Camera Traps: A Novel Method to Estimate Numbers of Nesting Sea Turtles

Abstract

Abundance estimates are difficult to obtain for many animal groups, yet essential for endangered species management and conservation. For sea turtles, estimates are made from ground counts of nesting females, tracks, and nests, but these are challenging at remote locations. Here we explore the applicability of using camera traps to monitor and estimate sea turtle nesting tracks at a green turtle (Chelonia mydas) rookery in the Western Indian Ocean. Camera traps (n = 13) were deployed to photograph turtle tracks daily along a 2.8 km beach in Diego Garcia, Chagos Archipelago in 2021 and 2022. Foot patrol surveys were conducted in April and May 2021 and August 2022 (14, 13 and 20 days, respectively). Track counts were compared from both methods to validate the use of cameras. From foot patrol surveys, we observed an increase in track counts around neap tides (mean ± SD: 5.0 ± 4.0 tracks per day; n = 131 tracks) compared to spring tides (2.4 ± 1.8 tracks per day; n = 51 tracks). Mean track longevity was similar during neap (2.9 ± 2.0 days; n = 39 tracks) and spring tides (2.7 ± 2.6 days; n = 20 tracks). Mean daily track counts were comparable during neap tides (camera traps: 7.3 ± 12.9 tracks cf. patrols: 5.0 ± 4.0 tracks) and across the tidal cycle (camera traps: 5.5 ± 13.1 tracks cf. patrols: 3.9 ± 3.4 tracks). Using simulated data, we found track count variability decreased in a power-law relationship with increasing coverage by cameras. The disparity in track counts between methods would likely decrease if beach coverage increased from 5% to 20%. Camera traps provide a complementary tool to fill data gaps at remote sites that would otherwise have little to no assessments. Furthermore, the increased temporal coverage from cameras can help identify changes in nesting phenology and trends in nesting numbers.