Presuming pathways of sea turtles using stable isotopes from shells of epibiont barnacles

Understanding the migratory pathways of sea turtles is critical for effective conservation efforts, however, direct tracking in vast oceanic habitats remains challenging. This study explores the use of stable isotope analysis (SIA) of epibiont barnacle shells as a method to infer the migration history of a sea turtle. Oxygen (δ¹⁸O) and carbon (δ¹³C) isotopic compositions were analyzed along the growth axes of barnacle shells collected from five barnacles attached to a loggerhead sea turtle (Caretta caretta) stranded in Jeju Island, Korea. Using Secondary Ion Mass Spectrometry (SIMS), high-resolution isotopic measurements were obtained, and regionally calibrated growth curve coefficients were derived to reconstruct the turtle's migratory pathway. The isotopic values ranged from -6.56‰ to 2.17‰ for δ¹⁸O_calcite and from -5.06‰ to 2.98‰ for δ¹³C_calcite. The regional growth curve coefficients indicated larger and faster growth in the study area compared with previously reported regions. The analysis suggested that the turtle migrated through the Taiwan Strait, the Philippines, the Ryukyu Islands, and waters near Jeju Island before stranding. The same approach was subsequently applied to infer the migratory pathways of two other sea turtles: a loggerhead specimen, that was inferred to have moved between the Philippines and Jeju Island via Taiwan Strait and the Ryukyu Islands, and a hawksbill turtle, that might have migrated north toward Jeju Island through the Ryukyu Islands. These findings demonstrate the potential of barnacle-based SIA to provide valuable insights into sea turtle movements, particularly in regions where traditional tracking methods are impractical.