NEW ORLEANS (Reuters) - At least 150 sea turtles have washed up dead or dying along the U.S. Gulf Coast since the giant oil spill off Louisiana, a higher number than normal for this time of year, a leading wildlife expert said on Monday.

The toll among sea turtles has been steadily rising since the deep-sea well ruptured last month, and the stranding count began to reach an unusually high level in the past week, said Dr. Michael Ziccardi, a veterinarian overseeing some of the area’s wildlife rescue teams.

Several days ago, when the number of dead turtles stood at just over 100, federal wildlife officials said that was still considered typical for the season.

Wildlife officials are especially concerned for the well-being of sea turtles in the Gulf following the spill because all five species native to the region are endangered, and they are just heading into their spring nesting season.

This is a time of year when dead or debilitated turtles would normally begin to show up with greater frequency, but the 156 found since April 30 along the coasts of Louisiana, Mississippi, Alabama and Florida “are in higher numbers than you would expect,” Ziccardi said.

None of the animals had obvious signs of oil contamination. But, because of their proximity to the spill, they are being treated as possible victims of the crude oil that has been gushing from the ruptured wellhead since April 20, he said.

The 12 confirmed dead dolphin strandings along the same four Gulf Coast states, Ziccardi said, were “more or less in line” with what would normally be found for the same period of time without an oil spill.

No outward signs of oiling were detected on the dolphins either, and only one full necropsy has been performed to date.

Ziccardi, director of the Oiled Wildlife Care Network in California, a hub for the world’s leading experts in capturing and caring for oil-soaked sea life, is the chief coordinator for teams across the Gulf tracking the status of turtles and marine mammals. He spoke to Reuters outside his office at a spill response command center near Houma, Louisiana.

No whales or manatees have been reported dead since the spill, though several sperm whales were spotted swimming in and around the oil slick, Ziccardi said.

Wildlife specialists said there has been relatively little impact on bird life so far.

Energy giant BP Plc said it has begun to have some success in containing the oil in the last two days.

A FORENSIC INVESTIGATION

Of 156 turtles collected as of Monday, eight were still alive when found, and six of those survived and are undergoing rehabilitation. “They’re looking good,” he said.

The rest range from relatively complete carcasses to specimens consisting of little more than a shell. Turtles known to have died before the spill, or thought to have been sick or injured beforehand, are not included in the tally.

Necropsies, the animal equivalent of an autopsy, have been performed on 40 turtles so far. And tissue samples taken from as many specimens as possible are being analyzed for abnormally high chemical levels associated with oil contamination.

Initial necropsy results are expected in a few days, but laboratory tests of the tissue samples will likely take weeks to complete. In many cases these results are needed to make a conclusive finding about the cause of an animal’s death.

In some instances, it may be difficult to rule out other possible causes of death, such as illness or exposure to some toxin other than oil.

“It’s a complex puzzle, and what we’re doing is putting together all the pieces and trying to get the correct answer of whether or not the animals have been oiled,” Ziccardi said.

One possible reason for the higher-than-normal count of dead turtles could be the intensive effort to monitor the region’s wildlife with hundreds of individuals patrolling the shoreline for dead or distressed animals.

He acknowledged that some cases will remain inconclusive, and it will ultimately be up to damage assessment teams to determine what the likely death toll was due to oil pollution, based in part on mathematical “modeling.”

The “chain of custody” for each sample collected is rigorously documented, the way evidence is for a criminal forensic investigation, Ziccardi said.