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Near-Real Time Satellite Tags
How Do Satellite Tags Work?
Near-real time satellite tags are especially exciting because they allow us to track the sharks day by day and almost on a real-time basis. Every time the shark comes to the surface of the sea, the tags send a radio signal that is picked up by passing polar-orbit satellites carrying the Argos System. The satellites then re-transmit this message to land-based Argos Centers where the information is processed. The global position of the tag and any data it might have sent is then sent directly to our computers via email, or we can get it directly from web-based database. The entire procedure takes about an hour or two, thus the 'near' real time name of these tags. In this way, we can keep an up to date track of where the white sharks are day to day with only a few hours delay.
 This research uses two types of near-real time satellite tags, SPOT tags and SDR tags. SPOT tags (Smart Position and Temperature Transmitting tags) give us information on the position of the sharks and on the ambient temperature; SDR tags (Depth Recording Satellite tags) as their name indicates, in addition give us information on the depth where the shark has been in the 24 hrs previous to the message being sent. More precisely, every time the shark's fin breaks the water surface a 'salt-water' switch senses if the instrument is out of the water, then allows the tag to transmit its signal to the satellite in a matter of milliseconds, thus saving precious battery time. However, for a given transmission to be received, a satellite needs to be passing over the shark at the same time that the transmission from the tag is being sent. This is why many times days go by without us hearing from the sharks.
A world's first in shark health-care and safety
The method used by the research team to attach near-real time satellite tags to the dorsal fins of white sharks is very involved. This technique required a lot of careful planning and development to ensure that the handling of the sharks is done in a safe and efficient way that minimizes stress and avoids permanent or major damage to the sharks or threatens their health or life, while making sure that the tags are securely attached and the safety of the researchers is also guaranteed.
 An important part of this complicated process is the metal cradle used to lift the sharks out of the water and restrain their movements while the scientific team fits them with the satellite tags. This cradle was specially designed by the research team and built in South Africa, based on the experience of other shark scientists who kindly shared their knowledge.
Once the sharks are in the cradle there are one or two veterinarians taking care of the sharks' health at all times. First, a hose with pumped seawater is placed in the shark's mouth providing a constant flow of water over the gills and maintaining the sharks' oxygen supply. Then the veterinarians inject them with several drugs, antibiotics and vitamins to help them overcome the stress of capture, making this the first research of its kind to include veterinarians on board to take care of the sharks and reduce the impacts of the tagging. During this time the rest of the scientific  team securely fixes the satellite tag to the dorsal fin of the shark using nylon pins that pass through holes that are drilled into the shark's dorsal fin, and which are secured with metal nuts and washers. The tags have been designed not to remain permanently attached to the sharks: the combination of metals chosen for the nuts and washers ensures that they will corrode after several months eventually causing the tag to fall off.
Because the sharks are caught on bait and hook, before returning them to the water the hooks are cut and completely removed, leaving only a small wound that will heal shortly leaving no permanent damage to the sharks. Because the health of the sharks is a top priority, the team has a highly skilled fisher that places the hook right on the corner of the jaw and brings the shark into the cradle after a minimum struggle and in great shape. Overall, the team has managed to consistently handle the sharks in only about 14-16 minutes total time, from hooking to the freeing of the shark with transmitters in place; typical times out of the water in the cradle run from 3-7 minutes only.
What Does the Data Tell Us?
The daily movements of the white sharks that have been tagged with SPOT or SDR tags can be followed because the satellites that receive the transmissions from either tag can determine the accurate global position of each tag. By following the sharks day by day we can learn a lot about their movements and gather information about the areas they frequently visit and how much time they spend on each area, as well as determining any specific routes they might use for their long distance movements. Over larger periods of time we will also be able to see how far they move, how frequently they leave the protected waters of South Africa and what other countries they travel to. In addition the messages received from the SDR tag provide information about the different depths the shark reaches during dives and how much time is spent at each depth, allowing us to gain insight into what areas of the water column they utilize and to what extent. All of this information is essential in determining the effectiveness of protection in South Africa and the need for further protection of the species elsewhere, as well as for better understanding the basic ecology of great white sharks.
Where are South African great white sharks going?
Since the field work began, in mid-2002, a total of 18 sharks have been successfully fitted with near-real time satellite tags in South Africa; 4 in May 2003, 3 in November 2003, and 11 in May 2004. During 2002, the team had attached 4 real time satellite tags to white sharks in Mossel Bay. However these set of tags were not attached to the sharks' dorsal fins, instead the tags, which were in flotation devices shaped like torpedoes, were attached to the back of the shark using a small dart. Unfortunately these tags did not stay attached to the sharks and fell off after a few days. Due to the low success rate using this attachment method, the team decided to switch to the method currently used and described above.
The results from the May 2003 field campaign have shown among other things, that two sharks underwent long distance movements mostly along the coast, towards the north east part of South Africa. One of the sharks, a juvenile female of 283 cm in length and nicknamed Megan, was followed for a total of 77 days as it moved from Mossel Bay where it was tagged on May 31, to the waters off Durban. Its trip covered a minimum of 1,600 km (1,000 miles)!
Last time we heard from Megan was on October 1, but this transmission was not good enough for the satellite to estimate her position. The tracking of two other sharks showed less dramatic results, with a female of 252 cm in length moving out of Mossel Bay to the west and making a round trip in offshore waters of 500 km (310 miles), returning back to Mossel Bay after 58 days.
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