Protagonists of great migrations: Caretta caretta turtles

Sea turtles are reptiles adapted to aquatic life, but depend on the terrestrial environment for laying and incubating their eggs. Each female-turtle lays an average of a hundred eggs and, of those hatched eggs, only few individuals manage to reach adulthood.

Sea turtles are very long-lived animals, usually nesting in the place where they were born, making very long migrations across the Mediterranean Sea, due to their exceptional orientation skills.

The three main species of sea turtle reported also in the Mediterranean Sea are the loggerhead turtle (Caretta caretta), the green turtle (Chelonia mydas) and the leatherback turtle (Dermochelys coriacea).

Caretta caretta (C. caretta) is by far the most widespread species in the Mediterranean Sea, moving from the Eastern basin of the Mediterranean Sea to the waters of the Tyrrhenian and Adriatic Seas during the feeding and wintering season. They play an important role in the ecological balance of marine ecosystems, occupying the top of the food chain and thus contributing to maintain the numerical balance of marine flora and fauna populations. They are considered excellent environmental indicators and sentinel species of the health of the seas. As well as it is considered as excellent environmental indicators and sentinel species for the environmental assessment of plastic pollution worldwide because they are long-lived species with a migratory behaviour and a high propensity to ingest plastic.

The C. caretta sea turtle is now classified as vulnerable by the IUCN Red List: a species of lesser concern in the Mediterranean region, but vulnerable on a global scale. Anthropogenic marine activities seriously affect the lives of sea turtles and the most common causes of stranding and death include the ‘entaglement’ of limbs with nets, fishing lines and/or macroplastics, accidental ingestion of fishing line and hook, and impact with boats. In fact, the impact of accidental catches by fishing gear such as trawlers, demersal longliners and fixed nets, brings the estimated total catch of turtles in the northern Adriatic Sea to more than several thousand per year.

These turtles migrate over long distances, thus extending their distribution area and habitats, which is why it is important to investigate their migratory routes and geolocation. The distance travelled and the mode of movement depend on the life cycle. It has been shown that movements of the species C. caretta change significantly depending on the size of the individuals, habitat characteristics, variations in surface temperature and water currents.

In the ecology and migrations (from the Atlantic to the Mediterranean) of C. caretta turtles, the Strait of Gibraltar, located between Morocco and Spain, plays a very important role. Currents in the Strait can trap sea turtles in the Western Mediterranean, making this region a specific feeding area. Ocean currents and wind direction can determine the locations of beachings.

In addition, juveniles and sub-adults migrate to coastal areas, usually far from the hatching site, to feed. While, adult females migrate from foraging areas to nesting grounds, often hundreds or thousands of kilometres away.

C. caretta turtles show a certain constancy in their migratory routes towards feeding, wintering and breeding sites, choosing the eastern Mediterranean coasts as their main nesting sites: most frequently they locate at beaches of Greece, Libya and Turkey, as well as some areas of Egypt, Israel, Lebanon, Tunisia and Italy. Regarding the main wintering and foraging sites in the Mediterranean, the most populated are the Gulf of Gabès in Tunisia, the Libyan coasts, the central-north Adriatic, the Turkish coasts and, to a lesser extent, the coasts of Egypt, Spain and Greece.

To this end, satellite tracking is now used not only to analyse the migratory routes of adult sea turtles in the Mediterranean Sea, an important foraging area, but also to identify foraging, breeding, nesting and wintering hotspots.

Hart and colleagues (2023) conducted a study on the migratory routes of C. caretta and Chelonia mydas, where subjects captured in Biscayne, Dry Tortugas, Everglades National Parks, Broward County beaches, within the Florida Keys National Marine Sanctuary, and North Carolina. Through the analysis of satellite data, it was demonstrated for the first time how the monitored individuals used Biscayne National Park, Florida, throughout the year as a foraging area.

The analysis of satellite data on the routes and diving behaviour of specimens of C. caretta, released on the Aeolian archipelago, made it possible to gather information on the environment in which these turtles were moving: some headed for the Sicilian channel, others headed for foraging habitats on the Tunisian plateau and the Gulf of Amvrakikos, in Greece. Similar data were found from the analysis of turtle routes monitored by satellite devices by CRe.Ta.M. (National Reference Centre on the Welfare, Monitoring and Diagnosis of Sea Turtle Diseases).  The longest route was travelled by a turtle released along the coast of Sciacca and followed between May and October: during its migration it stopped along the coasts of Greece, Turkey and then ended up along the coast of Laodicea (Syria) (Fig. 1), confirming these places as ideal foraging habitats for this species.

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