Nearshore wave characteristics as cues for swimming orientation in flatback turtle hatchlings

Waves are thought to provide an important directional cue for hatchlings of marine turtles to navigate through the nearshore zone and to facilitate dispersal to oceanic waters. As the flatback turtle (Natator depressus) is the only species of marine turtle that lacks an oceanic juvenile stage and remains on the continental shelf throughout the entire life cycle, it is possible that hatchlings of this species do not use wave cues for early dispersal. Here, we used a wave flume to examine the response of flatback turtle hatchlings to waves as a cue for nearshore dispersal. We exposed hatchlings to two types of waves (sea and swell waves) differing in height and period commonly experienced at nesting locations and monitored swimming direction (orientation). Hatchlings oriented towards shorter period (3 s) sea waves with both small (6 cm peak to trough) and large (12 cm peak to trough) wave heights. Orientation of hatchlings to longer period (8 s) swell waves only occurred with large (16 cm peak to trough) and not with small (7 cm peak to trough) wave heights. Acceleration generated by waves was the strongest predictor of directional preference in hatchling bearings. There was a positive relationship between maximum acceleration and directional preference, with hatchling bearings more concentrated (r-value 0.98) towards the oncoming wave direction with waves that produced the highest acceleration (sea waves with largest wave heights). These waves were similar to sea waves generated from onshore winds, which was the most common type of wave we measured at a flatback turtle nesting beach. Our study has confirmed that despite lacking an oceanic development stage, flatback hatchlings, like other species of sea turtles, can detect and respond to wave cues, and that the accelerations generated by waves may be a key characteristic of waves that drives orientation.