High Spatiotemporal Diversity of Sand Temperatures and Estimated Hatchling Sex Ratios of Green Turtle in an Important North Pacific Rookery

Abstract

Sea turtles exhibit temperature-dependent sex determination with warmer incubation temperatures producing more females. Evidence from some regions indicates that global warming may already be increasing the proportion of female hatchlings, but this trend may not be uniform across all areas. Therefore, it is fundamental to understand the spatiotemporal diversity of sand temperature profiles and hatchling sex ratios within each rookery when developing conservation measures. The Ogasawara Islands (27°1′–11′ N, 142°9′–14′ E), Japan, are a highly isolated oceanic archipelago and one of the most important rookeries for green turtles (Chelonia mydas) in the North Pacific. The islands show high inter-beach environmental differences, but to date no study has assessed how these differences translate into spatiotemporal diversity of sand temperatures and hatchling sex ratios. In the present study, we measured the sand temperature at different depths and estimated the hatchling sex ratios from the thermal profiles at seven important nesting beaches of the green turtle in the Ogasawara Islands during 2018–2019. Results showed a marked spatiotemporal diversity in sand temperatures, with estimated hatchling sex ratios varying from male-biased to female-biased among the seven beaches. The spatiotemporal diversity is likely associated with the environmental characteristics between beaches, such as degree of shading, ground vegetation cover, and sand color, as well as seasonal temperature shifts. Nest depth is likely irrelevant for estimating hatchling sex ratios in the Ogasawara Islands. Continuous monitoring of the nesting environments is needed for efficient conservation of green turtle resources.