Ten year shark study comes to an end
Great white sharks often dive to great depths and navigate in straight lines - but scientists don't know why.
The findings came from a 10-year joint project by NIWA and the Department of Conservation which saw 95 sharks tagged and tracked, mainly from the Chatham Islands and Stewart Island.
The initial aims of the project were to find out how mobile the sharks were, how far they travelled and where, and their habitat requirements, NIWA fisheries scientist and shark expert Dr Malcolm Francis said.
The information would be compared with commercial fishing distribution to figure out where and when the sharks were at the greatest risk of being inadvertently caught by fishing gear.
Researchers found most great whites migrated to the tropics during winter.
"They go between May and July and return between December and March, spending more time out of New Zealand waters than in,” Francis said.
New Zealand tagging showed the sharks travelled in a straight line on their migrations, averaging about 100km a day, but could also reach 150km a day.
In the afternoons they tended to spend time at the surface, but also made regular dives to between 200 and 800m. The record depth was 1246m.
"We don't know why they're doing that, we assume they're feeding," Francis said.
"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."
The project had raised a series of other questions, including what the sharks did close to mainland New Zealand.
Department of Conservation shark scientist Clinton Duffy said the researchers had confirmed that juveniles inhabited shallow coastal waters and harbours around New Zealand, feeding mainly on fish.
Once they grew to about 3m long, they began to feed on marine mammals.
"They continue to feed on fish and on squid but they tend to aggregate near seal colonies, so there are large behaviour changes," Duffy said.
The tagging programme used acoustic tags to determine how sharks used their habitat near seal colonies, pop-up tags that gathered data on light, depth and temperature, and electronic tags attached to a shark's dorsal fin to track their movements.