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Humans Not Solely to Blame for Sea Turtle Declines

28 April 2011 1:00 pm
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At loggerheads. Natural ocean cycles may affect the nesting success of loggerhead sea turtles more than people do.

Humans are pushing sea turtles to the brink of extinction by entangling them in fishing gear, tossing plastic garbage into their habitats, and building resorts on prime nesting beaches, among other affronts. That's the going hypothesis, anyway. But a new study suggests that our transgressions are peanuts compared to natural oceanic cycles, at least for loggerheads. The findings don't let people off the hook, the authors say, but they do provide new insight into the ways climate can shape turtle populations.

Loggerheads lay their eggs on subtropical beaches around the world. After hatching, baby sea turtles head out to sea where they spend years maturing. When females reach breeding age—25 to 35 years old for loggerheads—they clamber ashore to lay eggs on the beach. Nest counts are the main source of demographic data for sea turtles, but it's hard to estimate population size from these counts. Between the mid-1990s and 2006, loggerhead nests in Florida—one of the species' nesting epicenters—declined from roughly 55,000 per year to around 30,000. That drop and declines elsewhere prompted U.S. federal agencies to propose upgrading most loggerheads from threatened to endangered under the Endangered Species Act.

Numerous studies have shown that fishing bycatch kills an alarming number of sea turtles each year, and the threats posed by many other human activities are well documented. But ecologists Kyle Van Houtan of the National Oceanic and Atmospheric Administration's Pacific Islands Fisheries Science Center in Honolulu and John Halley of the University of Ioannina in Greece wondered if other factors were also at work. In the new study, published online this week in PLoS ONE, the duo measured the effect of certain ocean conditions on loggerhead nesting, using nest count data from Japan and Florida stretching back as far as the 1950s. Specifically, they looked at two long-term warming and cooling cycles whose effect on sea turtles hadn't been investigated, one in the Pacific where Japanese turtles spend their formative years, and another in the Atlantic, where young Floridian turtles live. They also looked at recent ocean conditions, in particular the temperature of the sea surface near Japan and Florida the winter before a given breeding season. Such conditions have been shown to influence whether females are robust enough to make the long migration to shore and produce hundreds of eggs.

Using mathematical models, Van Houtan and Halley found a strong correlation between the nest counts in a given year and the state of the long-term oceanic cycles some 3 decades earlier. That's when most nesting females would have been in their first year of life, the researchers say. Van Houtan and Halley think the two oceanic cycles—the Pacific Decadal Oscillation (PDO) and the Atlantic Multidecadal Oscillation (AMO)—can make or break a young turtle's survival; favorable cycles bring food and good weather, for example. The number of survivors seems to strongly affect the number of nests.

For Van Houtan, this correlation conjures an astronomical analogy. "When you're counting nesting turtles, in some ways it's kind of like watching the stars," he says. "When you look out across a starry night, you're observing something that was set into motion a long time ago, that happened … far away."

The ecologists also found a modest role for ocean-surface temperature just prior to the breeding season. Together, the past and recent oceanic conditions explained about two-thirds—and as much as 88%—of loggerhead nesting activity.

Based on the data, Van Houtan and Halley predict continued declines in the Japanese loggerhead population over the next 25 years, but a rebound in the Florida population. While most loggerhead nesting variation appears to be natural, Van Houtan insists that humans bear a good share of the blame. He suspects that our actions may account for much of the remaining nesting variation. And he says that global warming could alter the PDO and AMO to the turtles' disadvantage.

Rebecca Lewison, an ecologist at San Diego State University who studies sea turtles, says the findings are important and likely apply to other sea turtle species. One key take-away is that biologists can't look to annual changes in nesting activity as a way to gauge the success of any particular management strategy, she says.

Tony Tucker, a sea turtle biologist with Mote Marine Laboratory in Sarasota, Florida, is more critical. While praising the models' sophistication and the paper's illumination of the link between climate and nesting, Tucker says he is not convinced that baby-turtle survival is more important in determining nest counts than the number of adult females in good breeding condition. By extension, he says, he doubts that ocean conditions long ago and far away have a bigger effect on turtle nesting than recent ones do.

Elizabeth Griffin Wilson, a marine scientist with the international conservation group Oceana, points out that the new paper does not specifically investigate the effects of factors like fishing bycatch or habitat destruction, so she urges caution in comparing the human and natural toll on sea turtles. Regardless of the final balance, people still have a responsibility to minimize the damage we inflict, she says.

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