Every month at Costa Rica’s Ostional National Wildlife Refuge, tens to hundreds of thousands of female sea turtles arrive, often within a few days of each other, to lay their eggs on the beach.

Biologist Vanessa Bézy has been studying this remarkable phenomenon, known as a mass arrival, or arribada in Spanish, for years. As part of her research, she filmed the reptiles, mostly olive ridley sea turtles, aggregating in the ocean before reaching shore. One lucky day, she recorded, via a drone, an immense gathering of the creatures—the greatest density of sea turtle species ever recorded.

“I immediately knew there was something special going on,” Bézy says of filming the turtle swarm in November 2016. “To this day I’m still blown away by the video. They look like bumper cars out there.”

Ostional is one of the only places in the world where such large arribadas occur; it’s thought only Mexico’s Escobilla beach in Oaxaca receives more turtle visitors.

Bézy says she’s releasing the footage now because the turtles are increasingly threatened by growing numbers of tourists, who can crowd the beaches at critical times, and, potentially, development. The video helps show just how unique this place is—and that it needs to be protected, says Bézy, a National Geographic Explorer.

“This is the only time I’ve seen a video capturing this phenomenon in the water,” says Roldán Valverde, scientific director of the Florida-based Sea Turtle Conservancy and a biologist at Southeastern Louisiana University. “Most of the photography documenting this occurs on the beach.”

The drone footage shows the animals gathering in a density of something like one turtle per square meter, which is unheard of elsewhere. Over the course of the wet season, an average of 250 turtles per square kilometer has been measured—which is likewise off the charts. Furthermore, the video also shows turtles regularly rising into view, suggesting that there are actually more beneath the surface than above.

Bézy hopes her research will help reveal why and how so many turtles gather here, primarily between August and October. It may have to do with sea currents, beach orientation, sand type, and the like. Such arribadas may also provide “strength in numbers” for females and their eggs, possibly maximizing their chances for survival, Bézy adds.

Olive ridley sea turtles are one of the six sea turtle species considered under threat by the International Union for Conservation of Nature, and such mass aggregations are a vital part of their life cycle, she adds.

“It’s an enigmatic natural phenomenon,” she says. “We don’t know how the turtles coordinate this and why.”

Making plans

Settlements surrounding and within Ostional are growing quickly, and regulations are needed to ensure they expand in a responsible manner, Bézy says.

One of the communities abutting Ostional, the municipality of Nicoya, is working on a set of guidelines for development in a “buffer zone” surrounding part of the wildlife refuge. About 80 percent of comments received from residents are supportive of these proposed rules, says Francisco Jimenez, with the Nosara Civic Association, which works closely with Nicoya officials to develop the regulations.

But some developers are not thrilled. Jeffrey Grosshandler, CEO of The Gilded Iguana Hotel and co-owner of a real estate franchise in the region, says the regulatory process could create “legal insecurity” for developers. He claims that the proposed guidelines have not been vetted by the federal government, and applies a “one-size fits all” approach to areas that would, ideally, be zoned according to granular differences in demographics or geography.

View Images Olive ridley sea turtles, massing just off the coast of Ostional National Wildlife Refuge. Photograph by Vanessa Bezy

Jimenez contends the process is appropriate, and that the proposed rules have been repeatedly shared and tweaked based on public input. The commission will soon introduce the latest draft regulations in the coming weeks, and the public will then have 10 business days to comment, either in person, at an upcoming meeting, or via email. He hopes this process can be completed sometime in early 2020.

Bézy says the regulations limit building height, light usage, and other factors that could impact the environment and the turtles. She regards the regulations as being relatively lenient for developers, but she says they’re better than nothing since the area is growing so quickly.

Safe harvest?

Development is already not allowed on the first 200 meters of the beach from the water, as is true elsewhere in protected areas of the country.

During the first two days of arribadas, people can legally harvest some eggs that are likely to be trampled by turtles that arrive later. Locals sell the eggs, and some of this income helps support community projects like infrastructure, security guards, and beach cleanups. (Related: Over 100,000 sea turtles nest at the same time. How?)

Valverde’s research suggests the legal harvest is sustainable, he says, in part because there are some established rules that limit when and where eggs can be taken. While the olive ridley population at Ostional appears stable, it also appears to have declined somewhat over recent decades, he says. (Learn more about how sea turtles are surviving—despite us.)

However, illegal harvesting of the eggs also takes place, says Bézy, who formed an NGO called TortuGuiones Sea Turtle Conservation Project, which uses citizen science input to measure the level of harvest and nest disruption. Bézy also created another group, called the Wildlife Conservation Association, which works “to protect what’s here,” she says, for example by partnering with tour groups to responsibly view the turtles without disturbing them.

She hopes the public will care more about the turtles after seeing the film—and will support protecting the areas.