Great white shark project reveals surprising facts to NZ researchers
A 10-year project to find out more about the great white sharks that inhabit New Zealand waters is coming to an end as scientists tackle the final phases of data analysis.
The project was run jointly by the New Zealand National Institute of Water and Atmospheric Research (NIWA) and the NZ Department of Conservation project to fisheries scientist and shark expert Dr Malcolm Francis is one of several scientists who have spent the past decade tagging great whites – a protected species – and following their movements.
Dr Francis said the initial aims of the project were to find out how mobile the sharks were, how far they travelled and where to, and their habitat requirements.
The tagging programme involved the use of acoustic tags, determining how the sharks use their habitat near seal colonies, pop-up tags that gather data on light, depth and temperature, and electronic tags attached to a shark’s dorsal fin to track their movements.
Since 2005, the team has tagged 95 sharks, mainly at the Chatham Islands and Stewart Island and have made some surprising discoveries.
“We found that most migrate to the tropics during winter,” he said. “The first one or two we saw do this came as a real surprise. They go between May and July and return between December and March, spending more time out of New Zealand waters than in.”
A 3.3m-long great white named Pip by the researchers, arrived in New South Wales near Sydney, with tracking data showing she took 20 days to travel 2,020km from the southern Snares Shelf. She has since moved steadily northwards, and recently entered Queensland waters.
By comparison, great white sharks that live around the southern Australian coast travel mainly up the east or west coasts, but do not often venture into the open ocean.
The New Zealand tagging also showed that the sharks travel in a remarkably straight line on their migrations, averaging about 5km/h or 100km/day, but have done up to 150km a day.
In the afternoons they tend to spend time at the surface but also make regular dives between 200 and 800m – the record depth is 1,246m.
“We don’t know why they’re doing that,” said Francis. “We assume they’re feeding. We also don’t know how they navigate in a straight line or why. It’s a big puzzle and not one we are likely to work out.
“While we’ve found out some answers to some questions, our work has also raised a series of other questions. We’ve found out about the large scale, and where and when they hang out around northern Stewart Island, but not what they do close to mainland New Zealand.
Clinton Duffy, shark scientist with the Department of Conservation, said, “We confirmed that juveniles inhabit shallow coastal waters and harbours around New Zealand, feeding mainly on fish.
“Once they grow to about 3m long, the sharks begin to feed on marine mammals. They continue to feed on fish or squid but they tend to aggregate near seal colonies, so there are large behaviour changes.”
The information gathered on the distribution of sharks will now be compared with the distribution of commercial fishing to figure out where and when the sharks are at greatest risk of being inadvertently caught by fishing gear.
See Warwick Lyon, a marine biologist on the project, talking about two of the sharks they have been monitoring, Pip and Scarface, in the videos.