Near-shore Dispersal of Papuan Leatherback Hatchlings Using Acoustic Telemetry

Abstract

As part of a multi-year effort to study the oceanic dispersal of leatherback hatchlings departing the nesting beaches of Papua’s Bird’s Head Peninsula (Indonesia), a pilot study was carried out to determine the most efficient tracking method and yield preliminary data on near-shore dispersal. Both acoustic and VHF (radio) tags were tested in the field, using stationary buoys and live hatchlings. The results show the superiority of acoustic tags in terms of directionality, the most important parameter when tracking small organisms at sea. This led to the development of an optimum setup for acoustic tracking and the first recorded tracks of western Pacific leatherback hatchlings. Fifteen trajectories with lengths of up to 7 km and 6 hours duration show the feasibility of the method and provide preliminary information on swimming behavior and surface currents that mediate dispersal. The high temporal resolution of the data enabled us to measure the impact of (1) the drag of the tracking set (bobber and transmitter) and (2) the influence of age at release (time elapsed between natural emergence and release for tracking) on displacement speed and swimming direction (offshore bearing). The results suggest that the tracking set slows down the displacement speed of the hatchlings by 38 %, however, when entrained in a Westward surface current, there was no difference in displacement speed of turtles dragging the tracking set vs. turtles swimming freely. Overall, hatchlings’ swimming direction was not affected by the tracking set. The same results were obtained when comparing the displacement speed of turtles that emerged naturally from their nests vs. hatchlings dug up from hatched/predated nests (“stragglers”). Refining the analysis to take into account the age at release on displacement speed, it was found that hatchlings less than 24 hrs old swam significantly faster than hatchlings of 48 hours or more. Interestingly, “stragglers” performed better than 48+ hatchlings. The results suggest that when entrained in currents matching or exceeding their sustained maximum swimming speed, the tracking set did not influence hatchlings’ swimming behavior. These findings furthermore suggest that hatchling dispersal is primarily influenced by their initial ability to become entrained in the strong near-shore current. Conversely, in calm waters, hydrodynamic drag becomes an issue. The impact of the hatchling’s age on displacement speed provides additional evidence supporting the immediate release of hatchery turtles