New Zealand scientists are baffled by the 13,300km migration of a mako shark tagged in a landmark study.
The shark, a 1.8-metre mako named Carol, has been tracked by Niwa scientists using a satellite tagging device. The tag was attached to Carol in the Bay of Islands six months ago.
The tag has provided the scientists with previously unknown details of the timing and long-distance migratory movements of the species.
"Conventional plastic identification tags tell us little about the timing of mako shark movements, the route that they take or distance travelled," said Dr Malcolm Francis, who is leading the Niwa research.
So far, Carol has travelled over 13,300km, averaging 60km per day and exceeding 100km per day during some parts of her migration. She has swum to Fiji and back and has worked her way past the Bay of Plenty and Hawke Bay heading south.
She was last heard from heading down the east coast of the South Island past the Kaikoura Peninsula.
The tag has revealed that Carol is spending a lot of time at the ocean's surface, reporting her location to the satellite several times daily.
Dr Francis said the researchers were surprised by the findings.
"We have never tracked one in real time before so anything we are getting is really detailed and it's all new information," he said.
"What really surprised us was Carol took off to Fiji once, got about halfway there and turned around and came straight back to New Zealand, and then hung around the 90 Mile Beach area for about six weeks and then she did go to Fiji.
"We knew from game fish tagging - game fishermen putting little plastic tags on sharks - that New Zealand sharks do end up in the Fiji/New Caledonia area. We thought she'll be up there for the winter, getting away from our cold water, but she pretty well turned around and came straight back to New Zealand."
Dr Francis said the researchers had no idea why the shark had spent so little time in the warmer waters.
"One thing about studies like this is we find out all this new information and we know what they're doing, but why they're doing it we just can't get at."
He said mako sharks feed on schooling fish, such as skipjack tuna and mackerel, and suggested Carol could not find enough food to eat so returned to New Zealand to feed.
The study was funded by the Ministry for Primary Industries and was undertaken in collaboration with Dr Mahmood Shivji at the Guy Harvey Research Institute at Nova Southeastern University in Florida, who are funding the electronic tags and Argos satellite time.
The study is to be expanded with many more makos to be tagged off east Northland in February.
"We are really keen to get a few more tags out this year and figure out what other sharks are doing - whether they are doing something similar or whether they are all randomly running around the ocean doing strange things."
Dr Francis said the study is important to help identify the geographical distribution and stock levels of the species.
To follow Carol's travels on an interactive website go to: http://www.nova.edu/~johnmatt/makosharks.htm
The mako shark - There are two species of Mako shark, the long fin and the short fin, but only the shortfin is found in New Zealand.
- The shortfin mako, Isurus oxyrinchus, is the world's fastest shark. It has been recorded swimming at speeds of about 100km/h.
- Mako, the Maori name for the shark, has been adopted worldwide, although it is sometimes called mackerel shark and blue pointer.
- Mako has short pectoral fins, a tiny second dorsal and anal fin, a crescent-shaped caudal fin, and indigo-blue dorsal surfaces and white undersides. It can weigh up to half a tonne.
- At full maturity, male mako sharks are about 200cm long and females 300-310cm. Males mature at eight years and females at 20 years.
- Mako are oceanic rather than coastal, swimming as deep as 650 metres deep. However they occasionally enter coastal waters but rarely attack humans.
- The species is listed as "vulnerable" on the International Union for Conservation of Nature list of threatened species.
- Most of the commercial catch of mako sharks is taken by tuna longliners and most of the catch is processed. New Zealand mako shark fisheries are managed under a quota system which limits the amount of sharks caught. Current catches are well below the quota.