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).