Federal officials have counted 153 of the turtle nests in Texas, which is the largest U.S. nesting ground for the animals outside Mexico. A similar drop also has been detected in Mexico.
Last year, 209 nests were tallied in Texas.
"I'm very concerned about it because it is something we weren't expecting," Tom Shearer, the U.S. Fish and Wildlife Department Kemp's ridley coordinator, told the Houston Chronicle.
Scientists had expected the number of nests for the smallest sea turtle species to grow annually by 12 to 17 percent.
The recovery of the Kemp's ridley is measured by the annual count of nests from roughly May to July. New nests typically don't show up after mid-July, according to Donna Shaver, chief of the National Park Service division of sea turtle science and recovery at Padre Island National Seashore.
An estimated 40,000 nests were laid in 1947. But by the 1960s, the Kemp's ridley turtles were nearly extinct, due in large part to commercial fishing and shrimping that trapped the animals in nets. Introduction of turtle excluder devices on nets has helped a population rebound.
The prime nesting ground in Mexico, at Rancho Nuevo, Tamaulipas, fell to a low of 702 in 1985. Officials said the number of nests there as of July 22 had fallen from 21,797 last year to 16,200.
Ben Higgins, a biologist with the National Marine Fisheries Service in Galveston, said 2009 and 2010 were both cold years that could have killed hatchlings.
"An event like that where you could potentially wipe out a lot of turtles can show up seven years later," Higgins said.
Others suggest the 2010 BP oil spill may have had an effect. Recently released research shows female Kemp's ridleys regularly forage after nesting in areas off the Louisiana coast that were hit by the spill.
"I think it's got to be suspect," Carole Allen, director of the Sea Turtle Restoration Project's Gulf Coast office, said. "We don't know for sure what BP did to them and how bad it was, but they hurt them and they hurt them bad."
In 2010, the number of nests fell in Texas to 139, down from 197 the year before; in Mexico the count dropped to about 13,000 from 22,000.
The cold winter could have combined with the April 2010 BP spill to affect the turtles at the start of the nesting season.
It's also possible the decline could be a natural fluctuation or was brought about by an event even longer ago, since female turtles take 12 to 17 years to reach breeding age.
"There could be something else out there we don't know about," Tasha Metz, a sea turtle biologist at Texas A&M University at Galveston, said. "We really can't jump to conclusions."
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