Sharks swim back to Stewart Island after 4000km voyage during summer months. Photo / Supplied
Marine scientists have confirmed that New Zealand's deep south has its own great white shark population, some of which swim 4000km round trips during summer before returning to exactly the same spot.
Great whites seen in New Zealand waters were until recently thought to be strays from Australia's warmer oceans.
But a National Institute of Water and Atmospheric Research (Niwa), Department of Conservation and University of Auckland research trip to northeast Stewart Island has consolidated the suspicion that these sharks call New Zealand home.
DoC shark expert Clinton Duffy said while tagging 27 sharks at the Titi (Muttonbird) Islands in April, and observing another 14, researchers found 18 of them had been seen near the island in previous trips. Researchers made a preliminary population count of 82.
Niwa principal scientist Malcolm Francis said the sharks, which grow to 6m and 2000kg, took "tropical holidays" at Great Barrier Reef, New Caledonia, Fiji and Tonga during summer.
One shark, named Grim, was traced 2500km to Fiji before returning exactly a year later to within 1.5km of where he was last tagged near the Titi Islands.
Mr Duffy said the satellite tags were staying in New Zealand "for anywhere between a week and five months." The great whites are believed to be drawn to Stewart Island by the rich fish life and the fur seal colony.
Mr Duffy said it was not known how the sharks navigate with such precision. Experts speculated that sharks' ultra-sensitive organs which detected heat and electric fields could allow them to navigate according to the earth's magnetic field or even the stars.
The satellite tagging allowed the marine scientists to trace a rough path of the great whites' migration patterns.
Swimming between the surface and a depth of 1200m, most departed New Zealand's coastline at Fiordland, before swimming across the Tasman via the Lord Howe Rise, where the water is shallowest.
Some headed to New South Wales waters but others turned north, and travelled to Pacific islands. New Caledonia was the most common destination.
The sharks have been tagged with more sensitive acoustic devices, which will track their finer movements around Stewart Island.
