Last year in July, we deployed 10 micro-satellite tags on juvenile loggerheads in partnership with the Aquarium La Rochelle in France. Of this cohort, all of which are named after European scientists, the tags on turtles Archimede, Nicolas Copernic and Charles Darwin have continued to transmit for an astounding 312 days and counting (a new record)! As we prepare to release another set of turtles, we wanted to take a moment to reflect on some of the observations from this first deployment, and how they help us to take the next steps in our research.
The tracks of all 10 turtles released in July, 2024. Map by Tony Candela / Upwell.
The travels of Archimede are very interesting, since the turtle did not leave the Bay of Biscay even as temperatures dropped to 12-15 celsius (53.6-59 fahrenheit). Hard-shelled turtles can become cold-stunned, or hypothermic, when temperatures drop below 10˚C (50˚F) which is how many of the tagged turtles ended up at the Aquarium La Rochelle in the first place. However, Archimede stayed in the south of the Bay along the coast of Spain where water temperatures are slightly warmer and did not drop below 10˚C (50˚F), even in the winter months.
The movements of Archimede transmitted in the months of December 2024 through March of 2025 with a background of average sea surface temperature. Map by Tony Candela / Upwell.
The waters of the Bay are productive, with multiple rivers flowing in bringing high concentrations of nutrients that support a population of gelatinous organisms (jellyfish) throughout the year. This means that Archimede probably had a lot of opportunity for snacking on prey like molluscs like clams, crustaceans like crabs, along with softer foods like sea jellies and sponges. However, along with the abundant food comes abundant threats from fisheries and vessel traffic from transit, tourism, and recreation.
While we cannot draw conclusions from the behavior of a single turtle, Archimede's travels provide an interesting data point to researchers questioning if the Bay of Biscay is a year-round habitat for sea turtles or an ecological trap.
The other turtles in the cohort all seemed to head out of the Bay and into the Atlantic. The tags of Nicolas Copernic and Darwin continue to transmit, and show that they are meandering in the open ocean between mainland Portugal and the Azores. We have seen loggerheads tagged and released in both mainland Portugal and the Azores utilize this area as well.
The movements of Nicolas Copernic and Darwin with a background of the surface currents of the ocean. Animation by Tony Candela / Upwell.
While you may think that this is far enough from land to make them safe from anthropogenic (human created) threats, it is unfortunately not the case. The pelagic habitats are frequented by many fisheries targeting commercially valuable species such as tuna and swordfish, whose habitats often overlap with those of sea turtles.
Darwin’s tag has a pressure sensor that collects data on his diving behavior in addition to location. His behavior is in line with what we have seen from other young loggerheads - staying mostly within the first 5 meters (15 feet) of the ocean, but making occasional deeper dives. When we analyze this data further, we hope to learn more about how these dives may be related to environmental factors or other loggerhead behaviors. Keep an eye out for forthcoming publications on these analyses!
As Darwin, Archimede, and Nicolas Copernic’s tags continue to transmit, we are preparing another set of loggerheads rehabilitated at Aquarium La Rochelle for tagging and release. With increasing sample size, we are able to generate more robust statistics and develop stronger hypotheses. We can then integrate environmental datasets to examine correlations in turtle’s horizontal and vertical movements in relation to environmental factors such as sea surface temperature, productivity or concentrations of chlorophyll, the speed of currents, sea surface height, or bathymetry. The tracking data and analyses give us rare insight into the time juvenile loggerheads spend foraging and growing (up to 15 years) at sea, which is often referred to as their “Lost Years.” We use this insight to calibrate various models that can help us predict sea turtle habitat use or even population trends - models that aim to support effective and targeted conservation efforts.
Will this next set of turtles join Archimede in the Bay, or head out to the high seas like the others? We are excited to find out!