Dataset DOI: 10.5061/dryad.6hdr7srd7

Description of the data and file structure

Study area

Diego Garcia (7.42°S, 72.45°E) is the largest and only inhabited island in the Chagos Archipelago and has 72.1 km of coastline of which 40.5 km (56%) is deemed suitable nesting habitat. Our study was undertaken along a 2.8 km stretch of beach (Index Beach) identified as one of the highest nesting density areas (Figure 1a). In the Chagos Archipelago, green turtles (Chelonia mydas) nest year-round, mostly between June and October with a peak in August, whilst hawksbills (Eretmochelys imbricata) nest between October and February (Mortimer et al., 2020).

Foot patrol surveys and track counts

Turtle tracks, defined as the imprint a turtle leaves in the sand, were counted during foot patrol surveys. The surveys were conducted during the months of April and May 2021 and August 2022 (14, 13, and 20 survey days within each month, respectively). For every survey, all tracks were counted, and a line drawn through the track to avoid double counting. Mean track width was recorded from three measurements using a flexible tape measure and used along with track characteristics to identify species. Green turtles leave a symmetrical track (usually > 100 cm wide) in the sand while hawksbills leave an asymmetrical track (typically < 95 cm; see Pritchard & Mortimer, 1999 and Mortimer et al., 2020). Mean track count per day was calculated using the number of fresh tracks recorded on survey days (n = 47 days).

Camera trap survey design and settings

Camera traps (Apeman H70; n = 13) were attached to tree trunks or large branches (e.g., 166 Tournefortia argentea; Figure A1a) lining the nesting beach. Cameras were positioned to be equally spaced along the survey area as far as possible and to capture the length of beach. Camera placement was affected by (a) vegetation available for attachment and avoidance of (b) vegetation obstructing the camera’s field of view, and (c) overlap of images (Figure A1b). There was equal potential for cameras to be placed in areas of higher or lower nesting activity. The distance observed from each camera trap was measured by creating ‘turtle tracks’ in the sand every 5 m until the tracks were no longer visible (range from a single camera trap = 5-30 m; Table A1). The number of ‘turtle tracks’ visible from each camera was counted both in person and from images (Figure A1b). Estimated distances of beach coverage differed from each camera (e.g., due to variability in vegetation obstruction along the beach) and from the same camera over time (e.g., due to the camera falling slightly and altering the angle and view from the camera; Table A1; for more detail see Appendix 1). Trigger settings were disabled, and camera traps were set to take daily images on timelapse with 30 min intervals between 0700 and 0800 h. In 2022, ten of the cameras were set to take images every 30 s between 0700 and 0830 h from July 13.

Files and variables

File: Foot_patrol_track_count_2021_2022.csv

Description: Sea turtle track counts from foot patrol surveys.

Variables

• Date: Date of the foot patrol survey (dd/mm/yyyy).
• Fresh_track_count: The number of fresh tracks counted on the survey day. NA indicates no survey day.
• Tidal_state_daysafterspringtide: Day 0 = spring tide. Spring tide being the highest tide with the greatest tidal range on or after the most recent full or new moon. Subsequent days are the number of days after spring tide.

File: Camera_trap_April_Sept_2021_2022.csv

Description: Sea turtle track counts from camera trap surveys.

Variables

• Date: Date of the camera trap survey (dd/mm/yyyy).
• CT_ID: Camera trap ID number (01-13).
• Track_0_1: 0 = no track, 1 = track. If more than one track was visible in the same photograph, the track was recorded in a separate row.
• Longevity_days: The number of days the turtle track persisted on the beach recorded from the camera trap images.
• Tidal_state_daysafterspringtide: Day 0 = spring tide. Spring tide being the highest tide with the greatest tidal range on or after the most recent full or new moon. Subsequent days are the number of days after spring tide. 
• Camera_distance_m: Total observable distance in an image from a camera trap.

Code/software

All analyses were conducted in the free software R.