The journey of loggerhead turtles from the Northwest Atlantic to the Mediterranean Sea as recorded by the stable isotope ratios of O, C and N of their bones.

Loggerhead turtles, Caretta caretta, born on the nesting beaches of the Northwest Atlantic Ocean (US eastern coast) undertake a transoceanic migration immediately after birth, traveling eastward in association with the Gulf Stream and reaching the coasts of Europe and northwestern Africa when two or three years old and 20-30 cm in curve carapace length. Once there, they may remain in the eastern Atlantic or enter the Mediterranean Sea before eventually returning to the western Atlantic several years later. However, the timing of entry into the Mediterranean and the length of the period spent inside are poorly known. To study this, skeletochronology was combined with the analysis of the stable isotope ratios of oxygen (δ¹⁸O), carbon (δ¹³C) and nitrogen (δ¹⁵N) in the cortical bone of the humerus of 31 juvenile loggerhead turtles of Northwest Atlantic origin found dead stranded in the Balearic Islands. Incremental bone layers were sampled to assess changes in habitat through the movement across isotopically distinct water masses and the existence of any ontogenetic change in the diet. Although the incremental layers corresponding to the very first years of live were missing in all individuals, the wide range of δ¹⁸O values of the remaining layers suggested that these juveniles moved between water masses differing in salinity, from the eastern Atlantic, the western Mediterranean, and the much saltier eastern Mediterranean, without any consistent temporal pattern. Nevertheless, upon reaching ten years old, loggerhead turtles seem to settle in low salinity areas of the western Mediterranean, such as the Algerian Basin or the Alboran Sea, likely preparing for their return towards their natal beaches in the Northwest Atlantic. Finally, the changes observed in the δ¹³C and δ¹⁵N values were small, suggesting only minor ontogenetic changes in their diet throughout the analysed life stages.