Comparison of green turtle Chelonia mydas sex ratios at two time-points over 20 years at a foraging ground in Yaeyama Islands, Ryukyu Archipelago, Japan

Abstract

Sex ratio is an important factor in population dynamics as it influences the production of offspring; understanding the sex ratio of a given population is vital for the conservation of endangered species. In sea turtles, the sex of hatchlings is temperature dependent, with warmer incubation temperatures producing more female hatchlings. The rise in temperatures due to global warming may skew the sex ratios towards females. We studied the sex ratio of immature green turtles at a foraging ground in the Yaeyama Islands from 1997-1999 and from 2016-2017, using laparoscopy. The overall proportion of females was 68.2% (N = 314), and proportions were not significantly different between 1997-1999 (69.9%; N = 183) and 2016-2017 (65.6%; N = 131). Thus, sex ratios have not changed at this site over the last 20 years, suggesting minor or no effect of global warming/environmental influences on this population. The small size class (<55 cm straight carapace length [SCL]) was more female biased than the large size class (≥55 cm SCL) during both periods, which suggests the possibility of movement into or out of the foraging aggregation. Therefore, future research must clarify how the initial recruitment into the foraging aggregations, and the subsequent migrations affect sex ratios.