Trialling seawater irrigation to combat the high nest temperature feminisation of green turtle Chelonia mydas hatchlings

Ce Smith,A Crosby,Ca Madden-Hof,Mn Staines,Jd Miller,Dt Booth,H Versace

doi:10.3354/meps13721

Abstract

Global increases in beach sand temperatures are predicted to skew hatchling sex ratios of marine turtle populations towards female bias. Currently, shade structures and freshwater irrigation are management strategies used to cool nest temperatures, but require resources that are limited at remote rookeries. Here, we report on a novel investigation that used seawater irrigation as a sand-cooling method. Various volumes of seawater were applied to sand to determine the optimal application required to lower sand temperatures at nest depth to produce male-biased sex ratios. We performed these experiments during the 2019-2020 nesting season at Heron Island on the southern Great Barrier Reef, Australia, and at Panasesa Island, Papua New Guinea. We found the amount of cooling at nest depth was site dependent and varied with the seawater irrigation regime used. At Heron Island, we used a one-off application of the equivalent of 120 mm of rainfall either as freshwater or seawater to the sand above incubating clutches 18 d after oviposition to determine whether this affected the hatching success of green turtle Chelonia mydas eggs. Both treatments had higher hatching success (83.8 ± 3.5% and 71.2 ± 6.3%, respectively, means ± SE) compared to control clutches (63.5 ± 6.0%). Our results indicate that a one-off application of seawater may be an effective management option for reducing nest temperatures during the sex-determining period of marine turtle clutches incubating in situ. Seawater irrigation could be used in areas where populations are at highest risk of feminisation caused by a hot drying climate where freshwater is not available for irrigation.

Highlights

Induced climate change has placed intense pressures on ecosystems around the world (Descamps et al 2017)
We examined the effectiveness of seawater irrigation at 2 green turtle nesting beaches, using: (1) different irrigation regimes to lower sand temperature, and (2) a one-off irrigation of seawater to test its impact on green turtle clutch hatching success
During the sexdetermining period (SDP), nest temperature increased from 28.2°C to 31.5°C, and by the end of incubation, nest temperatures reached between 35.0°C and 36.4°C (Fig. 5)

Summary

Introduction

Induced climate change has placed intense pressures on ecosystems around the world (Descamps et al 2017). For many populations around the world, the warming climate has pushed marine turtle hatchling sex ratios towards complete feminisation (Hays et al 2014, Urban et al 2014, Jensen et al 2018). This feminisation process is predicted to cause a decline in the hatchling output and genetic diversity and eventually lead to population extinction (Witt et al 2010, Hawkes et al 2013, Jensen et al 2018, Tanner et al 2019, Blechschmidt et al 2020). We urgently need practical methods of cooling nest temperatures to a level that results in greater male hatchling production

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Conclusion