The Kemp's ridley sea turtle lay belly-up on the metal autopsy table, as pallid as split—pea soup but for the bright orange X spray-painted on its shell, proof that it had been counted as part of the Gulf of Mexico's ongoing “unusual mortality event.”

Under the practised knife of Dr. Brian Stacy, a veterinary pathologist, the specimen began to reveal its secrets: First, as the breastplate was lifted away, a mass of shrivelled organs in the puddle of stinky red liquid that is produced as decomposition advances. Next, the fat reserves indicating good health. Then, as Stacy sliced open the esophagus, the most revealing clue: a morsel of shrimp, the last thing the turtle ate. “You don't see shrimp consumed as part of the normal diet” of Kemp's ridleys, Stacy said.

This turtle, found floating in the Mississippi Sound on June 18, is one of hundreds of dead creatures collected along the Gulf Coast since the Deepwater Horizon oil rig exploded. Swabbed for oil, tagged and wrapped in plastic “body bags” sealed with evidence tape, the carcasses — many times the number normally found at this time of year — are piling up in freezer trucks stationed along the coast, waiting for scientists like Stacy, who works for the National Oceanic and Atmospheric Administration, to begin the process of determining what killed them.

Despite an obvious suspect, oil, the answer is far from clear. The vast majority of the dead animals that have been found —1,387 birds, 444 turtles, 53 dolphins and one sperm whale — show no visible signs of oil contamination. Much of the evidence in the turtle cases points, in fact, to shrimping or other commercial fishing, but other suspects include oil fumes, oiled food, the dispersants used to break up the oil or even disease. The trail of evidence leads from marine patrols in Mississippi, where more than half the dead turtles have been found, to a toxicology lab in Lubbock, Texas, to this animal autopsy room at the University of Florida in Gainesville.

The outcome will help determine how many millions BP will pay in civil and criminal penalties — which are far higher for endangered animals like sea turtles — and provide a wealth of information about the little-known effects of oil on protected species in the gulf.

In a laboratory at Texas Tech University in Lubbock, Jennifer Cole, a graduate student, was slicing a precious chunk of living dolphin tissue into 0.3-millimetre sections. Supervised by Celine Godard-Codding, an endangered species toxicologist, Cole was studying cytochrome P450 1A1, an enzyme that breaks down hydrocarbons. Tissue samples are one of the only ways to learn more about toxins in marine mammals and sea turtles, whose protected status limits the type of studies that can be done.

Oil — inhaled or ingested — can cause brain lesions, pneumonia, kidney damage, stress and death. Scientists working on the BP spill have seen oil-mired animals that are suffering from extreme exhaustion and hyperthermia, with the floating crude reaching temperatures above 130 degrees, Stacy said. Far less is known about the effects of dispersants, either by themselves or mixed with oil, although almost 2 million gallons of the chemicals have been used in the BP spill. Studies show that dispersants, which break down oil into tiny droplets and can also break down cell membranes, make oil more toxic for some animals, like baby birds. And the solvents they contain can break down red blood cells, causing haemorrhaging.

When Lt. Donald Armes of the Mississippi Marine Patrol heard about the rash of dead sea turtles littering the state's shores, his first thought was not of oil but of shrimp boats.

“Right off the bat, you figure somebody's gear was wrong,” he said recently. By gear, Armes meant turtles excluder devices, which shrimp trawlers are supposed to have. Without them, trawls can be one of the biggest dangers for turtles, which can get trapped in the nets and drown. The devices provide an escape hatch. Another kind of shrimp net, called a skimmer, is not required to have an excluder device _ instead, the length of time the skimmers can be dragged is limited to give trapped turtles a chance to come up for air.

When shrimp season began in Mississippi on June 3, the Marine Patrol inspected all the boats and found no violations involving the excluders, Armes said. But June 6, 12 dead turtles were found in Mississippi in a single day. Similar spikes have occurred when parts of Louisiana waters were opened to shrimpers, and since most of the waters in the spill area have closed, turtle deaths have subsided.

Shrimpers emerged as a prime suspect in the NOAA investigation when, after a round of turtle necropsies in early May, Stacy announced that more than half the carcasses had sediment in the airways or lungs _ evidence of drowning. The only plausible explanation for such a high number of drowning deaths, he said, was, as he put it, ``fisheries interaction.''

Environmentalists saw the findings as confirmation of their suspicions that shrimpers, taking advantage of the Coast Guard and other inspectors being busy with the oil spill, had disabled their turtle excluder devices. Officials in Louisiana and Mississippi say turtles die in shrimp season even when shrimpers follow the law, from boat strikes and other accidents. They also say there have been far fewer shrimpers working since the spill, in part because many have hired out their boats to BP. That should mean fewer, not more, turtle deaths.

But there has also been illegal activity. In Louisiana, agents have seized more than 20,000 pounds of shrimp and issued more than 350 citations to commercial fishermen working in waters closed because of the oil spill.

Diagnosing difficulties

In the necropsy lab in Gainesville, Stacy was slitting open the turtle's delicate windpipe, looking for traces of sediment, a telltale sign of drowning. He found none there, so he examined a crinkled papery membrane barely recognisable as lungs. Nothing.

In a sense, the necropsies have posed more questions than answers, demonstrating how oil has become just another variable in a complex ecosystem. Late in June, a dolphin examined at the Institute for Marine Mammal Studies in Gulfport, Miss., showed signs of emaciation, but its belly was full of fish, suggesting that it may have gorged itself after a period of difficulty finding food.

Another dolphin, its ribs broken, was hit by a boat, a catastrophe that dolphins are normally nimble enough to avoid. The veterinarian, Dr. Connie Chevis, found a tarlike substance in the dolphin's throat. The substance will be analysed to see if it is oil, but one theory is that the animal could have been disoriented by oil exposure, which can have a narcotic effect, rendering it incapable of avoiding a boat strike. Lori Deangelis, a dolphin tour operator in Perdido Bay, said the dolphins on her recent tours have been ``acting like they've had three martinis.'' The results raise questions about oil's indirect effects. Is crude, for example, responsible for what anecdotal reports say is a steep increase in turtles in Mississippi and Louisiana waters? The population of Kemp's ridleys has been rebounding thanks to years of protective measures. But some scientists have speculated the spill is driving wildlife toward the coast, crowding areas where there is more boat traffic and setting the stage for fatal accidents. — New York Times News Service

More In: Comment | Opinion