Arrested Recovery

The spectacular comeback of the Kemp’s ridley sea turtle has hit a roadblock, with the 2010 Gulf oil spill fingered as a primary suspect

07-31-2017 // Laura Tangley
Kemps_Ridley 1
National Park Service biologist Donna Shaver releases newly hatched Kemp’s ridleys at Padre Island National Seashore in Texas. Park staff shield hatchlings from gulls while tourists cheer on the tiny turtles racing to sea.

ON A WINDY, BLISTERINGLY HOT APRIL DAY several years ago, Pat Burchfield saw a rewarding sight: hundreds of Kemp’s ridleys—the world’s most endangered sea turtles —swarming ashore en masse to nest on a strip of beach in the state of Tamaulipas, Mexico. It was a clear sign that the critically endangered species he had worked so long and hard to save was finally on the road to recovery.

When Burchfield, director of the Gladys Porter Zoo in Brownsville, Texas, began to run the U.S. arm of a binational plan to save the species 30 years earlier, in 1981, “you never saw more than a handful of ridleys on the beach at a time,” he says. Scientists estimated the entire population of breeding females at no more than 400 individuals. For the following four years, the ridley’s numbers continued to plummet. By 1985, fewer than 300 turtles laid eggs during their spring-to-summer nesting season—a more than 99 percent decline from the number that nested on the same beach along the Gulf of Mexico four decades earlier.

But just a few years later, the U.S.-Mexican effort started to pay off. In 1988, ridley numbers stabilized and by the mid-1990s began increasing exponentially at an average rate of 18 percent a year—peaking at more than 20,000 nests dug by some 8,000 females in 2009. Based on population models, scientists predicted that in little more than a decade, there would be enough ridleys that the government could change the species’ status from endangered to threatened. “I figured I’d eventually be out of a job,” Burchfield recalls.

Fast-Forward to Reverse

Then came the spring 2010 nesting season. In a year when nest numbers were again projected to increase significantly, they instead dropped by 35 percent. Since then, the figures have fluctuated up and down, but they have never returned to their exponential rise. “For a species so recently on the brink of extinction, even leveling off is bad news,” says Ryan Fikes, staff scientist for the National Wildlife Federation’s Gulf of Mexico Restoration Program.

The same spring, another significant event took place in the Gulf of Mexico. On April 20, 2010, an offshore oil rig called Deepwater Horizon exploded, setting in motion the worst offshore oil disaster in U.S. history. By the time the well was capped almost three months later, nearly 170 million gallons of crude oil had spilled into the northern Gulf. “Tens of thousands of sea turtles were present in the oil-contaminated area,” Fikes says.

The northern Gulf is critical foraging and migratory habitat for ridley adults and juveniles as well as developmental habitat for hatchlings. Satellite tracking by Donna Shaver, chief of the National Park Service’s Division of Sea Turtle Science and Recovery, reveals that nesting females from Texas south to Veracruz, Mexico—the species’ current nesting range—come to this zone to feed after laying eggs. “We also discovered that the turtles are faithful to foraging habitats,” Shaver says, with some individuals tracked for four seasons returning to the same areas year after year. “If a turtle is hardwired to a specific foraging habitat,” she adds, “it’s very worrisome when that area becomes contaminated by oil.” 

Published in late 2015, research conducted for the spill’s Natural Resource Damage Assessment (NRDA) verified the death of thousands of Kemp’s ridleys during the oil disaster. Those deaths included as many as 20 percent of adult female ridleys. Yet even such major losses “may not fully account for the lower-than-expected nest numbers we’ve observed since 2010,” says Shaver, an NRDA principal investigator who has run a Kemp’s ridley conservation project at Padre Island National Seashore in Texas since 1986.


The 2010 Gulf oil spill killed thousands of the turtles, including the juvenile above.

“Riddle of the Ridley”

Uncertainty surrounding the ridley’s derailed recovery adds another mystery to the story of an animal that has been mysterious since it first was discovered by scientists. Though the species, Lepidochelys kempii, was named and described in 1880, nearly everything else about it—particularly where the turtle breeds and nests—remained an enigma for more than eight decades. Because neither biologists nor fishermen had reported seeing a nesting adult ridley or hatchling, one theory popular for many years was that the reptile was a cross between a loggerhead and a green sea turtle, earning it the nickname “bastard turtle.”

That hybrid hypothesis did not sit well with the late, world-renowned sea turtle biologist Archie Carr. In his book The Windward Road, he wrote: “It bothered me that the ridley should be such a distinctive and original-looking creature, with his traits his own and nothing about him that seemed intermediate between the other species.” In the late 1930s, Carr embarked on a decades-long quest to solve “the riddle of the ridley,” a creature he dubbed “the most mysterious air-breathing animal in North America.”

The riddle finally was cracked in 1961, when Texas fisheries biologist Henry Hildebrand presented at a scientific meeting (with Carr in the audience) a 16-millimeter film shot on a beach near the town of Rancho Nuevo, Mexico. Recorded on June 18, 1947, by Mexican rancher Andrés Herrera, the movie shows an enormous gathering of Kemp’s ridleys nesting together, a phenomenon known by the Spanish word arribada. The now-famous film “not only provides the earliest documentation of the Kemp’s ridley nesting beach but also is the first documentation of arribada nesting in sea turtles,” says Thane Wibbels, a University of Alabama at Birmingham biologist who has studied the species since the early 1980s. A recent reanalysis of the film by Wibbels and his colleagues, published in 2016 in Ecosphere, estimates that the arribada it shows contained more than 23,000 turtles.

