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Friday October 31st, 7:45 am. On the quay in front of the Hôtel de la Collectivité, five men are busy. They are unloading crates of equipment from a car and placing them aboard a small zodiac. The mechanics are well oiled; this morning choreography has been going on for six days.
One of the pieces of equipment brought in is a strange pink-and-black torpedo-shaped contraption. "It's an autonomous underwater vehicle (AUV)," explains Jean-Baptiste Favier, a technician from the Creocean design office, which is in charge of the ongoing project with the Agence territoriale de l'environnement (ATE).
The mission undertaken by the two organizations is to map the seabed of Saint Barth from a depth of 18 to 30 meters. Similar work, down to 18 meters, had already taken place in 2022. But without the use of the robot. "The aim is to identify sensitive areas with high ecological stakes," explains ATE director Sébastien Gréaux.
The data collected will be shared on the agency's website by early 2026. They will be used by the Collectivité in its installation of sargassum nets. The data will also be available on the free navigation application "Donia", which sends an alert signal to boaters and fishermen when they are about to anchor in an area rich in biodiversity.
Robot and towed dives
A pre-mapping had already been established by Creocean during a remote detection process via satellites. "Our work now consists of verifying and improving this pre-mapping with actions in the field", adds Sébastien Cnudde, the office's environmental studies manager. In particular, the researchers are carrying out towed dives. "At the rate of two per day for two divers", says Hilaire Dufournier, marine ranger at ATE.
But the experts are counting above all on the star of the project, the autonomous underwater robot. The approximately one-meter-long tool is programmed before each mission to know how deep it will descend and in what type of environment. There's no need to operate it remotely from the boat. "It's equipped with a side-scan sonar," describes Jean-Baptiste Favier. This means that the device is capable of sending out sound waves which, thanks to the echo, enable it to locate itself underwater. This means it doesn't run the risk of bumping into landforms as it moves forward on its own. And it can map the seascape it travels over.
The crew goes out to sea every day, from morning until late afternoon. Once launched, the robot returns to the surface every 30 minutes. Creocean members then take the opportunity to download the data collected. "It takes a photo every five seconds," comments Sébastien Cnudde. Once the mission is over, the scientists will have around 6,000 photos. They will have to analyze these images and their precise location in order to proceed with the mapping.
The last such grid took place in 2013. Since then, the ocean floor has changed. "A very invasive weed has settled in Saint Barth," explains Sébastien Gréaux. It colonizes the sandy bottoms and has spread considerably. There has also been a regression of the reef zone. "The ATE director stresses the importance of carrying out an assessment every ten years or so. And the specialist concludes: "If we know our seabed well, we can preserve it and raise awareness. ".
