Camera traps: A novel method to estimate numbers of nesting sea turtles

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
complimentary 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.