Especially vulnerable to cold-stunning due to their small size, early-stage juvenile hard-shelled turtles can quickly lose heat when water temperatures drop. When juvenile turtles can be rescued and rehabilitated after cold-stunning, we have a unique opportunity to gain insight into their lives at sea by outfitting them with small satellite tags upon release.

Upwell’s Lost Years Initiative is focused on understanding how juvenile turtles use their ocean habitat. Since 2020 we have deployed various models of Lotek micro-satellite tags—the smallest one a mere 2.5 grams—on juvenile turtles with institutions engaged in sea turtle conservation around the world. This summer, we welcomed The Aquarium La Rochelle in France to our Lost Years Initiative.

The Aquarium La Rochelle sits in the middle of France’s Bay of Biscay coastline. Their Centre d'Études et Soins pour les Tortues Marines (CESTM) or Center for the Study and Care of Marine Turtles in English, receives turtles reported in distress from a network they coordinate along France’s entire Atlantic coast. Upon arrival at the Centre, these turtles, suffering mainly from hypothermia, undernutrition (which can be related to plastic ingestion), or vessel strike injury, receive all the care necessary for their rehabilitation. CESTM cares for and studies marine turtles with the aim of reintroducing them into the natural environment and improving knowledge of endangered marine animals. To date, the Centre has rehabilitated and returned 138 sea turtles to the ocean in 2024. 

The Bay of Biscay is a “trap” for sea turtles. During warm-weather months the water temperatures and abundant nutrients create a favorable environment, but in the winter, temperatures can drop to a deadly (for hard-shelled turtles) 10°C (50°F). CESTM has previously conducted research to learn more about the turtles they care for post-release using satellite tags to track 28 turtles between 2008 and 2020. The turtles in this experiment had surprising trajectories, with larger turtles seemingly swimming west, possibly towards nesting habitats while smaller turtles stayed behind to forage in the colder (and productive) waters of the bay. This led the CESTM team to ask further questions about how sea turtles of various age ranges are utilizing the Bay of Biscay Habitat. 

Upwell’s Executive Director, Dr. George Shillinger, recognized that the rehabilitated early-stage juveniles at CESTM were perfect candidates for Lotek microsatellite tags and that the tags could help answer many of the CESTM team’s questions. He invited CESTM to join Upwell’s Lost Years Initiative, kicking off the collaboration with the tagging of 10 juvenile loggerheads. 

The collaboration was facilitated through Upwell’s ongoing partnership with researcher Dr. Philippe Gaspar at Mercator Ocean International, who works with Upwell on projects associated with Mercator’s Sea Turtle Active Movement Model.

George says, “We are thrilled to have Aquarium La Rochelle joining The Lost Years Initiative, and are looking forward to future releases that give us further insight into where these turtles go and to applying that information to advance management and conservation efforts.”

George traveled to La Rochelle in July, joined by collaborators on the Lost Years Initiative, including Tony Candela, Oceanographer and Marine Ecosystem Modeler for Upwell, Mercator Ocean International and CESTM and Frederic Vandeperre, Coordinator of the COSTA Project, University of the Azores Okeanos Institute.

They met with CESTM lead Florence Dell’Amico to prepare the tags and attach them to the turtles using methods described within Upwell’s recent collaborative manuscript describing Lotek microsatellite tag performance (Candela et al., 2024) on juveniles of multiple sea turtle species. The turtles in CESTM’s prior research ranged from 1.8 to 93.2 kilograms; but, thanks to Lotek’s micro-satellite tags enabling tracking of even smaller turtles, the turtles CESTM tagged and released in partnership with Upwell were 482 grams to 1.326 kg. 

The aquarium staff decided to give the turtles, which had been rescued from strandings both at sea and on land, names of famous European scientists. When they were tagged and ready for release, the team headed about 40 km offshore to return them to their ocean habitat. The maps below show the satellite tracks of the turtles since their release on July 24, 2024. The tracks are overlaid on a background indicating the surface current velocity and average water temperatures during the tracking period.

To date, the performance of the tags deployed in La Rochelle has been unprecedented: 80% of the tags are still transmitting after 70 days! They are providing the first-ever post-release movement data for rehabilitated cold-stunned loggerheads of this age and size class. Over the past two months, the turtles have shown a range of behavior from staying in the Bay to heading south to Spain or west into the Atlantic. They have been making surprisingly deep dives for turtles of such a young age.

Reflecting on the release, Florence Dell’Amico said, “For 35 years, the CESTM has rehabilitated and released juvenile marine turtles found stranded or in distress at sea on the French Atlantic and Channel Coast. Since 2008, we have deployed satellite tags to highlight their behavior in the Bay of Biscay, their survival rate after their release and to describe the lost years. It is so exciting to partner with Upwell and track the littlest turtles that we have never tracked before. Since we don’t know what they are doing once they are at sea, every little piece of information could be a huge insight!”

Here is a closer look at two the turtles and their journeys so far: 

Galilée:

Named for Italian scientist Galileo Galilei, Galilée was brought to the Aquarium after being found stranded on the beach of Pornic in April. One of the bigger turtles, Galilée grew to 1,036 g with a Straight Carapace Length (SCL) of 18.2 cm before release. Being the fastest swimmer, Galilée covered 1,313.4 km in about 70 days. After release, Galilée moved straight southwestwards and left the Bay of Biscay to rapidly reach the North Atlantic Ocean. This turtle spent about 77% of its time underwater and mostly (97%) in the first 5 meters of the ocean. However, it appeared that Galilée performed deep dives, reaching an average of about 19 meters each day, with a record down to almost 70 meters!

Archimede:

Named for the Greek mathematician and physicist, Archimede was brought to the Aquarium after being found stranded on Ré Island in May. One of the smaller turtles, Archimede grew to 546 g with a SCL of 15.3 cm before release. Archimede did not travel as much as Galilée, covering “only” 762.9 km during the same time. After release, Archimede moved southwards at about 60 km from the French Atlantic coast and remained on or near the continental shelf until it reached the northern coast of Spain. From there, this turtle moved westwards along the continental shelf, looped a little bit with the current but stayed within the Bay of Biscay and along the northern Spanish coast.

The data from these tags is being analyzed by Tony Candela, who, in addition to his role as Oceanographic Researcher for Upwell, is completing a doctoral degree at Toulouse University. Tony’s co-supervision team includes Dr. George Shillinger, Dr. Fred Vandeperre, Florence Dell’Amico and Dr. Patrick Lehodey (Mercator Ocean International). Tony looks at each turtle’s location data paired with real-time environmental data like ocean currents or the surrounding water temperature to try to understand the reasons behind their movements. For example, in the maps below you can see surface currents, temperature, and bathymetry (ocean depth). Tony sends his analysis in a weekly update to our international team of collaborators until all tags cease transmitting.

Tony also examines the travels of the loggerheads released from La Rochelle in the context of other data collected as part of the broader Lost Years Initiative. This collaborative initiative includes juvenile loggerhead releases in the Azores Archipelago with the COSTA Project, mainland Portugal with Zoomarine Algarve, South Africa with Two Oceans Aquarium, and Florida with Florida Atlantic University, all working together to understand the movements, dispersal, and habitat use of North Atlantic loggerhead turtles.

Drawing conclusions takes time, but each tag deployed and each dataset provides another small piece to the puzzle of juvenile turtles’ time at sea. Further understanding this mystery is essential to our efforts to mitigate human-related threats and better protect early-stage juvenile turtles during this vulnerable period.