Topping out at two feet long, the gray-green Kemp’s ridley (Lepidochelys kempii) is the smallest and most endangered sea turtle in the world. Despite half a century of conservation efforts, Kemp’s ridley nest numbers have remained lower than expected by researchers, even as other, similarly protected species’ numbers have continued to increase.
New analysis, however, suggests an alternative theory that offers a more positive interpretation of the Kemp’s ridley population data – and shines a light on the complex, long-term effects of climate on sea turtles. The research findings were recently published in the peer-reviewed journal Scientific Reports by researchers at the South Carolina Department of Natural Resources (SCDNR) and a former College of Charleston professor.
“The climate angle is not as simple and linear as we thought,” said first author and SCDNR assistant scientist Dr. Mike Arendt. “There are lag effects at play, and what happens today may have been set in motion years ago. But another important takeaway is that conservation is working. If people were not protecting these eggs for as long and as diligently as they did, we wouldn’t have Kemp’s ridleys at all.”
Although they rarely nest on South Carolina beaches, Kemp’s ridley sea turtles are not uncommon in coastal waters. Young turtles take refuge in the state’s vast networks of salt marsh tidal creeks, and older juvenile and adults are common in coastal waters, where they feed on crabs and other marine invertebrates. SCDNR data suggest high survival in coastal waters in the decades since turtle excluder devices became required aboard shrimp boats – but Kemp’s ridley sea turtles remain vulnerable to boat strikes and being hooked by anglers, which can result in injury and/or death.
Most of the world’s Kemp’s ridleys, including South Carolina’s, originate in one place: the state of Tamaulipas on Mexico’s Gulf coast. Here, and to a lesser extent along Texas’ Gulf coast, these sea turtles engage in an unusual nesting phenomenon referred to as ‘arribada,’ or synchronized mass nesting. Mass nesting historically made Kemp’s ridleys especially vulnerable to poaching, and in the mid-twentieth century, their numbers dropped dramatically.
The Kemp’s ridley, which matures at a much younger age than other sea turtles, was expected to rebound more quickly from the conservation measures that began to be implemented in the 1960s.
“Researchers anticipated a far more robust increase in annual nesting than what transpired,” the paper reports. Kemp’s nesting has essentially plateaued since 2006 but remains a small fraction of a nest count recorded in 1947, leading to a consensus that the population must be faring poorly.
The modeling in this analysis, however, incorporated statistical elements that led to an alternative conclusion: that the population has finally stabilized and continues to rebuild.
“After decades of egg poaching and high at-sea removal of adult females based on firsthand accounts from those pioneering conservationists, the population nearly disappeared,” Dr. Arendt said. “But after 40 years of effective protections, population growth stabilized around 2007.”
Arendt also wanted to use statistical techniques to better understand how climate might be influencing population numbers. To tease out potential impacts, he and the team looked at six different climate datasets and used multiple linear regression to analyze the relationship between climate and annual nest count and hatchlings per nest from 2006-2022.
The results surprised Dr. Arendt. Kemp’s ridley nest counts in the Gulf indeed showed a strong connection to climate – it could explain as much as 83% of nest number variability from year to year – but with an unexpected lag.
The analysis suggested that the number of Kemp’s ridley nests laid in a given year – as well as the average number of hatchlings per nest that year – were correlated to the climate roughly a decade prior.
How is that possible? The explanation isn’t yet clear, but the research team has some ideas.
“It might have to do with ocean circulation, which impacts food availability and the distribution of floating algae habitat during the first year or two of life,” Dr. Arendt said. Climate plays a major role in where, how much and what types of food are available to sea turtles. That, in turn, can impact the long-term health of mother sea turtles, which can influence her nesting down the road.
Modeling suggests that nest counts will continue to fluctuate in the coming years – and that’s ok, as long as long-term average numbers remain stable or increase.
“Saving as many turtles as possible still matters to best position this species as we all look ahead to an uncertain climate future,” Arendt said.
South Carolinians can help Kemp’s ridley sea turtles by being cautious boaters and reporting any hook-and-line-caught animals to SCDNR’s 24-hour wildlife hotline at 1-800-922-5431.
Disclaimer: The views and opinions expressed on South Carolina Wild are solely those of the authors, and do not reflect official policies, positions, or endorsements of activity or products by the South Carolina Department of Natural Resources.