Unveiling the "lost years"
Insights into the past, present and future of the North Atlantic loggerhead sea turtle population.
“Another one!” someone shouts, pointing at a dark dot on the sea surface a couple hundred metres ahead of our boat. As we approach, a sea turtle unravels on the surface. The animal, apparently resting, is completely unaware of our presence. “It’s the third loggerhead today”says Frederic Vandeperre, a researcher from the University of the Azores who has dedicated its research to sea turtles. Before we could say anything else, the turtle realized it was not alone and vanished down into the deep blue, flapping its powerful fins. “That was a large one!”, says Vendeperre. “Shouldn’t take much longer until it goes back to Florida”, replies Christopher Pham, also researcher at the University of the Azores.
An estimated 50 to 70 thousand annual spawnings of the loggerhead sea turtle (Caretta caretta) take place in the southeastern United States of America, particularly on the beaches of Florida. These numbers translate into approximately six to eight million eggs, assuming an average of 115 eggs per laying, making this region the largest nesting area in the Atlantic, and possibly the world.
After approximately two months of incubation in a nest dug by the mother, a few centimetres below the surface of the sandy beaches, the small and restless turtles, not exceeding five centimetres of shell, emerge from the ground and rush towards the sea. A short but risky journey, proving lethal for many of them, unable to avoid the onslaught of sea birds, crabs, reptiles, and small mammals, which do not hesitate to grab an easy meal. The fastest, or luckiest ones, manage to reach the water where they have to overcome the surf zone through vigorous, albeit uncoordinated flapping of their small fins. They then disappear into the immense blue, without looking back, with a promise to return one day.
The moment the newborns enter the sea marks the beginning of a phase of oceanic life for these animals. A phase that will last until adulthood, and which, for many decades, was characterised by almost total ignorance. Nobody knew for sure where the individuals were going, what they were eating, how they interacted with their surroundings and what dangers they faced. It was as if they disappeared without a trace. The scientific community called this period “the lost years”.
Faithfull to their promise, they would eventually return as adults, settling and becoming a regular presence in the coastal waters of the region they had left behind several years earlier.
In the 18th and 19th centuries, scientists were struggling with another mystery, involving the same species of turtle, but on the opposite side of the Atlantic: its great abundance, particularly in the waters around the Azores archipelago. Reports date back to 1595, when the Dutch captain Van Linschoten, wrote in his navigation routes: “… when passing from 36° to 39 1/3°, you will spot the Island of Flores with many turtles floating in the water”. During oceanographic campaigns in the Azores region aboard the ships ‘Hirondelle’ and ‘Princess Alice’ in the 19th century, Prince Albert I of Monaco also published some observations of these animals in the region. In his notes, the prince suggested that their origin might be the West Indies or Florida and that the turtles travelled all the way from the western Atlantic through the Gulf Stream. In 1972, the Dutch zoologist Leo Brongersma published a remarkable study on European sea turtles, mentioning the large number of small individuals around the Azores and suggesting, for the first time, that the animals found in European waters originated from the nesting beaches of the western Atlantic.
This theory could explain part of the mystery that had for so long kept American scientists awake at night. However, scientific evidence was needed to support it. According to Brongersma, such evidence could be achieved through the implementation of a turtle tagging programme that would allow to begin to understand their movements.
This programme started in the Azores in 1982, promoted by the researcher Helen Rost Martins, from the Department of Oceanography and Fisheries (DOP) of the University of the Azores and the naturalist Dalberto Pombo. “One day, in a café in Madalena on Pico Island, I noticed a flyer from Pombo asking local fishers to look for tagged turtles,” says Martins. “I contacted him straight away.”
Pombo had been in contact with American scientists for over a decade in order to obtain turtle tags, while attempting to locate animals he had already tagged, with the help of the fishers. This contact between Martins and Pombo turned out to be decisive for the start of the tagging programme: “With the support of the University of Florida, we were able to compensate local fishers to bring us turtles for tagging,” explains Martins. “And so, the programme began,” she concludes.
That year, a fundamental research step was taken when Martins shared data on the lengths of the shells of the first turtles tagged in the Azores with the Department of Biology of the American University, where Archie Carr, a professor at the University of Florida, one of the biggest names in sea turtle research and the creator of the expression “lost years”, was working. By analysing shell length of turtles in the Azores, Carr found that they fell into size classes that were not represented in American waters. As there were no known nesting colonies of loggerhead turtles in the eastern Atlantic that could translate into such an abundance of juveniles, the scientist concluded that the ‘Azorean’ turtles belonged to the population that hatched on the beaches of the American southeast and that they were swept away as hatchlings by the Gulf Stream. Integrating into the North Atlantic Subtropical Gyre, would then take them across the Atlantic, passing through the Azores, Madeira and the Canary Islands, before later returning as almost adults to their place of origin, leaving the oceanic environment (pelagic phase) and settling on the coast (neritic phase).
This idea, published in 1986, addressed the mystery of the “lost years”, and meant the unveiling of one of the greatest enigmas of the natural history of turtles at the time. Carr died the following year, but two of his students, Karen Bjorndal and Alan Bolten, continued his legacy.
Continuing their collaboration with the University of the Azores, Bjorndal, Bolten and Martins carried on with studying these animals, demonstrating not only that the size classes found filled precisely the missing classes in the western Atlantic, but also that the turtles moved from the Azores to Madeira, with the remaining route proposed by Carr (that the animals followed from Madeira to the Canaries and then started their journey back to the western Atlantic) being later confirmed by the tagging programme.
The definitive confirmation would come in 1997, in a publication involving the same authors and using genetic tools, at a time when these had just appeared. This new study concluded that practically all turtles found in the Azores were from the southeastern USA.
The continuation of this collaboration allowed for a full and detailed characterisation of their life cycle, namely regarding the duration and geographical distribution during the oceanic phase, something that had long remained unclear.
It was now known that the animals hatched on the beaches of the southeast USA between the end of June and the beginning of November and ‘embarked’ in the Gulf Stream, starting the oceanic phase. They could reach the Azores in less than a year, measuring around 10 centimetres. One individual, for example, arrived in 220 days. They remained there or drifted in a unidirectional clockwise migration to Madeira, Canary Islands and Cape Verde, for around nine to 12 years. After that, with an average carapace length of 55 centimetres, they started the migration that would take them back to the southeast of the USA which would mark the end of the oceanic phase and the start of their life cycle’s coastal phase.
Analysis of variables that could influence the survival rate of turtles in the oceanic phase, whether environmental or anthropogenic, was carried in the following years, resulting in a remarkable increase in knowledge in this area. It was recognised that the beginning of the oceanic phase was absolutely critical for the survival of this species, a period being characterised by a high mortality rate. Poor swimming capacity makes these animals passive swimmers, making them an easy prey, but also allowing them to be swept into oceanic waters by the Gulf Stream, where predators are less abundant than near the coast.
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This feature appears in ISSUE 22: The wild isles of Oceanographic Magazine
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