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Critically endangered Kemp’s ridley sea turtles are in even more trouble than scientists thought

  • Bilateral conservation efforts between the U.S. and Mexico led to an exponential recovery rate of Kemp’s ridley sea turtles in 2009 that were expected to continue for decades.
  • In 2010, marine biologists noticed that nesting levels had plunged sharply; approximately 14,000 nests were laid in the 2015 season, a 34 percent decline over 2009.
  • New research suggests that the species’ recovery has stalled at less than one-tenth of historic levels.

Kemp’s ridley sea turtles, found mostly in the Gulf of Mexico, had been pushed to the brink of extinction by the 1980s, but bilateral conservation efforts between the U.S. and Mexico led to an exponential recovery rate by 2009.

In 2010, however, marine biologists noticed that nesting levels had plunged sharply, and now new research suggests that the species’ recovery has stalled at less than one-tenth of historic levels.

In the journal Ecosphere this month, researchers with the University of Alabama at Birmingham (UAB) published the results of their study on the conservation status of Kemp’s ridley sea turtles (Lepidochelys kempii).

The UAB researchers were tasked with determining just how much trouble the turtles are in by the International Union for Conservation of Nature (IUCN), which currently lists the turtles as critically endangered. But there was very little information available on the species’ nesting habits and population numbers between 1880, when they were first discovered, and 1978, when the U.S.-Mexico conservation efforts began.

This meant that there wasn’t enough information to establish historical population levels, and hence how far the turtles have come towards a full recovery. Even the exact location of the nesting grounds for Kemp’s ridley sea turtles had remained a mystery to science until a Mexican sportsman named Andres Herrera discovered their primary nesting beach in the western Gulf of Mexico, near Rancho Nuevo, Mexico, in 1947.

A nesting female Kemps ridley sea turtle at Rancho Nuevo, Mexico. Photo by Hector Chenge.

UAB biology professor Thane Wibbels and doctoral student Elizabeth Bevan studied film of a mass-nesting event shot by Herrera in 1947 in order to determine a baseline population size prior to the species becoming endangered. They calculated that there were around 26,000 sea turtles on a 1- to 2-mile stretch of beach on the day the film was taken.

From there, Wibbels and Bevan estimated that 120,000 to 180,000 nests must have been laid over the entire 1947 nesting season — far more than the approximately 14,000 nests laid in the 2015 season, which itself represents a 34 percent decline over 2009, when populations were experiencing exponential growth that was expected to last for decades.

Wibbels said that the current Kemp’s ridley population in the Gulf of Mexico is just 10 percent of historic levels, and that there are a number of theories as to why the turtle’s recovery has come to such a drastic halt.

“The Gulf of Mexico today, may not be the Gulf of Mexico 70 years ago,” Wibbels told Mongabay. The explosion and subsequent oil spill at BP’s Deepwater Horizon drilling rig in 2010, which dumped 4.9 million barrels of oil into the Gulf of Mexico, certainly impacted the species, Wibbels said. But the species’ decline could also reflect more long-term changes in the Gulf, such as the fact that blue crabs, a preferred prey of the Kemp’s ridley, have significantly declined in some areas.

“Kemp’s ridley represents a ‘signature’ species for the Gulf of Mexico, and being a higher tropic level predator, may well be a ‘canary in a coal mine’ for the Gulf of Mexico,” Wibbels told Mongabay. “Historically it may have been one of the most abundant sea turtles in the Gulf until it almost went extinct in the 1980’s.”

Donna Shaver, who heads the Division of Sea Turtle Science and Recovery for the U.S. National Park Service, told Mongabay she agrees with the UAB researchers’ conclusion that nesting is no longer increasing exponentially, as it was prior to 2010, and that nest numbers in Texas are following the same trend observed in Mexico.

“Research is on-going to try to identify the cause of the stalling,” Shaver told Mongabay.

“There could have been impacts to the species from the Deepwater Horizon Oil Spill through mortality of turtles, sub-lethal impacts to the turtles, or impacts to their food resources. Various other hypotheses regarding the decline of nesting in recent years have included mortality from fisheries interactions, and reduced fitness and fecundity due to cold winters, reduced freshwater inflow, reduced preferred forage resources (e.g., blue crabs and fisheries by-catch), density dependence, and combinations of the above.”

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