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

Brief description

We used a wave flume to determine if flatback turtle (Natator depressus) hatchlings can detect and respond to wave cues and examined how different wave types influenced their orientation. A total of 55 hatchlings were collected from the field at Bell’s Beach in the Pilbara region and flown to Perth for experiments. Trials were conducted on 2 nights (26 January and 29 January 2017). For each trial we examined the swimming direction of hatchlings in the flume in total darkness with no waves and two different wave types (sea and swell waves) with high (> 0.10 m) and low (< 0.08 m) wave height (Hs). A total of 11 hatchlings were released in each treatment. Each hatchling was placed in a harness with a tether which was attached to the centre of a bar placed horizontally across the flume ~20 cm above the water. The tether was long enough for hatchlings to swim in any direction without touching the side of the wave flume. Waves approached from 0°. The trials were recorded using a downward facing infra-red (IR) video camera mounted directly above the bar. Hatchling orientation was assessed in relation to a compass with 10° sections which was placed across the top of the flume walls directly above the tethering point to show orientation of hatchlings on the video. The footage was viewed via a live video link to a computer located outside of the darkened area and the orientation of the hatchling to the nearest 10° was recorded every 30 s for a period of 5 min after a 2 min period of acclimation. 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 was variable; it only occurred with large (16 cm peak to trough) and not with small wave heights (7 cm peak to trough). Wave steepness (the ratio of wave height and wavelength) was the strongest predictor of variance in hatchling bearings and there was a positive relationship between wave steepness and consistency in orientation, with hatchling bearings more concentrated (r-value 0.98) towards the direction of waves when the steepest waves were present (sea waves with large wave heights). The waves in our steepest treatment 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 steepness may be a key characteristic of waves that drives orientation.

Lineage

Maintenance and Update Frequency: notPlanned

Statement: Data description: Hatchling bearings Recordings of trials Data from the pressure sensor in the wave flume Field collected oceanographic data Funding Body: Department of Biodiversity, Conservation and Attractions (DBCA). P. Wilson was supported by an Australian Government Research Training Program (RTP) Scholarship and a top-up scholarship from The University of Western Australia (UWA).

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Credit
University of Western Australia (UWA), Australia

Credit
Department of Biodiversity, Conservation and Attractions (DBCA), Australia

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Pendoley Environmental, Australia