It’s night, on a tropical beach, and a 300-pound green turtle is hauling herself up a beach. When she passes the high-tide line, she starts scooping sand away with her hind flippers, creating a deep hole. She fills it with eggs—100 to 200 leathery, white spheres, which she then buries. After an hour or two of hard work, she returns to the sea.

And then, she turns around.

A single female will lay her eggs at several places within the same nesting ground—a reproductive spread-bet that prevents her from losing an entire generation to, say, a storm or an industrious predator. Scientists have assumed that green turtles lay an average of 3.5 clutches each, and counting these clutches helps scientists estimate the global turtle population. But after tagging green turtles in the Indian Ocean, Nicole Esteban from Swansea University has shown that each female lays around twice that number. And if that holds true across other nesting beaches, it means that we might have overestimated the population of this endangered and declining animal by a factor of two.

Sea turtles are enigmatic animals, and for much of their lives, their whereabouts are a mystery. This means that if you want to count the world’s population, an underwater census is a non-starter. The best approach is to wait till they come ashore to lay their eggs. Scientists can canvas these nesting beaches and count the tracks of the females. If you divide that by the number of nests that each female makes, you get the total female population. For example, if you get 300 tracks, and you assume three nests per turtle, you get a total of 100 females. “But if you think the number of nests per individual is six, it’s a very different story,” says Esteban.

She arrived at that answer not by counting tracks, but by following actual turtles. In October 2012, she and her colleagues patrolled the beaches of Diego Garcia Island, waited for the turtles to finish laying their eggs, and then accosted them. They carefully cleaned the shell and then stuck on a state-of-the-art satellite tag—a flattened, waterproof, Tupperware-like box, which they painted with black antifouling paint to stop marine microbes and larvae from growing. The team waited for the paint to dry, and released the turtles.

A green turtle with a (barely visible) satellite tag on its back.
(Courtesy of Nicole Esteban)

After tagging eight turtles, Esteban realized that they were laying far more nests than anyone had expected. So her team returned to Diego Garcia in July 2015, to tag ten more animals at the very start of the breeding season. And they confirmed that the females were laying an average of six clutches each, with a range of two to nine.

It’s hard to draw firm conclusions from just 18 turtles, but Esteban isn’t the only researcher to have shown that track-counting overestimates turtle numbers. Nicola Weber from the University of Exeter came to a similar conclusion in 2013 after tagging green turtles in Ascension Island, as did Anton Tucker from the Mote Marine Laboratory in 2010, after tagging loggerhead turtles in Florida.

Pamela Plotkin from Texas A&M University says that she had similar experiences in the 1990s after tagging leatherback and olive ridley turtles in Costa Rica. For decades, beach counts have been “the predominant method for monitoring sea turtle species” she says, despite its many problems.  “Hopefully, the people who manage sea turtle nesting beach programs will be open to trying new approaches.”

A green turtle.
(Courtesy of David Loh)

Few people would argue with a call to use satellite tags more broadly, says Jeanette Wyneken from Florida Atlantic University, “but in practice, such use will be limited.” That’s because the tags are incredibly expensive. The model that Esteban used are about $4,000 each, and it costs another $200 per month to download the data. The latter bit isn’t optional, either, which adds an unpredictable cost to such studies. “One tag was working for 19 months, so even if you don’t want to study the turtle for that length of time, you still have to download the data as long as the tag is working,” says Esteban.  

Fortunately, there’s a cheaper alternative. In her study, Esteban found that the green turtles nest like clockwork, creating new clutches every 10 to 11 days. “If you know when they arrive and leave at a nesting beach, you can work out how many nests they’ve laid, even if you’re not recording their exact location,” she says. And that means scientists could afford to use cheaper, simpler tags.

There’s an urgency to these discussions. If we don’t know how many turtles there are, it’s hard to accurately plan conservation measures. And such measures are sorely needed: Of the seven species of sea turtle, three are vulnerable (the leatherback, loggerhead, and olive ridley), one is endangered (the green), and two are critically endangered (the hawksbill and Kemp’s ridley).

These animals aren’t just charismatic and endearing. Green turtles keep seagrass meadows healthy by nibbling the stems and preventing them from overgrowing; the meadows in turn provide habitats for manatees, seahorses, and many species of commercially important fish. Hawksbill turtles graze sponges and prevent them from outcompeting corals, keeping coral reefs healthy. A world without turtles is one in which many familiar and commercially important habitats fall apart.  And we may be closer to that world than we thought.