Sea Turtles Are in Much Worse Shape Than Previously Thought
A commonly used counting method might have overestimated their numbers by a factor of two.
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.
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.
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.
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.”
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.
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.