Where do sea turtles forage in the Mediterranean sea? Filling the gaps for a regional assessment of migration routes

Sea turtles are highly migratory species that can travel vast distances between their feeding and breeding sites. Considering the vulnerability and conservation concern of most sea turtle species, it is essential to establish connections between foraging and nesting areas to link threats at seas with the affected populations. Previous studies analyzed the population of origin for loggerhead turtles (Caretta caretta) in most known Mediterranean foraging areas, while knowledge on green turtles (Chelonia mydas) foraging areas is much scarcer. In this context, Kuşadası Bay, an important foraging zone located in the Aegean Sea, is inhabited by the two species all year round, although their population composition is unknown. Here, we evaluated the populations of origin of turtles foraging in Kuşadası Bay from 2017 to 2021 to perform a regional assessment of the migratory routes of both species. We genotyped 31 green turtles, using four mitochondrial DNA (mtDNA) short tandem repeats (STRs), and 61 loggerhead turtles, using a segment of the mtDNA D-loop (control region). The mixed-stock analysis (MSA) revealed that turtles from Kuşadası Bay come mainly from Turkish nesting populations, with the loggerheads mostly from western beaches and green turtles from easternmost beaches. By integrating all previously published studies of Mediterranean foraging grounds, we delineate the main foraging routes of both species in the Mediterranean that were consistent with prevailing currents. However, significant differences were generally found when comparing the foraging grounds observed composition with those predicted by particle modeling, indicating that juvenile and adult turtles may combine passive drifting with active swimming during their migrations in most of the foraging areas. This study highlights the importance of using genetic tools to identify the origin of sea turtles and to gather information from a wide geographic scale to refine the knowledge of the main foraging migration routes in Mediterranean Sea turtles at regional level.