Context. Marine turtle eggs incubate in dynamic beaches, where they are vulnerable to both saltwater and freshwater flooding. Understanding the capacity for marine turtle eggs to tolerate flooding will aid management efforts to predict and mitigate the impacts of climate change, including sea-level rise and increases in coastal flooding.

Aims. Evaluate the interactive effects of flooding duration and incubation stage on the hatching success of loggerhead turtle (Caretta caretta) eggs.

Methods. Groups of 20 eggs from multiple clutches were incubated in plastic containers in a beach hatchery. Eggs at six stages of incubation (0, 1, 2, 4, 6 and 7 weeks post-oviposition) were excavated from the hatchery and exposed to saltwater or freshwater flooding for seven durations of time (0, 1, 2, 3, 6, 24 or 48 h). Containers of eggs were either submerged in a bucket of water or left outside of the bucket (control; no flooding) for their designated duration, allowed to drain, then reburied in the hatchery. Following hatchling emergence, the hatching success of each group of eggs was evaluated.

Key results. Freshly laid eggs and eggs on the verge of hatching exposed to any flooding and all eggs exposed to extended periods of flooding (24 and 48 h) suffered complete mortality. Eggs at 20–80% development exposed to short periods of flooding (1–6 h) maintained high hatching success that was statistically equivalent to control eggs, while eggs at <20% and >80% development exhibited significant decreases in hatching success.

Conclusions. Marine turtle eggs in the middle of incubation can tolerate saltwater and freshwater flooding for up to 6 h. Outside of this period or when flooding is longer, disruption of gas concentrations and osmotic gradients in the egg chamber can lead to embryonic mortality. These findings have reinforced concerns regarding the capacity for marine turtle populations to continue to function as rising sea levels and increases in coastal flooding alter the hydrology of nesting beaches.

Implications. As current and predicted climate change threatens the suitability of the incubation environment used by marine turtles, corrective actions to maximise hatching success need to be taken before the eggs are flooded.