Effects of swimming behaviour and oceanography on sea turtle hatchling dispersal at the intersection of two ocean current systems

The knowledge gap on the early life-history of sea turtles during the "lost years" continues to hinder research and conservation of this critical life stage when mortality rates are the highest. An oceanic model was used in combination with a Lagrangian particle tracking framework to simulate and identify potential post-hatchling dispersal trajectories of loggerhead and leatherback turtles in the South Western Indian Ocean. The study aimed to investigate the effect of hatchling swimming behaviour on hatchling dispersal and survival probability. To our knowledge, this study provides the first estimate of neonate sea turtle dispersal in the SWIO, by combining a particle tracking model with in situ hatchling behavioural data. The model revealed that most virtual hatchlings are transported south-westward in the Agulhas Current with three distinct final locations after a year-long simulation (each zone comprising on average > 20% of the total amount of particles): the Agulhas Return, the SE Atlantic and the Southern Ocean zones. However, because loggerhead hatchlings are less strong swimmers compared to leatherbacks, they can be advected northward in the near-shore coastal current. Simulations revealed that initial active swimming (frenzy) as well as variability in oceanic conditions strongly influenced dispersal of virtual hatchlings. Furthermore, variability in oceanic conditions dispersed virtual hatchlings into different areas where threats, like fisheries bycatch, might also influence their survival. Lastly, the results of this study have potentially broad implications for climate change if turtles adapt by nesting earlier/later during the nesting season or further south which may influence hatchling locomotor performance and ultimately survival at early life stages.