A la carte turtles
On the French islands of the Indian Ocean, Jérôme Bourjea, researcher atIfremerresearcher Jérôme Bourjea has been unravelling the workings of sea turtles for over 15 years. From the migrations of adults, to the coastal diet of juveniles, to the impact on their health, biologging pierces the shell of these mysterious animals.
Green turtles or hawksbill turtles, the French islands of the Indian Ocean are lucky enough to see these great migratory birds return to their beaches every year. " Reunion Island, Mayotte and the Eparses Islands are hot spots for sea turtles, which come to breed before leaving to feed elsewhere," explains Jérôme Bourjea, researcher at the Marbec marine biology laboratory. To find out where they eat, in 2010 scientists fitted Argos tags to the shells of around a hundred turtles, using a heating epoxy resin, and tracked them against the wind and tide for almost a year.
Turtles hide to eat
The results were impressive," recalls Jérôme Bourjea, "we were able to see them spread out like fireworks: Mozambique, South Africa, Seychelles, Madagascar, Kenya... They spread out all over the south-west Indian Ocean!" Valuable information for the conservation of these endangered species. "This will enable us to better manage the French turtle heritage by cooperating with our neighbors, where they migrate to eat.
Building on these initial successes with adults, the researchers used biologging to learn more about the juvenile turtles that settle on the coast and feed there for the first ten years of their lives. "We see them, but do they always stay in the same place? Do they circle the island? Do they mix?" asks the biologist, who this time used GPS beacons to pinpoint their geolocation to within 5 or 10 meters.
The data collected revealed different feeding strategies in juvenile turtles, depending on the islands but also on the individuals. Some turtles are very loyal and will use an area no bigger than a soccer pitch, probably because they find good food there," explains the researcher. Others are more exploratory, moving to different areas after a few weeks.
The GPS readings were then cross-referenced with maps of the seabed, such as areas of seagrass beds, red algae and so on. "It's interesting, but GPS can't tell us whether the turtles are eating or sleeping. So we fitted them with cameras, but with an autonomy of just 2 hours per individual, this data is more qualitative than quantitative."
The images have nevertheless provided new information on the turtles' feeding strategies, but also on their social behavior. " We realized that they had social interactions that we had previously thought were totally non-existent", explains Jérôme Bourjea, who will be heading off to Aldabra, a virgin island in the Indian Ocean, in a few days' time, to establish the link between turtle diet, health and the environment, and to demonstrate "that turtles can be true sentinels of the environment".
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