Where and when do white sharks occur in New Zealand waters, and how can fisheries bycatch be reduced?
Why do we want to tag white sharks?
White sharks (Carcharodon carcharias) became a protected species in New Zealand waters in 2007, but many of them are still caught incidentally each year in set nets and other fishing gear.
The status of white sharks in New Zealand is unknown, but because of their low reproductive and growth rates, there is concern that fishing mortality may be causing a population decline. Little is known about the habitat requirements of white sharks, their seasonal behaviour patterns, or their interactions with fisheries. An improved understanding of where they are at what times, and their migratory patterns would allow strategies for reducing bycatch to be developed.
Hi-tech electronic tags
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
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 two of these tags have been deployed in New Zealand.
New insights into the behaviour of white sharks
Since 2005, 35 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.
The results (click righthand map to enlarge) 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 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 four years, and many others have been seen more than once. This indicates that white sharks have a sophisticated navigation mechanism that enables them to make major oceanic migrations, and then return to precisely the spot they left from.
During their travels, white sharks experience a wide range in temperatures, from 3 degrees C to 27 degrees C: very unusual for a fish. During their ocean crossings, they can travel 150 km per day, and average about 5 km per hour.
They spend two-thirds of their time at the surface, and the rest making repeated deep dives to 200-800 m. Most sharks reach maximum depths exceeding 800 m, and the record depth achieved was 1,200 m. We do not know what the sharks are doing in these depths - where there is no light - but we presume that they are searching for prey such as deep water squids and fishes.
Now that the large-scale migrations of white sharks are better understood, the focus of the research has shifted to determining their fine-scale movements and local temporal and spatial distributions. In March 2011, 25 white sharks were tagged with acoustic tags around north-eastern Stewart Island, and 25 data loggers were deployed at strategic locations around northern Stewart Island, Ruapuke Island and Foveaux Strait.
The data collected will tell us when individual sharks are present at the Stewart Island ‘hot spots’, enabling us to determine their residency periods, seasonal occurrence patterns, and which sharks return each year to the same place. Arrays of data loggers set up around the coastlines of Australia and New Caledonia should also detect our sharks if they migrate to those places.
Data collected from acoustic tags and dorsal fin tags in the future should reveal much more about where sharks are in New Zealand coastal waters, when they are there, and how long they stay for. This information will then be compared with the distribution of set net and longline fishing effort around New Zealand to identify locations and seasons where there is a greater risk of sharks being caught by fishing gear.