3 Great White Sharks drop tags in Tonga [1]
Friday, November 14, 2008 - 16:01. Updated on Wednesday, July 29, 2015 - 16:07.
By Mary Fonua
Three Great White Sharks - each over 4 metres long - have visited Tongatapu waters very recently, dropping their transmitting tags and surprising scientists who say that it is the first time they have known these creatures to come here.
"All three sharks may still be in Tongan waters or they may have left," said Clinton Duffy, a scientist with the Aquatic & Threats Unit in New Zealand's Department of Conservation, who confirmed that three tags have popped up in Tongan waters in the last two months.
The 4m long male and two 4.5m females were among six great whites that were tagged off the Chatham Islands in April by his collegue Dr Malcolm Francis from the NZ National Institute of Water & Atmospheric Research Ltd.
Clinton is looking for a shark tag that might have washed up on Ha'ateiho Beach on the southern coast of Tongatapu, last week. The tag that looks like a microphone sent out its last GPS signal from there at 4:33 pm on November 7.
Rare
Great White Sharks are rare to the point of being unknown in Tongan waters, and are found in the cooler waters off New Zealand and Australia.
Clinton said the PAT tag attached to the sharks only transmits after it has released from the shark. "We use the data transmitted back from the tag to work out where it has been," he said.
"We think the sharks may return to New Zealand and programmed the tags to stay on the sharks until January to see if that was the case. So, unfortunately, all of the tags that have reported from Tongan waters have come out prematurely. They may have grown out of the sharks, or caught on the bottom and pulled out. When this happens the tag will only drift for three days before it begins transmitting. This is triggered by the constant depth setting in the tags program -so if the depth does not vary by more than 4m in three days the tag automatically begins to transmit data. So there is no way that the tags could have drifted into Tongan waters," he said.
Visitors
Details of the sharks that have travelled to Tonga from the Chathams are:
(i) 4m long male tagged at Star Keys, 14 April 2008, pop-up location Ha'ateiho Beach, Tongatapu;
(ii) 4.5m long male tagged at Star Keys, 15 April 2008, pop-up location c. 50 km northwest of Tongatapu;
(iii) 4.5m long female tagged at Te Awapatiki, 19 April 2008, pop-up location c. 135 km south of Eua. (Clinton said that this shark may have visited Pelorus Reef but the tag actually started transmitting over deep water east of the reef, so it could have visited any of the sea mounts in that area).
"We tagged 6 sharks at the Chathams in April and two tags have yet to pop-up."
Protected
Great white sharks are protected in New Zealand waters because their numbers are in gradual decline and Clinton said the aim of the study is to identify important great white shark habitats in New Zealand waters and links between the New Zealand great white shark population and those found in other countries.
The females mature between 4.7-5.2 m total length and have a maximum of only 10 pups in a lifetime. They do not start giving birth until halfway through their life when it is thought they give birth every two to three years, said Clinton.
"Great whites are also listed on Appendix I and II of 'The Convention on the Conservation of Migratory Species of Wild Animals' (CMS, or Bonn Convention). Appendix 1 requires signatories to the convention to do everything they can to protect the species listed on it, and as New Zealand is a signatory to CMS the government was obligated to protect white sharks," he said.
Clinton hopes that someone will find the lost shark tag on the beach and that they can recover it.
"It is a Mk10 pop-up archival transmitting (PAT) tag manufactured by Wildlife Computers. It was attached to the shark and transmits data.
"An Argos satellite emails the data summaries to me, but if we get the tag back we are able to access all of the data collected. From this we could reconstruct a much more detailed picture of the shark's diving behaviour and temperature preferences. The information on light levels is used to roughly estimate the path the shark travelled along.
"This is the first time we have tracked great whites to the waters of Tonga. Previously we have had great white sharks tagged at the Chathams travel to New Caledonia and Vanuatu. We think they may be travelling to these areas to feed on humpback whales that die on migration or during birth. It is also possible they may attack sick, injured or orphaned calves. These areas also support deep water snapper fisheries and the sharks may also be feeding on fish aggregating around seamounts," he said.
Rare sight
Meanwhile, Poasi Fale, a scientist at Tonga's Ministry of Fisheries in Nuku'alofa agreed that the Great White is unknown here.
"To my knowledge the Ministry of Fisheries has not encountered or sighted this kind in our waters. We are tropical so that's why the great white don't like to come here. It is rare to be sighted," he said.
Teeth
The Great White Shark is the world's largest known predatory fish and like many other sharks, has rows of teeth behind the main ones, allowing any that break off to be rapidly replaced. According to Wikipedia, a typical adult great white shark measures 4 to 4.8 m (13 to 16 ft) with a typical weight of 680 to 1,100 kg (1,500 to 2,450 lb), females generally being larger than males. With a single bite, a great white can take in up to 14 kg (31 lb) of flesh, and can gorge on several hundred kilograms or pounds of food.