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.

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Raine Island, located in the northern Great Barrier Reef, hosts the largest green turtle nesting aggregation in the world, but typically experiences low hatching success (20–60%, depending on the number of females visiting the island to nest). To determine whether the low hatching success of green turtle eggs at Raine Island might be explained by local sand characteristics, we investigated the physical properties of Raine Island sand and compared it to sand from other eastern coast Australian sea turtle nesting beaches that have high hatching success (>80%). We also measured the water, salt and organic material content of sand within nests at Raine Island to see whether any of these variables were correlated with the proportion of early embryo death or hatching success. The physical characteristics of Raine Island sand were similar to those of other eastern coast Australian nesting beaches, so it seems unlikely that inherent physical sand properties, water content, salt or organic matter explain the relatively low hatching success observed on Raine Island compared to other Australian green turtle nesting beaches. However, we found that nests that were inundated twice with seawater during spring high tides at the end of their first week of incubation experienced greater early development mortality and lower hatching success than did non-inundated nests, suggesting that embryos drowned during the inundation. Last, we found that hatching success declined towards the end of the nesting season, suggesting that the beach sand in the nesting areas of Raine Island changes in some way, and/or that egg quality decreases as the nesting season progresses.

All sea turtles exhibit temperature‐dependent sex‐determination, where warmer temperatures produce mostly females and cooler temperatures produce mostly males. As global temperatures continue to rise, sea turtle sex‐ratios are expected to become increasingly female‐biased, threatening the long‐term viability of many populations. Nest temperatures are dependent on sand temperature, and heavy rainfall events reduce sand temperatures for a brief period. However, it is unknown whether these short‐term temperature drops are large and long enough to produce male hatchlings. To discover if short‐term temperature drops within the sex‐determining period can lead to male hatchling production, we exposed green and loggerhead turtle eggs to short‐term temperature drops conducted in constant temperature rooms. We dropped incubation temperature at four different times during the sex‐determining period for a duration of either 3 or 7 days to mimic short‐term drops in temperature caused by heavy rainfall in nature. Some male hatchlings were produced when exposed to temperature drops for as little as 3 days, but the majority of male production occurred when eggs were exposed to 7 days of lowered temperature. More male hatchlings were produced when the temperature drop occurred during the middle of the sex‐determining period in green turtles, and the beginning and end of the sex‐determining period in loggerhead turtles. Inter‐clutch variation was evident in the proportion of male hatchlings produced, indicating that maternal and or genetic factors influence male hatchling production. Our findings have management implications for the long‐term preservation of sea turtles on beaches that exhibit strongly female‐biased hatchling sex‐ratios.