Are stocks of immature loggerhead sea turtles always mixed?

• Juvenile sea turtle stocks are usually composed of individuals of different origins.
• mtDNA of loggerhead turtles in Argentina were studied to elucidate possible origins.
• Turtles at this foraging ground are exclusively originated from Brazilian rookeries.
• Loggerhead turtles at this area are originated from nearby nesting places only.
• Homogeneous stock in sea turtles differ from mixed stocks commonly found elsewhere.

Sea turtles perform extensive migratory movements between feeding, developmental and nesting areas. Developmental areas for immature loggerheads turtles (Caretta caretta) are usually composed of individuals from multiple distant rookeries. Thus, impacts on such stocks usually affect multiple colonies, which require international efforts for conservation. This study describes the molecular genetic composition of the more austral foraging and developmental grounds of loggerhead sea turtles in the Atlantic Ocean and infers the possible origin and dispersal patterns of the species. Analyses were performed using shorter (380 bp) and larger (760 bp) sequences of the control region of mitochondrial DNA (mtDNA) from 24 samples of bycatch and 37 samples of stranded loggerhead sea turtles on the coast of the province of Buenos Aires, Argentina. All specimens (N = 61) were adults and sub-adults (mean ± standard deviation curved carapace length CCL = 68.3 ± 13.4 cm; range: 52.0 to 107.0 cm, N = 41 individuals measured). Both shorter and longer mtDNA sequence analysis showed that in the foraging grounds of the Argentinean coast only haplotypes from Brazilian nesting areas (CC-A4 = 98% and CC-A24 = 2% for shorter sequences, and CC A4.2 = 81%, CC A4.1 = 17% and CC A24.1 = 2% for longer sequences) were found. The homogeneous stock located relatively close to the rookery where individuals originated contradicts the paradigm of immature loggerhead sea turtles forming mixed stocks in foraging and developmental areas. The conservation of the stock in coastal areas of Argentina could benefit the nesting population in the nearby Brazilian rookeries, and could be achieved by conservation actions between these two countries, as well as Uruguay in between.