Natural shading is helpful but not sufficient for mitigating warming in green sea turtle nests in the tropical South China Sea

Over the past decade, lowering nest temperatures has been identified as a conservation strategy for mitigating the effects of global warming and for balancing the sex ratios in sea turtle populations. However, the effectiveness of shading sea turtle nests under the current warming climate has not been fully elucidated, especially in tropical habitats. We recorded environmental conditions of nesting beaches and evaluated the effects of natural shading on clutch temperatures and hatchlings of the green sea turtle Chelonia mydas through high-frequency daily field surveys on Taiping Island in the South China Sea. The entire incubation period equally divided into three windows, first, middle and last, and three levels of sun exposure, low, medium and high, were explored. Natural shading was related to an increased proportion of hatched clutches from 42.86 % to 66.67 % under high and low sun exposure, respectively, accompanied by a reduction in clutch temperature from 31.94 ± 2.02 °C to 30.38 ± 1.72 °C. Nevertheless, shading had no effect on increasing hatch and emergence rates, and incubation temperatures still exceeded the usual pivotal value (29 °C) during the middle incubation window as the sex determination period by a mean temperature of 31.53 ± 1.97 °C, which resulted in potential feminization of green sea turtles on the island and in this sea area. The probability of hatchling abnormalities was the lowest (9.68 %) under low sun exposure, and the likelihood of these abnormalities was strongly correlated with clutch temperatures, particularly during the first and middle incubation windows. Severe abnormalities, such as deformities, weakened as self-righting responses. These findings reveal that natural shading alone is insufficient to lower temperatures to mitigate female-biased sex ratios in green sea turtles in tropical habitats under the current climate and indicate that in situ monitoring and investigations of nesting habitats are necessary for establishing more appropriate regional conservation strategies for sea turtles.