World’s largest known turtle nesting site found in the Amazon

Researchers from the University of Florida have uncovered the largest known nesting site for the threatened giant South American river turtle (Podocnemis expansa). How did they find over 41,000 nesting reptiles? By deploying a meticulous squadron of drones. The turtles were found gathered along the Amazon’s Guaporé River between Brazil and Bolivia. This innovative use of drones opens up new avenues for conservationists, as detailed in a study recently published in the Journal of Applied Ecology.

“We describe a novel way to more efficiently monitor animal populations,” study co-author and University of Florida ecologist Ismael Brack said in a statement. “And although the method is used to count turtles, it could also be applied to other species.”

Researchers from the New York-based Wildlife Conservation Society (WCS) have been monitoring the giant South American river turtle in Brazil, Colombia, and Bolivia for several years. Also known as the Arrau turtle, these freshwater reptiles live in rivers and lakes along the Amazon and Orinoco River basins. Some can have shell lengths up to 35 inches long and weigh upwards of 130 pounds. They are omnivorous, eating leaves, seeds, and fruit, hunting for crabs, shrimp, and fish, and also scavenging for dead fish. Arrau turtles’ primary threats include poachers who sell their meat and eggs, habitat loss, dams, and pollution. These reptiles are also considered exceptionally social.

Every year in July or August, female Arrau turtles gather to nest in the Guaporé River’s sandbanks between Brazil and Bolivia, and last year, a joint team from the University of Florida and WCS set out to count just how many come to nest. It’s a monumental task in this part of the Amazon, that’s home to jaguars, the Bolivian river dolphin, giant armadillos, and numerous other species. 

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The University of Florida researchers met with WCS scientists at a conference and shared with them how they use drones to count turtles in and around Florida. With the imagery from the drones, the University of Florida team creates orthomosaics—highly detailed, high-resolution composite images made by stitching together hundreds of overlapping aerial photos. Tallying the animals shown in the orthomosaics is often a faster, more accurate, and less invasive method than simply counting the animals from the ground. However, orthomosaics don’t account for animals that sometimes move during observation periods.

To solve this, the team developed a new method that improves counting accuracy by eliminating multiple sources of error. These include when the same individual animal is counted multiple times (called a double count) and missed individuals. The team used white paint to mark the shells of 1,187 turtles that gathered on an island sandbank within the Guaporé River. Over 12 days, a drone flew overhead on a steady back-and-forth path four times per day, taking 1,500 photos each flight: That’s 6,000 photos per day. Using a photo software, the researchers stitched the photos together.

They then carefully reviewed each composite image, noting if a turtle’s shell was marked and whether the animal was walking or nesting when it was photographed. With this data in hand, they developed probability models that account for the turtles entering and leaving the area, the observed turtle behaviors, and the likelihood of detecting an identifiable shell marking.

According to the team, the models revealed several sources of error that could arise from traditional orthomosaic-based counts. For example, only 35 percent of the turtles that used the sandbank were present during the drone flights. On average, 20 percent of those detected walking appeared multiple times in orthomosaics. Some individuals were spotted as many as seven times—don’t say turtles don’t get around.

The observers positioned on the ground counted roughly 16,000 turtles. Those who reviewed the orthomosaics—not accounting for animal movement or shell markings—counted about 79,000 turtles. Once the new statistical models were applied, the study estimated that about 41,000 turtles were at that one site.

“These numbers vary greatly, and that’s a problem for conservationists,” Brack said. “If scientists are unable to establish an accurate count of individuals of a species, how will they know if the population is in decline or whether efforts to protect it are successful?”

The study also details ways to adapt and apply this technique to conservation efforts with other species that are monitored and surveyed using drone-derived orthomosaics. Past monitoring studies have involved clipping seals’ fur to make markings, attaching high-visibility collars to elk, and using paintball pellets to mark mountain goats to keep track of animal movement during counts.

The team plans to perfect their monitoring methods by conducting more drone flights at the Guaporé River nesting site and in other South American countries where the giant South American river turtles gather. These include Colombia and possibly Peru and Venezuela, according to Brack.

“By combining information from multiple surveys, we can detect population trends, and the Wildlife Conservation Society will know where to invest in conservation actions,” he said.

 

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