Efficacy of artificial nest shading as a climate change adaptation measure for marine turtles at Ascension Island

Abstract

Successful embryonic development and offspring sex ratios of marine turtles are determined by thermal conditions experienced during incubation, rendering them potentially vulnerable to anthropogenic climate change. With the rate of projected temperature rises likely to outpace the adaptive capacity of long-lived species such as marine turtles, there is growing interest in management interventions aimed at mitigating the effects of climate change at nesting grounds. In this study, we experimentally tested the impacts of artificial nest shading on the incubation temperature, hatching success, and predicted offspring sex ratio of green turtle (Chelonia mydas) clutches at Ascension Island. Clutches (n = 97) were sampled from 2 nesting beaches with naturally contrasting thermal environments (one hot; one cool) and either left as in situ controls or relocated to shaded or unshaded hatcheries on their beach of origin. Compared to unshaded experimental clutches, shading reduced mean incubation temperatures and sex-determining temperatures (i.e., middle third of embryonic development) by 0.5–0.9°C and 0.5–1.2°C respectively, with the reduction being greater on the hotter beach. Shading also differentially affected hatchling output across the 2 sites: on the hot beach, shading significantly improved hatching success by ~23% but had minimal effects on offspring sex ratio; whereas on the cooler beach, shading did not impact hatching success but resulted in ~12% more male offspring. Interestingly, mean incubation temperatures of in situ controls did not differ significantly from shaded clutches, and were significantly cooler than unshaded experimental clutches, suggesting relocation may have negated some of the benefits of shading. Our results demonstrated that artificial shading may be a viable approach for partially offsetting climate change impacts on nesting marine turtles; however, scalability will be a major challenge in achieving conservation objectives at high-density nesting sites like Ascension Island.