Scientists who study sea turtles in Texas and Mexico have a mystery on their hands.

After years of seeing populations of endangered species of sea turtles increase they are now trying to figure out why those numbers seem to be on the decline.

Every Spring and Summer volunteers head out in search of nesting sea turtles at the Padre Island National Seashore.

The protected coastline is the main nesting grounds for the endangered Kemp's ridley sea turtle in the United States.

This year 152 nests were located on Texas beaches, down from the record setting 209 nests discovered in Texas last year.

In Mexico, the main nesting grounds for Kemp's ridley turtles, scientists also recorded a similar decline.

"This was a surprise that we would take a downturn this year," said Dr. Donna Shaver, chief of the National Park Service division of sea turtle science and recovery. "We were disappointed to see the numbers drop. We had a decline in 2010 but were hoping that the numbers were going to continue to increase."

Shaver has been studying the Kemp's ridley for over 30 years, watching their remarkable comeback from the brink of extinction in the 1980's.

Shaver said despite another decline in 2010 the number of nests was projected to increase by 12 to 18 percent each year.

"We thought that we turned the corner," Shaver said. "The numbers were increasing rapidly and many people were calling it an endangered species success story in the making. We're still up compared to the low point population in 1985 but we're not as optimistic as we were just a few years ago."

One of the possible culprits driving the decline could be the 2010 BP Deepwater Horizon oil spill.

Satellite tracking data shows female Kemp's ridley turtles head to Louisiana waters to forage after nesting in Texas, putting them in the area of the spill.

"Most of them leave South Texas or Mexico and a very important area for them to go to forage is in the waters off the coast of Louisiana and Mississippi and Alabama and they also migrate through there. Some go as far as the West coast of Florida," Shaver said. "So there is concern about the turtles as they entered into that area and maybe as they remain there as well."

Shaver said it's too soon to point fingers, it will take several years of collecting data to determine what impact the oil spill has had on the turtles or to pinpoint any other possible causes for the decline.

"We're concerned at this point and we just need to collect more years of data in order to be able to continue to investigate," Shaver said. "It's still the world's most endangered sea turtle species. We need to continue our monitoring and we certainly need to continue our conservation efforts."

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