Unraveling Spatio-temporal genetic connectivity of loggerhead sea turtles across the North Atlantic

• Temporal shifts in juvenile stock composition were detected in key foraging grounds. • Florida remains a major source for Azores and Madeira juvenile turtle stocks. • Cape Verde is one of the main contributor to juvenile turtles in the Canary Islands. • Significant genetic structure detected between the historical and current periods. • Findings highlight the need for long-term genetic monitoring for conservation. The life cycle of North Atlantic loggerhead sea turtles ( Caretta caretta ) involves an oceanic juvenile dispersal stage and migration to foraging habitats in the Eastern North Atlantic, where juveniles from multiple nesting populations aggregate in a mixed stock. This study examined temporal shifts in the genetic composition and stock contributions of loggerhead sea turtles at these oceanic foraging grounds by analyzing the mtDNA control region fragment from sea turtles sampled between 2010 and 2018, alongside historical data (1990–2004). Mixed stock analyses revealed that the Central East and Southeast Florida management units (MUs) remain key contributors to juvenile aggregations in Azores and Madeira, while the Cape Verde rookery is one of the main source for juveniles in the Canary Islands. Temporal comparisons indicated a decline in the proportion of contributions from Florida rookeries and an increase from Mexico and Cape Verde, likely reflecting nesting population declines in Florida (between 2009 and 2012) and rising nest numbers in Mexico and Cape Verde. Significant temporal genetic structure was detected, especially in the Canary Islands. These findings highlight dynamic shifts in juvenile stock composition driven mainly by fluctuations in size of nesting populations. The study underscores the critical need for long-term genetic monitoring of both nesting and developmental habitats to inform targeted conservation strategies. Understanding temporal and spatial connectivity patterns is essential to safeguarding this migratory species, particularly in the face of ongoing anthropogenic threats and global environmental change.