Great white sharks (Carcharodon carcharias) are arguably the most recognisable sharks on the planet.
The subject of numerous movies, books, articles and scientific papers, these charismatic sharks don’t always get a good press due to their sometimes-bad eating habits. It could even be said that they have an image problem.
Everyone, it seems, knows lots about great whites, yet when we critically examine our accumulated knowledge, we know surprisingly little about their biology, behaviour and population status.
White sharks are apex predators (they eat large fish and marine mammals), and because of energy loss in the food chain, their population size must be substantially less than that of their prey. White sharks appear to be declining in most of their world range, but it is very difficult to count or monitor them, and there is no information on whether the New Zealand population is increasing, stable or declining.
Understanding shark populations
We do know that white sharks grow slowly and do not produce many young (females don’t mature until about five metres long, and only produce about five young per year), so they cannot sustain high catches. In New Zealand, white sharks are regularly caught as bycatch in coastal fisheries (mainly in set-nets), and there is a small catch by trophy hunters. In most other parts of the world where they are common (notably Australia, South Africa and the USA), white sharks have been protected. The New Zealand Government has also protected them (from April 1, 2007).
This begs the question of how effective protection will be. We currently do not know whether New Zealand white sharks form a distinct population or whether they are part of a more widespread stock, and might therefore move outside the range of any planned protection. In a collaborative research programme funded by NIWA, DoC and the Wildlife Conservation Society, we aimed to determine the spatial scale of movement of New Zealand white sharks over a period of months: are they home-ranging or do they rove beyond our EEZ?
In April 2005, we mounted a trial tagging expedition to the Chatham Islands. The Chathams were chosen because we knew we could find white sharks there, we had the support of local fisherman and shark-cage diving operator, Tim Gregory-Hunt, and we anticipated that any sharks moving away from the Chathams would undertake open-ocean movements, most likely towards the New Zealand mainland. Successful results at the Chathams would provide a platform for extending this research to white sharks around mainland New Zealand.
Modern tagging technology
Fish tags have come a long way since the days of simple plastic markers with a serial number printed on them. Modern electronic tags gather data on time, depth, light intensity (from which geographic location can be estimated) and temperature every few minutes, and store them onboard. Pop-up Archival Transmitting (PAT) tags can be programmed to auto-release from the shark (by sending a small current through a corrodible link) after a pre-determined time. They then float to the surface and transmit the data to a satellite, which beams the data directly to the scientist’s computer by email without the need to recapture the shark. All we needed to do was tag the sharks and then sit back and wait!
At about $5000 each, PAT tags are not cheap, and our budget allowed for the purchase of just four tags. However, these would be the first electronic tags deployed on any species of shark in New Zealand waters, so the opportunity for new discoveries was high.
After several days of watching bad weather streak by, we hit a window of fine weather and set off for a seal colony on some remote rocks. It’s true that seals attract great whites – surfing or diving in such locations is definitely not recommended! Within nine minutes of starting the berley trail, we had a shark at the boat. Our tuna baits were hungrily devoured, and several sharks returned repeatedly to the boat, allowing us to choose a good moment to tag them.
Tags are anchored under the skin by jabbing with a tagging pole (the needle tip is fitted with a stopper to prevent it penetrating too far). Sharks occasionally reacted to being tagged, but often returned to feed again, indicating that the tagging procedure caused minimal discomfort.
Over the course of three days, we deployed all four PAT tags, plus three conventional gamefish dart tags. We saw about 10 different sharks altogether, most of them about 3-4.5m long, although one large female greater than 5m long was clearly the big mamma of the bunch. Unfortunately, she was very cautious and wouldn’t come close enough to be tagged.
Our PAT tags were deployed for three to six months, leaving us a long nervous wait to see if they would pop up on schedule and transmit data. The four-month tag caused great disappointment when it came off prematurely after only a week and washed up on a beach on Chatham Island, never to be seen again. The other three tags ran their full course and transmitted data, although they all called home about 12 hours late, giving us much anxiety.
Trips to the tropics
The three ‘successful’ sharks remained near the Chatham Islands for two to five months after tagging. We suspect they stayed close to seal colonies feeding on young seals as they learnt to swim. Two of the sharks departed from the Chathams within a week of each other, and the third departed several months later. They all headed north or northwest.
