While the boom is encouraging and likely due in part to improved monitoring and nest protection, less benign reasons may also be a factor.

“Changing ocean conditions linked to climate variability may be altering the marine food web and creating an environment that favors turtles by reducing the number of predators and increasing the abundance of prey, particularly jellyfish,” says Larry Crowder, professor of marine biology at Duke University.

The study is published in the journal Ecological Applications.

With plenty of jellyfish to munch on, breeding-age female leatherbacks may be able to build up fat reserves more quickly, allowing them to nest more frequently, says Kelly Stewart, who received her PhD from Duke in 2007 and conducted the turtle research as her dissertation.

Reduced populations of large predators, including the collapse of shark populations in the northwest Atlantic over the past decade, may be playing an even larger role in the turtle boom by decreasing at-sea mortality rates for juvenile and young adult turtles, she says.

News for leatherback populations elsewhere is not so encouraging.  Populations have plummeted at eastern Pacific nesting beaches in Mexico and Costa Rica, which once hosted thousands of female leatherbacks each year. Extirpation, or local extinction of the species, may be imminent on those beaches.

“The good news here is that while most sea turtles continue to decline, some sea turtles are increasing.  We need to understand why they are increasing as much as why they are declining so we can transfer this understanding to other at-risk species, like Pacific leatherbacks,” Crowder says.

Nest counts are the most reliable way of assessing trends in sea turtle populations because they spend most of their lives in the open ocean,  where changes in abundance are difficult to detect.

Researchers at the University of Bath contributed to the study.

More news from Duke University: www.dukenews.duke.edu