PADRE ISLAND NATIONAL SEASHORE, Texas » It is nesting season here and just offshore, Kemp's ridley sea turtle No.15 circles in the water before dragging herself onto the sand to lay another clutch of eggs.

The sea turtle, affectionately nicknamed Thelma by a National Park Service employee, has already beaten some terrible odds. Still in the egg, she was airlifted here from Mexico in the wake of the 1979 blowout of the Ixtoc 1 rig, which spilled millions of gallons of oil into the Gulf of Mexico and covered the turtles' primary nesting place.

Now Thelma and others of her species are being monitored closely by worried scientists as another major oil disaster threatens their habitat. Federal officials said on Tuesday that since April 30, 10 days after the accident on the Deepwater Horizon, they have recorded 156 sea turtle deaths; most of the turtles were Kemp's ridleys. And though they cannot say for sure that the oil was responsible, the number is far higher than usual for this time of year, the officials said.

The Deepwater Horizon spill menaces a wide swath of marine life, from dolphins to blue crabs. On Tuesday, the National Oceanic and Atmospheric Administration expanded a fishing ban in the gulf because of the spreading oil. But of the endangered marine species that frequent gulf waters, only the Kemp's ridley relies on the region as its sole breeding ground.

Since the Ixtoc 1 spill, the turtles, whose numbers fell to several hundred in the 1980s, have made a fragile comeback, and there are now at least 8,000 adults, scientists say. But the oil gushing from the Deepwater Horizon could change that. More than any other large marine animals, the turtles may be vulnerable to the oil spreading through the gulf. An ancient creature driven by instinct, it is programmed to forage for food along the coast from Louisiana to Florida, in the path of the slick.

“;It lives its entire life cycle in the gulf, which is why we are so critically concerned,”; said Dr. Patrick M. Burchfield, a scientist at the Gladys Porter Zoo in Brownsville, Texas, who has studied the Kemp's ridley for 38 years.

The nesting season for the sea turtles runs until mid-July, and for most of that time they will remain off Padre Island and the beaches of Mexico, where there is currently no oil. But then things become more chancy, as new sea turtle babies go off to sea, floating on currents in the gulf or on seaweed patches that could be covered by crude. Hungry after egg-laying, adult females are known to go to the mouth of the Mississippi, a particularly rich feeding ground, to replenish themselves.

Andre M. Landry Jr. of the Sea Turtle and Fisheries Ecology Research Laboratory at Texas A&M University, Galveston, said that satellite radios have been attached to several sea turtles, including Thelma, for research. He hopes these will offer clues about what is happening offshore.

“;If she is beached, it is going to be constantly sending out a signal as opposed to the random signals they send out when they randomly come up to breathe,”; Landry said.

Barbara Schroeder, national turtle coordinator for NOAA fisheries, the government agency charged with assessing damage to offshore life, said that the agency was investigating the sea turtle deaths intensively, but did not have many answers yet.

She said that so far full necrospies had been performed on 50 turtles and partial necropsies on another 17. Internal inspections of the animals, she said, did not reveal oil. But she added that scientists still had to test tissue samples taken from some of the turtles for evidence of oil.

She cautioned that it might be hard to determine conclusively how the turtles died or even how the spill was affecting the species more generally.

“;People think this is like television, where the mystery is solved in one hour,”; she said. “;It is very complex. Most of the impacts occurring to turtles are out of sight. Most turtles never wash ashore.”;

The Kemp's ridley is millions of years old; its ancestors once swam with dinosaurs. A sandy-olive color, Kemp's ridleys are the smallest of the sea turtles, only about two feet across. Although the turtles have been sighted along the Atlantic Seaboard, they must return to the warm waters of the gulf to breed.

As recently as the 1940s, they were abundant in the Mexican gulf waters. Tens of thousands at a time would come ashore on the same day at Rancho Nuevo, a remote Mexican beach in Tamaulipas State, to lay their eggs in the synchronized pattern unique to their breed. But pollution, the collection of eggs for food and aphrodisiacs, and the nets of shrimp trawlers depleted their numbers.

Then came the blowout on the Ixtoc 1. The deepwater well dumped three million barrels of crude into the gulf, covering the beach at Rancho Nuevo. Nine thousand hatchlings had to be airlifted to nearby beaches. Although the role of the oil in killing the turtles was never confirmed, by 1985, there were fewer than 1,000 Kemp's ridleys left, and the International Union for Conservation of Nature declared the species one of the 12 most endangered in the world.

To prevent a single environmental catastrophe from sending the turtles into extinction, eggs from remaining turtles, including an egg that became sea turtle No.15, were brought here to Padre Island to begin a new colony.

At birth, the babies were set free in the surf down the road from the ranger station to allow them to imprint the beach on their memories, then captured again and protected until they were 9 months old and less susceptible to becoming prey.

“;We called it head start, after the school program,”; said Donna J. Shaver, chief of sea turtle science and recovery for the National Park Service at Padre Island, who has worked with the sea turtles there since 1980.

No.15 has returned to the island six times to lay clutches of eggs, burying her most recent round of 92 eggs in the sand by an enormous rusted, beached buoy only one and a half miles from where she was first put into the surf 24 years ago.

“;Their precision is really amazing,”; Shaver said. She and other scientists will be watching the radio blips from the tagged turtles closely, but the tracking device is not infallible.

The transmitters might stop functioning because of dead batteries. And even if a turtle is known to have beached, the carcass might never be found or might be found only after serious decomposition, and the cause of death might never be known.

Still, Shaver prefers to think positively until more results come in. “;When I got here, there were many who thought the species might not survive at all,”; she said. “;We've come so far.”;