One tag popped up near the Louisville Seamount Chain, northeast of East Cape, about 1,000km from the Chathams. The other two sharks, carrying five- and six-month tags respectively, travelled almost 3,000km into tropical waters north of New Zealand – one tag popped up in New Caledonia and the other in Vanuatu.
These results were a great surprise to us – white sharks have not previously been reported from the Louisville Seamount area, and although they have been seen in tropical waters, sightings there are rare. These three sharks sought warm seas during winter, a trait shared by many Kiwis. We are puzzled about the reasons for these movements, but are exploring several theories that might explain them.
We have only a small sample size so far, but it is clear that moves to protect white sharks in New Zealand waters may not extend far enough. Our three sharks all moved outside the New Zealand EEZ and away from our legal jurisdiction. This suggests that white sharks should be protected in the areas they visit frequently during their migratory movements, as well as in New Zealand.
Buoyed by the success of 2005, we carried out a second expedition to the Chatham Islands in March 2006, this time armed with more PAT tags and also some SPOT tags. SPOT tags are attached to the shark’s dorsal fin and have a small aerial that transmits position information to a satellite every time the dorsal fin breaks the surface. No more anxious waiting for months for a tag to pop off and transmit data! The main drawback with SPOT tags is that you need to catch the shark first and restrain it while attaching the tag!
Unfortunately, the March trip was completely unsuccessful. We lost almost half our available time to bad weather, and when we did get out to sea, the sharks seemed too full and were uninterested in our baits. We saw sharks on most days as they swam up our berley trail, but they couldn’t be enticed close enough to the boat to tag or catch. Perhaps we were just spoilt in 2005, or perhaps it was because we went a month earlier in 2006?
We had to wait another long year before we could mount a third expedition, this time to the Titi Islands, north-east of Stewart Island, in March 2007. Again we were plagued by bad weather and uncooperative sharks, but we did successfully PAT-tag a large 4.4m female, and this was followed by a 3.3m male, which we caught and manoeuvred into our tagging cradle on the afternoon of the last day of the trip. With a towel over its eyes, a hose pipe in its mouth to oxygenate the gills, and the loving attention of our vet – who administered anti-stress and antibiotic drugs – we managed to attach a SPOT tag to the dorsal fin and a PAT tag just below the dorsal fin. We had just deployed the first SPOT tag ever put on a shark in New Zealand.
Using two tags on one animal offers us the best of all worlds – SPOT tags give high quality location information in near real time whenever the shark surfaces, while PAT tags provide detailed depth and temperature records. We hope to find out if our three Chathams’ sharks were unusual or typical, or whether mainland sharks also make tropical migrations. The PAT tags on the two sharks were each deployed for nine months, so we have a long wait for them to pop up in late December – delivering a sack load of new data in time for Christmas!
The nine-month deployment also gives us a chance to monitor a ‘there-and-back’ migration, if such things occur, and find out more about what great whites do in the tropics.
One day after we tagged the male shark off the Titi Islands, the SPOT tag started transmitting locations to the satellite. We received many hits as he swam backwards and forwards through Foveaux Strait and to Ruapuke Island in the eastern strait. Interestingly, he did not return to the tagging site. We hope this tag will keep transmitting for many more months.
White sharks have proven difficult to study in the past, and the use of exciting new electronic technology is opening new windows into their behaviour.
Such information could not be obtained in any other way. We have only scratched the surface so far, but the results are already intriguing and astonishing. Elsewhere in the world, white sharks cross ocean basins. Tagged sharks have moved from California to Hawaii, and from South Africa to Australia and back, and a shark tagged in Australia with a conventional tag was recaptured near Ninety Mile Beach. It now looks like New Zealand sharks enjoy ocean travel, too.
PAT tags have also shown that during such ocean crossings white sharks spend most of their time near the surface (often shallow enough for the dorsal fin to break the surface), but they periodically make deep dives to hundreds of metres. We can only speculate as to what they are doing down there in the dark and cold!
No longer can we consider great whites to be stay-at-home, coastal sharks with a mundane lifestyle. They may stay put for extended periods of time (our Chathams’ sharks remained near the islands for up to five months), so they are ‘resident’ while it suits them, but they are also ocean-going rovers that can cover thousands of kilometres in just a few weeks.
Tagging white sharks isn’t cheap and isn’t always easy, but the potential rewards in terms of understanding the behaviour of one of the ocean’s greatest predators are enormous. We are slowly unravelling the enigma of great white shark behaviour.
This article is reproduced with permission of
New Zealand Fishing News
Sept 2007 - by
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