Artificial light on water attracts turtle hatchlings during their near shore transit

Abstract

We examined the effect of artificial light on the near shore trajectories of turtle hatchlings dispersing from natal beaches. Green turtle (Chelonia mydas) hatchlings were tagged with miniature acoustic transmitters and their movements tracked within an underwater array of 36 acoustic receivers placed in the near shore zone. A total of 40 hatchlings were tracked, 20 of which were subjected to artificial light during their transit of the array. At the same time, we measured current speed and direction, which were highly variable within and between experimental nights and treatments. Artificial lighting affected hatchling behaviour, with 88% of individual trajectories oriented towards the light and spending, on average, 23% more time in the 2.25 ha tracking array (19.5 ± 5 min) than under ambient light conditions (15.8 ± 5 min). Current speed had little to no effect on the bearing (angular direction) of the hatchling tracks when artificial light was present, but under ambient conditions it influenced the bearing of the tracks when current direction was offshore and above speeds of approximately 32.5 cm s(-1). This is the first experimental evidence that wild turtle hatchlings are attracted to artificial light after entering the ocean, a behaviour that is likely to subject them to greater risk of predation. The experimental protocol described in this study can be used to assess the effect of anthropogenic (light pollution, noise, etc.) and natural (wave action, current, wind, moonlight) influences on the in-water movements of sea turtle hatchlings during the early phase of dispersal.

Keywords: VR2W positioning system; acoustic telemetry; coastal development; green turtle; in-water movement; light pollution.

Figure 1.

Map of the study site at Wobiri Beach, North West Cape, Western Australia and inset map of Australia showing the position of the study site. Map shows acoustic receivers in black and reference tags in red. Three bathymetry contours are shown: 2 m in black, 3 m in dark grey and 4 m in light grey.

Figure 2.

A green turtle hatchling with the Vemco V5 acoustic transmitter attached.

Figure 3.

Tracks from each individual hatchling shown in different colours. Open circles represent the positions of each of the receivers in the tracking array and the beach is shown in beige at the bottom of the plots. The asterisk indicates the release site of turtles at the water's edge. (a,c) The tracks of turtles in the ambient treatments, with no light present and (b,d) the tracks of turtles in the light treatments with the position of the light indicated by the orange filled diamonds and dark grey arrows.

Figure 4.

Stick plots showing the current direction and speed (length of the arrow) in each of the treatments, with light treatments in red and ambient treatments in blue.

Figure 5.

Predicted values (mean and standard error) from the model used to explain the relationship between the time turtle hatchlings spent in the tracking array for each of the treatments (ambient and light) and nights of the experiment.

Figure 6.

Relationship between turtle hatchling bearing from the release point (relative to 0°) and the independent variables in the top model; the interaction between current direction and treatment (table 3). The predicted smoothers are shown as solid lines and the dashed lines and transparent polygons show the standard error.