Sharks dive deep
Jayne Cooper-Woodhouse on tagging great white sharks to monitor their migrations
A New Zealand great white shark has set a world record with a dive of 1200 metres, the deepest ever measured. Tagged near a seal colony off Stewart Island, ‘Shack’ the shark made his record deepsea plunge on his way to Brisbane. He also made several other deep dives to 1000 metres. It is not known how long great whites live, but Shack may be up to 40 years old. At 4.8 metres, he is the largest male tagged in five years of research; females grow even larger than males, reaching at least 6 metres.
Great white sharks are found all around New Zealand. NIWA scientist Malcolm Francis and Clinton Duffy, from the Department of Conservation (DOC) and the University of Auckland, have teamed up to investigate sharks’ long-distance movements and their diving behaviour.
The scientists have tagged 31 great whites around Stewart and Chatham Islands since the project began in 2005. “Before we started this work, it was thought that great white sharks were cold-water animals. But we have found them in water temperatures ranging between 3 and 27 degrees Celsius. It seems that, like many New Zealanders, they are taking tropical winter holidays, departing New Zealand between April and September for somewhere warmer,” says Dr Francis.
The maximum distance travelled by a tagged shark was 3300 kilometres. One shark returned to its Chatham Islands tagging site after spending six months at Norfolk Island. “Our sharks don’t cross the equator – but they do go as far as Queensland, New Caledonia, Vanuatu, Norfolk, and Tonga. They can travel 150 kilometres in a day, and it takes them just three weeks to get to Australia,” says Dr Francis.
First, tag your shark
Tagging a shark is no simple task. First the scientists must attract the shark to the boat, using an enticing chum of oil and minced tuna. Then they use a long pole with a needle tip to inject the tag’s anchor under the shark’s skin. The tag records the shark’s location, depth, and the water temperature it passes through. After a pre-determined time – usually a year – the ‘pop-up’ tag releases from the tether, floats to the ocean surface, and starts sending the data via satellite.
Tagged Australian sharks have turned up in New Zealand waters, which suggests that southwest Pacific great white sharks may comprise a single population. However, there has been no direct mixing between the Stewart Island and Chatham Islands tagged sharks, although there is some overlap of these sharks in Australia. “Most of the Stewart Island sharks head to the northwest of New Zealand, whereas most of the sharks tagged around the Chatham Islands head north of New Zealand,” says Dr Francis. “Their ranges overlap up in the tropics, but we don’t know whether they interbreed. We need more biopsy DNA samples to see whether they are separate genetic populations.”
In New Zealand, they are a threatened species, and great whites were given full protection by DOC in 2007. The scientists hope their research will help to understand the sharks’ habitat requirements and behaviour, and lead to reductions in the numbers being accidentally caught and killed in set nets and on longlines.
On a recent tagging trip to Stewart Island, scientists were able to record almost 30 different sharks using photoidentification techniques. One shark was tagged with a dorsal fin ‘SPOT’ tag, which sends messages to a satellite every time the fin breaks the surface, providing real-time information on the shark’s location. On the next research trip, the scientists want to attach acoustic tags so that they can track the sharks’ movements around Stewart Island and Foveaux Strait in greater detail. The tags will be detected by an underwater array of acoustic receivers.
The research is funded by DOC, the Foundation for Research, Science and Technology, and NIWA.
Written by Jayne Cooper-Woodhouse, a NIWA science writer.