From patchy records to basin-wide insight: Integrating six decades of Mediterranean sea turtle nesting data

Transboundary conservation of migratory marine species is frequently constrained by fragmented and locally isolated biodiversity data, limiting the detection of large-scale ecological change. In the Mediterranean Sea, one of the world's most politically complex and environmentally pressured basins, decades of sea turtle nesting observations have remained dispersed across countries and organizations, preventing a holistic assessment of population trends. Here, we integrate over sixty years of nesting records for Caretta caretta and Chelonia mydas into the first standardized, basin-wide database for the Mediterranean (MedSTN), harmonizing 2290 validated records spanning 1960–2023. By integrating this database into Generalized Additive Models (GAMs), we reveal coherent regional patterns invisible in local assessments. Our results show a fundamental structural disparity in population dynamics: while the Eastern Mediterranean remains the primary reproductive stronghold with stable nesting core areas, for both species, we detect a significant northward range expansion for C. caretta in the Western Mediterranean. This emerging nesting trend is characterized by an exponential increase in unique nesting sites following a distinct temporal breakpoint circa 2010, suggesting a potential northward range expansion that is consistent with climate-mediated thermal suitability and may be influenced by trans-oceanic immigration. Conversely, C. mydas nesting remains almost entirely confined to its Eastern strongholds, exhibiting long-term latitudinal stasis despite localized increases. By transforming a fragmented legacy into a policy-relevant evidence base, MedSTN demonstrates how basin-scale data integration can uncover regional ecological signals and support proactive conservation prioritization. Our framework provides a replicable model for unifying long-term datasets to guide the transboundary management of wide-ranging species under accelerating global change.