Climate change threatens eastern Pacific leatherback turtles

Thursday, Aug 2, 2012

 Erin Keene)Climate change could almost completely wipe out the eastern Pacific leatherback sea turtle by the end of the century, according to a new study published in the journal Nature Climate Change. The turtles have been around for 90 million years, but the study projects that warmer temperatures will lead to a 75 percent population decline by the year 2100.

The researchers examined turtle populations at a Costa Rican beach, Playa Grande, which hosted over 1000 nesting females per year in the 1990s. Now there are just forty or so per year. There are only four major beaches where eastern Pacific leatherbacks nest, and all show similar population declines, said Vincent Saba, the study’s lead author, a research fishery biologist with the U.S. National Oceanic and Atmospheric Administration (NOAA) National Marine Fisheries Service based at NOAA’s Geophysical Fluid Dynamics Laboratory (GFDL) and a visiting research collaborator at Princeton’s Program in Atmospheric and Oceanic Sciences. He conducted the study with researchers from Princeton, Drexel University, Indiana-Purdue University at Fort Wayne, and NOAA.

Intense turtle egg poaching and fishery bycatch - when turtles drown in nets – have likely led to a precipitous decline in the eastern Pacific leatherback population. But Playa Grande has been protected from poachers since the early 1990s, and still has not seen a recovery in its population. Saba and his team looked at climate vulnerability as a factor in the population’s trajectory in the future.

The research team paired climate models with twenty years of population data to project the future of the leatherbacks. Because female turtles typically return to the same beach to lay their eggs annually, trends in the number of nesting females likely reflect the larger population.

The team found that the number of turtles that will hatch can be predicted based on climate data. Cooling rain and lower air temperatures, associated with the La Niña weather pattern, mean higher turtle yields.

Drier conditions and warmer temperatures, during the El Niño phase, result in lower turtle yields. Eastern Pacific leatherbacks have a huge gender bias, as females make up 88-90% of the population. Cool and wet weather leads to more male hatchlings, while warmer and drier weather means more females. “While some people think warmer temperatures are dangerous for turtles because they will all be females, the real danger of hotter weather is that fewer eggs hatch,” explained Saba. The team used a model that shows how gender ratio can be modeled as a function of annual rainfall.

Although turtles have adjusted to climate changes over the past 90 million years of their history, said Saba, the rate at which the climate is warming is far faster than in the turtles’ past. A species that takes fifteen to twenty years to mature cannot adapt to high temperatures in such a short period of time. The eastern Pacific turtles are even more vulnerable than their Atlantic relatives because the Pacific Ocean has fewer nesting beaches and a starkly smaller population.

Saba, who has worked as a research scientist and project manager at Playa Grande, can attest to the drastic decline in the number of turtles. “I used to see up to fifty nesting females in a single night’s beach patrol,” he says. “But when I was there last year, we didn’t see any females for an entire week.”

The project required collaboration between American academics and the Costa Rican government. The partnership has benefits to both parties, as the government gains tourism while academics gather information. A significant ecotourism industry has grown up around the turtles, with many former poachers acting as tour guides.

Saba’s research will continue at the start of the nesting season, which begins in October. Researchers will take eggs to a hatchery to shade them, and will irrigate nests to see how this affects the temperature of the nest as well as the number of hatchlings. This technique has already been successfully employed in the Caribbean, resulting in higher hatching rates.

Saba wants to research why specific beaches are nesting areas for females. Understanding what factors make a beach a nesting spot would help researchers project which nesting beaches the turtles may frequent in a warmed climate.

Funding for the project was provided by the Cooperative Institute for Climate Science, Earthwatch Institute, The Betz Chair Endowment of Drexel University and The Leatherback Trust.

Saba, Vincent, Charles Stock, James Spotila, Frank Paladino, and Pilar Santidrián Tomillo. 2012. “Projected response of an endangered marine turtle population to climate change.” Nature Climate Change 2, no. 7.