Influence of horizon elevation on the sea-finding behaviour of hatchling flatback turtles exposed to artificial light glow

Abstract

Marine turtles are threatened globally by increasing coastal development. In particular, increased artificial lighting at the nesting beach has the potential to disrupt turtle breeding success. Few published data exist regarding the behaviour of the flatback turtle Natator depressus, a species endemic to Australia, in response to artificial light. Given the ongoing industrialisation of the Australian coastline, this study is a timely investigation into the orientation of flatback hatchlings exposed to light glow produced by lighting typically used in industrial settings. We recorded the orientation of hatchlings at the nesting beach on Barrow Island, Western Australia, exposed to 3 types of standard lighting—high-pressure sodium vapour (HPS), metal halide (MH), and fluorescent white (FW)—at 3 different intensities. The light array was positioned either behind a high dune (producing a high, dark silhouette; 16° elevation), or in a low creek bed (producing a low silhouette and bright horizon; 2° elevation). At medium and high light intensities of all 3 light types, hatchlings were significantly less ocean-oriented when exposed to light at 2° elevation compared to 16° elevation. This difference remained with glow from low-intensity MH light; however, there was no significant difference in orientation of hatchlings exposed to low-intensity HPS and FW light glow at either elevation. Our study emphasises the importance of horizon elevation cues in hatchling sea-finding. Since all species of marine turtles show similar sea-finding behaviour, our results have important implications for management of lighting adjacent to turtle nesting beaches in Australia and elsewhere, as coastal development continues.