Ocean Sciences News

Predicting Wave Wash Overs for Sea Turtle Nests

To better protect coastal species, researchers developed a model that predicts harmful wash overs with 83% accuracy.

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Like humans picking out the perfect spot on the beach to set up camp, sea turtles choose their nesting sites with similar priorities: out of the waves, not too far from the water, and in a nice, sandy locale.

But sometimes these locations can be swamped by wave wash overs, suffocating the nests with seawater and killing developing embryos.

Wildlife managers can mitigate flooding by moving the nests out of the reach of wave run-up. However, the decision to move a nest can be murky; it’s not always a certainty that waves will inundate the nest.

Now researchers have uncovered a better way to identify nests in the danger zone. Their work, published in Ocean and Coastal Management, identifies likely areas that are ripe for a soaking, and their predictions have been more than 80% accurate.

In Harm’s Way

Relocating turtle nests is an option for threatened eggs, but the move can come at a cost, explains Matthew Ware, a biological oceanography doctoral student at Florida State University in Tallahassee.

“If we guess right, we’ve now helped more of those hatchlings reach the water,” says Ware. “But [if] we guess wrong, there are some unintended consequences—we do lose a few embryos in that move.”

Ware says that scientists often used the previous night’s high-tide mark to decide whether to move the nest. “The idea being if the tide got here yesterday, it will probably get here again in the near future,” says Ware.

But in areas like the Gulf of Mexico, where tide ranges are small and beaches are wide, Ware says high tide might not be the best marker for nest safety. In Fort Morgan, Ala., where the researchers concentrated their efforts, the beach is a popular nesting spot for loggerhead sea turtles (Caretta caretta).

The team decided to use the beach slope and wave data near the shore to better estimate where wash overs might occur. Ware says their inspiration came from scientists calculating storm surge risk estimates for coastal cities.

Storm Surge Refocused

The beach in Fort Morgan, Ala., includes both protected and residential land along the Gulf of Mexico. (The protected land is the Bon Secour National Wildlife Refuge.) The researchers used data from 10 lidar surveys of the Alabama coast spanning 1998 to 2016 to get a detailed model of beach slopes.

Researchers combined their slope model with tide information and wave data from a nearby National Oceanic and Atmospheric Administration (NOAA) buoy, including wave height and length. The researchers divided the beach into 3- by 3-meter cells and calculated the likelihood of those squares being washed over.

Ocean waves wash over a cordoned-off triangle marked as a “sea turtle nest.”
Researchers studied a stretch of public and private beach in Alabama to measure the impact of wash over on sea turtle nests. Credit: Matthew Ware

The team then created a risk map that showed all areas with a high risk of wash over. Ware says the map would allow managers to pinpoint the location of a nest and get the probability of wash over at that exact location.

Testing the Water

The team tested the accuracy of their wash-over risk map during the 2016 nesting season. Monitoring over 100 sites, they found that their model correctly identified which nests would wash over in 83% of cases.

Ware adds that at Fort Morgan, the researchers found they wouldn’t need to redo the risk map every year. “It turns out that that time-averaged model did better than when we just used the most recent survey from 2016,” he says.

However, he did say the prospect of redrawing the risk map depends on what a specific site might experience during a season. “If you have a really nasty storm that came through that really reshaped your beach, then you’re probably going to need a new map.”

“I think that [in] places like this [in Alabama], one of the major benefits…is the fact that they can do these kind of studies and then provide information for places like where I work,” says Sara Ramirez, a conservation biologist at St. Kitts Sea Turtle Monitoring Network who was not involved with the study.

In her work on St. Kitts, Ramirez says the small island buzzes with tourism and development, making pristine and protected beaches a nonentity. She says this study is “an ideal scenario” for testing wave wash-over predictions.

Risk Maps in Action

Between 80 and 240 sea turtle nests (mostly loggerheads and a few Kemp’s ridleys (Lepidochelys kempii)) are dug on Alabama beaches every year. In 2016, Fort Morgan beach had more than 100 loggerhead nesting sites.

Ware says his team’s wash-over risk maps can help wildlife managers make critical decisions on nest relocation. “Now we can take a look at the map and say, ‘This has a 90% probability of being washed over, let’s go ahead and move it,’” says Ware.

The maps are also helpful for conservation work in a rapidly developing area like St. Kitts, says Ramirez. “If you’re speaking to developers, they have set setbacks for development,” she says.

However, Ramirez notes that if half the beach is in the high-risk zone, the setback doesn’t help those sea turtles because they are “only left with this little strip [of beach].” She adds that understanding the location of the danger zone is imperative in halting encroaching development.

Ramirez says in some cases, sharing actual numbers with stakeholders, instead of anecdotal evidence, makes a bigger impression for those trying to balance conservation and development. “Saying to someone ‘Well, I know the beach like the back of my hand’ might not be as impactful [as scientific data],” says Ramirez. Showing a map helps get the message across more clearly.

Ware and his colleagues plan to expand their mapping to other beaches, including the entire Alabama coastline and into Florida. He is also interested in expanding the model to look at high-hazard situations like hurricanes.

Inspired by the storm surge warnings NOAA sends out before a hurricane, Ware and his team want to use their predictions “to look forward to identify what parts of the beach are going to be most at risk with a particular storm coming in.”

—Sarah Derouin (@Sarah_Derouin), Science Writer

Citation: Derouin, S. (2019), Predicting wave wash overs for sea turtle nests, Eos, 100, https://doi.org/10.1029/2019EO123723. Published on 14 May 2019.
Text © 2019. The authors. CC BY-NC-ND 3.0
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