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Headcount of Sea Turtles Proves Elusive

15 July 2010 1:36 pm
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Government agencies don't have the data they need to accurately count populations of the six species of endangered and threatened sea turtles in the United States, says a report issued today by the National Research Council. And that will throw a wrench into ongoing efforts to figure out how badly the turtle populations that live and nest in the Gulf of Mexico have been hit by the oil spill, says report chair Karen Bjorndal, a marine biologist at the University of Florida in Gainesville who studies loggerhead and green turtles. The data gaps also hamper the government's ability to set sensible, "acceptable take" limits, the numbers of turtles deemed permissible for fishermen to accidentally catch, she says.

Currently, the National Oceanic and Atmospheric Administration's National Marine Fisheries Service (NMFS) and the U.S. Fish and Wildlife Service estimate sea turtle populations by counting nests on beaches, and, occasionally, nesting females. But this gives an incomplete picture, say the report's authors. For instance, scientists don't know much about the survival rates of hatchlings, or how likely they are to breed. Some species of turtles can take up to 30 years to mature and return to nesting beaches, so nest-based population estimates lag far behind what's happening in the water, says Bjorndal.

That fact "really came home for the agencies" when, by monitoring nesting beaches, they noticed a 43% decline in Florida's loggerhead turtle population over the past decade, she says. Because the only data collected have been on nesting sites, scientists don't know, for example, whether the problem lies with mature, nesting females or immature turtles that live in the eastern Atlantic and that might be caught on long lines meant for swordfish. "If you don't know what's causing the decline, it's very difficult to set up a management plan to counteract it," Bjorndal points out.

To fill in data gaps, the report recommends compiling the often-scattered data sets collected by academic scientists and stepping up the monitoring of turtles in the water with more tagging and aerial or boat surveys.

A first, rough estimate of the oil spill's impact will emerge when government scientists count nests next year, says a report author, Larry Crowder, a marine biologist at Duke University in Durham, North Carolina. But if the government sticks to that method alone, they won't know the fate of this year's endangered Kemp's ridley hatchlings, which seek out floating seaweed patches in the gulf after leaving their nests in Mexico, until those hatchlings mature, up to 15 years later. "Something like a Kemp's ridley has to live for a dozen years before it becomes a statistic," he says.

"If [government agencies] had the kind of data that we tell them they should obtain, we'd be in a much better position to judge the impacts on the sea turtles in the Gulf of Mexico and the repercussions down the line," says Bjorndal.

For more on the gulf oil spill, see our full coverage.

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