Identifying individual animals over a period of time can provide information on population size, and individual survival amongst other things and as such is a key tool for conservation. But how do you identify animals that basically look alike?
The Marine Conservation Society, Seychelles (MCSS) in collaboration with Ministry of Environment and the Kelonia Marine Turtle Observatory in Reunion, recently held two workshops funded by Mangroves for the Future for scientists and the public, explaining how photo-identification can be used on turtles and whale sharks.
Whale sharks are regularly visitors to Seychelles waters and in 1997 MCSS set up a long term monitoring programme to help learn more about these elusive creatures. Initially, sharks were identified with marker tags giving each tagged animal a unique and easily recognisable number. Later, however, a photo-identification technique developed for ragged-tooth sharks in South Africa that used the pattern of spots on the sides of the sharks was shown to be effective in identifying individual whale sharks also.
The area found to be most suitable, is the area behind the last gill slit on each side (see photo). Photographs of this area, which include the top and bottom of the last gill slit and the edge of the pectoral fin, provide ‘landmark’ points that allow the image to be digitally ‘fingerprinted’. These fingerprints can then be used to rapidly identify the individual sharks using a special computer program (I3S).
The MCSS has built up a database containing over 13,000 images of whale sharks taken around Seychelles and from these they have identified 447 individual sharks over the last 10 years. Of these, 99 sharks have been resighted in multiple years, the longest span being for 4 sharks identified in 2001 and resighted this year. These images and their fingerprints are shared freely with other researchers in the Indian Ocean and also on the global whale shark database and are helping to unravel the mysteries surrounding the lives of this the world’s largest living shark.
The spot patterns on whale shark can be used to identify individual animals in a population, the box shows the critical area needed to get a usable fingerprint. Photo Luke Riley.
In some places, such as the Aldabra Atoll World Heritage site in Seychelles, juveniles can also be caught on the reef flats for tagging and weighing. Mature adult males, however, are pretty much inaccessible not only because they occur in deeper waters, but also swim much faster and weigh significantly more, making it hard to catch them for weighing, measuring and tagging.
Given these limitations, a number of organisations around the world have looked at ways of using photographs, rather than marker tags, to identify individual turtles. Claire Jean, Project Officer at the Kelonia Marine Turtle Observatory, in association with the Information Technologies department at the University of Reunion, have recently devised a method which uses the number, location and shape of scales from the left and right side of the turtle's head to identify individuals in a population.
Results from Green turtles photographed by divers around Reunion, Mayotte and Glorieuse islands have shown that individual animals can be reliable identified in their foraging grounds using this method.
To date, 60 Green turtles have been identified by researchers at Kelonia, with at least 14 individuals being re-sighted a few months later. Photographs of turtles in Seychelles, submitted by MCSS, have added an additional 36 Hawksbill turtles to this Indian Ocean database.
One of the many Hawksbill turtles photographed in Seychelles waters, the box shows the critical area needed to get a usable fingerprint. Photo David Rowat.
HELP US KEEP TRACK OF OUR ENDANGERED SPECIES:
If you would like to assist with these programmes, please feel free to send us your photographs of whale sharks and turtles. Simply include: your name, the date and location of the photograph and the animal’s behaviour at the time (i.e. feeding, resting, swimming etc.) and we will include it in the rapidly growing database.
PO Box 1299,