White shark tagging off New Zealand

Regional Population Connectivity, Oceanic Habitat, and Return Migration Revealed by Satellite Tagging of White Sharks, Carcharodon carcharias, at New Zealand Aggregation Sites.

A joint NIWA/Department of Conservation (DOC) tagging programme was launched in 2005. Hi-tech electronic tags are being used to gather information on where the sharks are and when, and to record their depth and the temperature of the surrounding water. Three tag types have been used: popup archival tags, acoustic tags, and dorsal fin tags.

Popup tags are implanted in the muscle under the dorsal fin with a tagging pole, and record depth, temperature and location, storing the data for up to a year. They then release themselves from the shark, float to the surface, and transmit summaries of the data to a satellite. If the tags are physically recovered, the high resolution data collected at one minute intervals can be downloaded. Popup tags provide only approximate location data, so they are most useful for tracking long-distance migrations.

Acoustic tags send out coded, individually identifiable sound ‘pings’ that can be detected up to a kilometre away by acoustic data loggers. The tag batteries last long enough to monitor the presence of white sharks in the region in the vicinity of a data logger for two years. Acoustic tags provide accurate fine-scale information on sharks at specific locations.

Dorsal fin tags bridge the gap between popup and acoustic tags. They provide accurate location information by transmitting to orbiting satellites every time the shark is at the surface and the dorsal fin and the tag’s aerial are exposed to air. Their batteries can last for more than one year, so both fine scale and large scale movement patterns can be recorded. However, dorsal fin tags are more difficult to deploy: the shark has to be caught and restrained while the tag is attached to the dorsal fin. So far, only a few of these tags have been deployed in New Zealand.

Since 2005, 44 white sharks have been tagged with popup tags, mainly at the Chatham Islands and Stewart Island. These islands support large colonies of fur seals, which are a major food source for white sharks. Our research has focussed on Stewart Island since 2009. The population there is dominated by males (about 2.2 males for every female). Nearly all of the females are immature, being shorter than the female length at maturity of 4.5 to 5 m. Only about one-third of the males are longer than the male length at maturity of about 3.6 m. This indicates that the white shark aggregations at Stewart Island are not related to mating, and are most likely driven by the abundance of seals for food. Large, mature females are rarely seen anywhere in New Zealand and their distribution, habitat and behaviour are almost completely unknown.

Tagging results show that most New Zealand white sharks make annual migrations to tropical waters in winter, travelling as far as 3,300 km away. Sharks have migrated to the Great Barrier Reef in Australia, the Coral Sea, New Caledonia, Vanuatu, Norfolk Island, Fiji and Tonga. They don't cross the equator.

Most of the sharks from Stewart Island headed northwest of New Zealand, whereas most Chatham Islands sharks headed north. However, some sharks from the two tagging locations overlap in the tropics. Surprisingly, we have not detected any direct movement between Stewart and Chatham islands.

Some popup tags have remained on the sharks for long enough (up to one year) to reveal that the sharks returned to their tagging locations after their tropical holiday. This has been confirmed by photo-identification work at Stewart Island. Each shark has a unique colour pattern, particularly around the gills and on the tail. Some individuals have been seen at Stewart Island each year for multiple years. This indicates that white sharks have a sophisticated navigation mechanism that enables them to make major direct oceanic migrations, and then return to precisely the spot they left from. We do not know how they achieve this.