Yet as soon as the first scientists made their way to Rancho Nuevo in the 1960s, initially by horseback, it became clear that the enormous arribadas of the past no longer took place—and that the Kemp’s ridley was on the verge of extinction. A combination of the species’ restricted geographic range, particularly its nesting range, and unusual breeding biology made the animal especially vulnerable. Kemp’s ridleys are just one of two sea turtle species in the world that nest in arribadas and the only species that nests during the day, especially on windy days. These traits helped commercial harvesters find and collect huge numbers of turtle eggs that, marketed as aphrodisiacs, were (and in some places remain) wildly popular. According to Wibbels, diurnal arribada nesting probably evolved and worked well to evade nocturnal predators such as coyotes and jaguars, “but when it comes to human predators, there’s a flaw in the system.”

On a Mexican beach along the Gulf of Mexico, hundreds of Kemp’s ridleys crawl ashore together to nest in an event called an arribada.

Binational Conservation

Fortunately, scientists’ discovery of the Kemp’s ridley nesting beach quickly spurred conservation action. In the mid-1960s, the Mexican government began sending biologists and armed marines to guard the beach from egg poachers. U.S. government agencies joined in 1978, launching a larger effort that continues today. Throughout the six-month nesting and hatching season, legions of staff from both sides of the border conduct beach patrols several times a day. Many female turtles are tagged for future study and the majority of their nests moved to fenced “corrals” where incubating eggs are protected from people and wildlife. Come hatching time, participants escort tiny turtles as they crawl to the ocean, preventing interception by gulls, ghost crabs and other predators. Together, these efforts dramatically boosted survival of both turtle eggs and hatchlings.

The plan’s success also depended on protecting adults at sea. At the time, scientists estimated that hundreds to thousands of ridleys were dying every year when they accidentally were caught and drowned in the nets of shrimp trawlers. In 1990, the U.S. government began to require shrimpers in its waters to install turtle excluder devices, or TEDS, on their nets to keep the animals out. The Mexican government passed similar regulations in 1995. Combined with increased production and survival of eggs and hatchlings, the reduced mortality that resulted from these efforts led to the spectacular rebound of the Kemp’s ridley that continued up until 2010.

“We may never know why the ridley recovery hit a roadblock,” says Pamela Plotkin, director of Texas Sea Grant and organizer of the Second International Kemp’s Ridley Symposium in 2014. The most obvious culprit, of course, remains the oil spill. But other hypotheses abound, including unusually cold weather during the spring of 2010. Perhaps most troubling, some scientists suggest that aside from the spill, the Gulf of Mexico has become so degraded today that it no longer can support a Kemp’s ridley population anywhere near its historic size.

Just off the main nesting beach of Rancho Nuevo, Mexico, hatchling Kemp’s ridleys ply the waters of the Gulf of Mexico. Recovery of this endangered species may hinge on restoring the Gulf’s degraded habitats.

A Gulf Besieged

The ridleys’ most important prey, blue crabs, have been hit particularly hard by problems such as pollution, overfishing, oxygen-depleted “dead zones” and climate-change-associated droughts and floods. The latter either decrease or increase the amount of fresh water reaching the Gulf, which in turn cause aberrations in the salinity levels that blue crabs need to survive. Kemp’s ridleys simply may no longer have enough food to grow as fast or produce as many eggs as they once did, Plotkin says.

Supporting that hypothesis, Shaver, reporting from Texas, and Gladys Porter Zoo biologist Luis Jaime Peña, from Mexico, describe changes in ridley nesting behavior based on tagging and resighting individual turtles for many decades. On both nesting grounds, females are waiting longer to return to beaches to renest and digging fewer nests per season. “If a female does not have enough fat to yolk up her eggs, she is not going to lay them,” Peña explains.

When this issue went to press in mid-June, the 2017 nesting season was winding down—and the most recent updates sounded encouraging. In Mexico as well as Texas, turtles had started laying eggs early, and it looked like nest numbers would increase for the third year in a row. But whether we ever will witness the spectacular arribadas of decades past is unknown. Notes Wibbels: “The Gulf of Mexico we have today is not the Gulf of Mexico we had in 1947.” 


NWF Priority

Restoring the Gulf of Mexico

Since the day the Deepwater Horizon oil disaster began in April 2010, National Wildlife Federation staff, affiliates, partners and volunteers have been working on the front lines for recovery of the region’s waters, wildlife and human communities. Today, as a result of penalties and legal settlements stemming from the spill, as much as $16 billion will be available to restore the Gulf of Mexico during the next decade and a half—including $44 million for sea turtle conservation that will benefit the Kemp’s ridley. NWF and its partners remain active throughout the Gulf region, working to ensure these funds are spent on projects that will provide the greatest benefit to species such as the ridley along with the habitats they need to survive.

To learn more, go to www.nwf.org/gulf.


Senior Editor Laura Tangley worked with Kemp’s ridleys at Rancho Nuevo, Mexico, during the 1980s.


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