Modelling the dispersal of juvenile leatherback turtles in the Caribbean Sea

The mysterious period of the turtles’ life, between their first day at sea and when they come to lay eggs years later on the same beach, continues to elude research of this critical life stage when mortality rates are the highest. The conservation efforts on beaches need to expand to the pelagic (offshore) life stage to reduce the mortality rate. It has long been assumed that turtles drift passively with the current during this period. Recently, it has been discovered that one year old turtles have an active behaviour in the current. The ability of turtles to be active in ocean currents, especially at early life stages, can significantly influence their future dispersal. The NGO Upwell recovered data by acoustic monitoring during the first hours of leatherback hatchlings from Pacuaré in Costa Rica, an important leatherback nesting beach in the Caribbean Sea. I used the data of the SLIM oceanographic model, developed by UCLouvain, to obtain the dispersion of the turtles during their two first months of life in this region. Based on previous studies and Upwell’s data, I made simulations with different behavioural hypothesis including active swimming. The objective was to make an accurate map of the probability of turtle presence. My study suggests that the dispersion of leatherbacks in this region of the Caribbean Sea is influenced by annual local current conditions and by the swimming behaviour of the turtles. During the first days, the turtles were located alongside the coasts of Costa Rica and Panama before heading offshore, in the gyre near Pacuaré and in the Gulf of Darién. They moved gradually north towards the Gulf of Mexico. No turtle went east of longitude -74. The model predicts that the active swimming component is essential to the survival of the species since it brings the risk of stranding down from up to 50% to 0.19%. These findings open up research prospects that could have an impact on the defence of this species.