag from a 3.5m shark called Thomas had popped up at Swain Reefs, off Rockhampton at the southern end of the Great Barrier Reef in Queensland. Thomas had been tagged at Ruapuke Island in Foveaux Strait in February.
Dr Francis said another great white, called Kerri, tagged in Stewart Island, had turned up in December last year in a similar location off the Australian coast.
Kerri's recorded position was only about 100km from where Thomas had been located, after a journey of more than 3000km.
Dr Francis said it was previously thought that great whites were cold-water, coastal sharks.
"But we now know that many make trans-oceanic migrations to tropical waters."
Dr Francis said the reasons for their winter tropical holidays were still unknown, but probably related to hunting for food.
"We think they may be searching for newborn humpback whale calves, because all the tags have surfaced in or near known humpback calving sites."
Other tags had surfaced in New Caledonia, Vanuatu and halfway to Tonga.
While the great white sharks would be much smaller than the baby whales, it was quite conceivable they could kill them.
Dr Francis said some baby humpbacks had washed ashore in Australia with shark-bite marks.
Sharks were also known to feed on dead whales, he said.
It was thought the great white sharks stayed around the New Zealand seal colonies, such as those at the Chathams and Stewart Island, for three to five months while the seals were young and not such good swimmers.
The sharks then moved away after the seals matured and became harder to catch.
Dr Francis said the research using satellite tagging had started in 2005 in an international collaborative programme. It was a way of finding out whether New Zealand had its own stock of white sharks.
"We now know at least some go to Australia and the tropical islands so there is a south-west Pacific stock."
