A novel assessment tool to measure thermal variability on sea turtle nesting beaches.

Monitoring sand temperatures at sea turtle nesting sites is useful to help understand likely spatio-temporal patterns of hatchling sex ratios and overall population reproductive success. Typically, many temperature loggers are deployed in different nesting habitats over the course of multiple nesting seasons to quantify thermal variability. Here, we present a methodology to record a snapshot of thermal variability along a nesting beach. We used a temperature logger with a 1 m long stainless steel compost thermistor probe to record sand temperatures on four islands of the Chagos Archipelago, an important hawksbill (Eretmochelys imbricata) and green (Chelonia mydas) turtle nesting site in the Indian Ocean. Temperatures were recorded over ten days in 2021 and 2022 (March, July, August and September) in different habitats (full vegetation, partial vegetation and open) and at different depths (50 and 70 cm). The accuracy of this logger was comparable to that of loggers that take continuous readings, which are typically used in thermal variability studies. Our results informed on temperature differences between seasons, islands, sites along the beach, nesting habitats, and depths. The primary source of thermal variability at nest depths was seasonal variation, with a 2.9°C mean difference between warmer and cooler months. Sand temperatures along a nesting beach varied by up to 1.7°C, and temperatures between nesting habitats (i.e., shaded habitats vs exposed habitats) varied by 0.6°C. Further, our results showed generally low sand temperatures (i.e., mean = 28.9°C in the warmer season vs mean = 26.0°C in the cooler season). We propose that our novel assessment method can be used to gain immediate and low-cost temperature values at a nesting site. This approach is particularly important as quantifying thermal variability at nesting sites will continue to be a conservation priority in decades to come, given that incubation temperatures may increase worldwide in the context of climate change.