The big advantage of photo ID or marker tagging is that you do not need any specialist equipment to track the shark, you simply photograph the shark or check each shark for its tag number and can build up a picture of its movements by repeated sightings. This is good in theory but it is also the major weakness in that it requires someone to find the shark again, get into the water and to take the photo or read the tag; unfortunately experience has shown that these sharks are seldom seen and so tracking the sharks through re-sightings alone has limited scope.

This has heralded the use of high technology or 'smart tags' which can transmit information over distances so that it is not necessary to actual re-sight the shark to confirm where it is and what it is doing. The Seychelles programme is using three types of these advanced tags each giving remote access to information on the shark in different ways.

Once on the surface the tag transmits its data to the Argos satellite system from where it is decoded and sent via e-mail to the project researchers for analysis. The data gathered is translated by a computer program that compares the date and time of each recording with the depth and light readings and thereby gives an approximate geographical position of the shark's position at the time of each reading. From this analysis an overall picture of the sharks movements can be plotted as well as its daily diving and surfacing habits. Eight of these tags have been deployed by our programme to date.

The archival pop-off tags are very good for long-term monitoring but their position fixing is approximate at best; the Spot2 and Splash tag work differently and can give much more accurate position data. These tags will transmit whenever they are at the surface and by cross triangulation the receiving Argos satellite they can give accurate position fixes.

In order for the tag to transmit it has to have its aerial above the water and so the whole tag is encapsulated in a buoyant capsule. This makes the tags comparatively bulky, causing drag in the water, which in turn places more stress on the anchor point and makes it more difficult to keep them attached to the shark. They also require that the sharks co-operate by coming to the surface on a regular basis so that the tag can transmit. Attaching this type of tag is thus a matter of calculated compromise, the size of shark must not be too small nor too large for the unit, the length of the tether attaching the tag has to be long enough to allow maximum opportunity for the module to reach the surface but not be too long as to allow entanglement with other sharks, boats or reef structures. Six of these tags have been deployed in the project to date.

Acoustic Tags:

The last group of hi-tech tracking devices chosen for the project are designed for close quarters tracking; the acoustic tag has a transponder which sends out a 'sonar-like' signal which can be picked up by an in-water hydro-phone. By rotating the hydrophone and measuring the signal strength the distance of the tag from the receiver can be calculated and then plotted onto a chart, some versions also measure depth and transmit this as a coded pulse.

This system gives extremely accurate fine scale details of the sharks movements and diving / surfacing behaviour but is extremely resource intensive as it requires a boat with the tracking equipment and personnel to follow the shark for 24 hours a day, for normally 2 - 3 days. This can give excellent data but the logistical requirements and the shark's capability to travel up to 35 kilometres in a day tend to restrict the usage of this technology. Two of these tags were deployed in the first year of the project and provided some interesting data regarding the sharks inshore activities.