Female sea turtles around the world lay their eggs in low-lying coastal habitats. The eggs are buried deep in the sand, but well above the high tide line. This keeps the eggs hidden from predators and lets them breathe. Plus, they’re close enough to the ocean that emerging babies can find their way. But rising sea levels will increase the likelihood that these nests will be flooded. According to new work published in Royal Society Open Science, saltwater inundation directly lowers hatching success.
A trio led by David Pike from James Cook University collected 262 eggs from three female green turtles (Chelonia mydas) nesting on Australia’s Raine Island, the world’s largest nesting rookery for the species. This remote cay in the northern Great Barrier Reef, about 630 kilometers from Cairns, could lose as much as 27% of its area with sea level rise.
After incubating the eggs and then setting some aside as controls, the team covered the rest of the eggs with saltwater for varying lengths of time. Keeping eggs under water means depriving the embryos of oxygen. "We are trying to anticipate the early effects," Pike says in a statement. "In some places it only takes a small rise in sea levels, when combined with a storm or a king tide, to inundate what had previously been secure nesting sites."
In their tests, eggs that were immersed for one or three hours showed no significant level of mortality compared to controls. But keeping eggs underwater for six hours increased mortality by 30%.
A giant tide or storm surge washing over the eggs for up to six hours is a very realistic scenario. "We were actually surprised at how resilient the eggs were," Pike adds. "We thought after six hours the mortality rate would be higher."
The turtles that did hatch seemed to be normal, both physically and behaviorally. However, it’s possible that hypoxia during incubation could affect their learning or spatial orientation – which would affect survival later on.
Moving nests further inshore or building higher elevation sites could help, and these sorts of efforts are underway. But the turtles on Raine Island have been suffering a mysterious decline in the last couple of years at least. It may have to do with microbial levels or heavy metals contaminants. It’s likely that these as well as seawater inundation all play a part. After all, some of the control eggs that weren’t drowned turned out not to be viable either. "There's a bigger mystery," Pike tells ABC Science. "There's something else going on and we don't yet know what that is."
Images: David Pike.