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VIMS researcher discovers sea turtle-El Niño link

The reproductive output of western Atlantic leatherbacks is nearly twice that of eastern Pacific turtles
The reproductive output of western Atlantic leatherbacks is nearly twice that of eastern Pacific turtles This difference is attributed to more productive foraging areas in the Atlantic that are not subject to the high climate variability of ENSO as those in the eastern Pacific.
The relationship between multidecadal regime shifts
The relationship between multidecadal regime shifts indicated by the type and frequency of ENSO events, and the nesting response of eastern Pacific leatherbacks. This natural variability renders the eastern Pacific population more sensitive to human induced mortality and likely explains the precipitous decline in turtle numbers without any signs of recovery.
An adult male leatherback turtle
An adult male leatherback turtle feeding on a jellyfish in the northwest Atlantic Ocean.
Early morning
Early morning a female returns to sea after the rigorous process of nesting. Females typically nest at night but sometimes arrive in the dark morning hours and end up returning to sea during sunrise. Other female tracks can be seen in the background.
by David Malmquist |  May 14, 2008

Research by Dr. Vincent Saba of the Virginia Institute of Marine Science and colleagues provides the first evidence of a link between declining numbers of leatherback sea turtles in the Eastern Pacific Ocean and the climatic phenomenon of El Niño.

The research, which appears in the May issue of Ecology, has been selected as a “highlight of the recent literature” by the editors of Science magazine.

Saba’s findings, part of his Ph.D. research in the College of William and Mary’s School of Marine Science at VIMS, help explain why leatherback populations in the eastern Pacific have failed to respond to protections established on the turtles’ main nesting beaches in Costa Rica in the early 1990s. Leatherbacks, the world’s largest marine turtle with adults reaching up to 8 feet long and weighing up to 2,000 lbs, are an endangered species.

Similar protections along nesting beaches in the Caribbean and Africa have been credited with a recent increase in the numbers of leatherbacks in the Atlantic and Indian oceans.

“There’s a clear dichotomy in the size and trends of nesting populations of leatherbacks in the Atlantic, Indian, and Pacific,” says Saba, currently a post-doctoral fellow in ecosytem modeling at VIMS. “The number of nesting females in the Atlantic and Indian oceans is stable or increasing, while populations in the eastern Pacific have been sharply declining.”

Declines in the number of leatherbacks and other marine turtles (all 7 sea-turtle species are either threatened or endangered), are typically attributed to poaching of eggs, bycatch in fishing gear, and loss of nesting beaches. But those threats seem to be equally distributed around the world, failing to explain why leatherback numbers in the eastern Pacific continue to fall, while Atlantic and Indian leatherback numbers are on the rise.

By combining population data from nesting beaches with oceanographic data from satellites, Saba was able to show that leatherback declines in the Eastern Pacific reflect the inconsistency of their food supply in the region. A large part of that inconsistency is due to the periodic appearance of El Niño. A large-scale shift in atmospheric pressure and ocean currents, El Niño occasionally replaces the Eastern Pacific’s normally cool, nutrient-rich waters with waters that are warm and nutrient-poor.

Leatherback seaturtle nesting
Researchers watch as a female leatherback turtle returns to the sea after nesting at Playa Grande, Costa Rica, also known as Parque Nacional Marino Las Baulas (Leatherback National Marine Park). Photo courtesy Dr. Vincent Saba.

Female leatherbacks require large amounts of energy to produce eggs, and to make the long migration between their nesting and feeding grounds. The turtles’ favorite food is jellyfish, which they must consume in large quantities to gain sufficient energy. Jellyfish, in turn, depend on nutrient-rich waters to fuel the plankton that make up their diet. Saba’s research shows that when an El Niño appears, this food chain is severed, and leatherbacks must expend much more energy as they swim far and wide in search of prey.

“El Niño initiates a chain of events that forces leatherback females in the Eastern Pacific to expend more energy on finding food and less on reproduction,” says Saba. “That makes it more difficult for the population to rebound now that nesting protections are in place.”

In the North Atlantic, where food supplies are more constant, female leatherbacks grow larger, lay more eggs more often, and may reach sexual maturity at a younger age. “Females in the western Atlantic,” says Saba, “can afford to expend more energy on reproduction due to their consistent, high-quality foraging areas.”

Looking to the future, Saba cautions that both observations and computer models suggest that human-induced climate change may be leading to conditions in the Eastern Pacific that are less favorable for leatherbacks, including the more frequent appearance of El Niño events.

“It’s still unclear how global warming will affect El Niño variability,” says Saba. “If El Niño events do become more frequent, as some scientists suggest, the reproductive output of Eastern Pacific leatherbacks will remain low, further delaying population recovery.”

Saba co-authored the paper, Bottom-Up and Climatic Forcing on the Worldwide Population of Leatherback Turtles, with Professor Jack Musick of VIMS, James Spotila of Drexel University, and Francisco Chavez of the Monterey Bay Aquarium Research Institute.