Deployment of the network :
From September 16 to October 20, 2020, three members of Ifremer's Indian Ocean delegation traveled to the island of Europa located in the Mozambique Channel to set up the network of receiving stations and the satellite connection for the turtle tagging operations scheduled for the first half of 2021.
The old receiving stations deployed as part of the pilot project (pIOT) were dismantled and replaced by new structures. Indeed, the screws were in an advanced state of corrosion due to the strong sea spray and the extreme conditions in this region.
Three new receiving stations now replace the five pilot stations in the lagoon area. The team's technological advances and the results of the pIOT project have shown that only three stations, equipped with the new, more efficient boxes and powered by larger batteries and solar panels, are sufficient to cover the entire lagoon and receive the signals from the tags deployed on the turtles. The team has also installed a satellite internet connection that will enable the data collected on the island to be sent directly to the project's scientists and to remotely monitor (from Ifremer's premises on Reunion Island) the proper functioning of the equipment.
Cartographic surveys with the autonomous USV board :
During this mission, the autonomous motorised USV (Unmanned Surface Vehicle) board developed by Ifremer and IDOCEAN was deployed on different areas of the lagoon in order to carry out bathymetric and photogrammetric surveys of the lagoon bottom. The long-term objective is to compare the turtle positioning data obtained from the tags with the bathymetric and photogrammetric measurements and to provide information on the geographical occupation of habitats by juvenile turtles in Europa.
For this purpose, the board is equipped with a single-beam echo sounder that measures depth (bathymetry), an underwater camera that takes 30 images/second and a GPS RTK with a positioning precision of around one centimetre. Despite fairly difficult weather conditions, a large amount of data has been collected and a major analysis of this data has begun but will still require time for processing.
The team also took advantage of this long mission to carry out numerous in situ tests of the network using the autonomous board.
Experimental taggings :
One of the other objectives of this mission was to test and validate the robustness of the mechanical design and on-board algorithms of the tags in real conditions, but also to confirm the reception and processing of the data provided by the tags via the LoRa network deployed on the island. To do this, the team equipped two juvenile green turtles with a new version of the tags.
Although no problems were detected during the tests previously conducted in the ponds of Kelonia in Reunion Island, the two tags unfortunately encountered problems shortly after their deployment. However, the data collected made it possible to highlight certain turtle behaviours never observed in the basins on Reunion Island. Indeed, the juveniles in Europa remain much less at the surface which does not make it possible to obtain precise GPS positions. To overcome this problem, the onboard software of the tags will be quickly adapted to this behaviour.
At the same time, LoRa and GPS reception tests were carried out using a tag kept on the boat used to navigate in the lagoon or on a board towed by the boat in order to get as close as possible to the behaviour of the turtles when they come to the surface to breathe and thus simulate the data transmission. The tag was programmed to transmit LoRa messages at regular time intervals (about 1 second) associated with the GPS position measured at same time. Depending on the GPS position and therefore the distance of the tag from the receiving stations on land, the reception level of each of the receiving stations was measured and mapped. The closer a tag is to a station, the better the reception of the LoRa signal, but regardless of the position of the tag in the lagoon, the signal was always received by one of the three stations. The quality of the GPS positions obtained with the tags was also validated during the mission.
The network of receiving stations and the satellite link are now operational and ready for phase two of the project on Europa: the deployment of the final version of the tag on eight juvenile turtles, scheduled for the first half of 2021.
This first mission will have made it possible to test the various equipment developed for the project under real conditions and to make the final adjustments necessary for the successful completion of the project on the various study sites by 2021.
Ifremer would like to thank the French Southern and Antarctic Lands (TAAF), managers of the island, the Armed Forces in the southern Indian Ocean area (FAZSOI) and the French Navy for the logistical support without which the mission could not have been achieved.