Ridley-turtle hatchling
© Gary Braasch
Ridley-turtle hatchlings head into the Gulf in Tamaulipas, Mexico.
Every year, Rancho Nuevo, 900 miles southwest of the Deepwater Horizon blowout, sees a spectacular phenomenon: the arribada - mass nesting - of the Kemp's ridley sea turtle, which has already neared extinction. This year, thousands of baby ridleys swam off toward a deadly new enemy.

Of all the devastation in the Gulf of Mexico caused by the Deepwater Horizon blowout, no one single species is being directly affected as much as the critically endangered Kemp's ridley sea turtle. Only 8,000 adult females nested in 2009, and the adult males are thought to be even fewer. Those that remain have been hit hard. Most of the surviving juveniles inhabit the waters 20 to 30 miles from shore, feeding and growing in the same currents and gyres that collected the bulk of the four million barrels spewed by the now capped well. There were confirmed reports of ridleys being burned alive in the pools of corralled, concentrated oil that BP had been burning off during the spill.

Almost every gravid female ridley lays her eggs on a single beach in Tamaulipas, Mexico, coming ashore in a unique mass-nesting event known as the arribada - the arrival. Kemp's cousins in the Pacific, the Olive ridleys, also do this, but the other five sea-turtle species (and a small percentage of ridleys) are solitary nesters and don't always return to the same place. The arribadas happen at Rancho Nuevo - a beach 900 miles southwest from the blowout. It's only 200 miles south of Brownsville, Texas. Not a bad drive, only I'm told it's too dangerous because three warring factions of narcotrafficantes - the Gulf cartel, the Zetas (former hit men of the cartel), and a local mafia called La Maña - have been having shoot-outs along it. Instead, I fly to Tampico, the sleepy port where the opening scene of The Treasure of the Sierra Madre was filmed, which is 60 miles south of Rancho Nuevo. (Not that Tampico is immune to the violence; the week before I arrive, the naked bodies of five policemen were found hanging from one of its bridges, I am told by a fellow gringo who narrowly escaped being shaken down at one of the narcos' impromptu roadblocks right in the city.) I'm met at the airport by two people from the federal agency that manages Mexico's protected areas, and they whisk me to the nearby Hampton Inn for the night.

In the morning we are driven to the Rancho Nuevo beach reserve by its director Dr. Gloria Tavera. Its 20 miles of wild white sand are patrolled three times a day by guards on A.T.V.'s, and 20 times a day or more during nesting season. Dr. Tavera tells me that the arribadas are over, but that the white ping-pong-ball-size eggs, having incubated for 45 days, are starting to hatch.

Sure enough, at five a.m. on the second morning, we jump onto four-wheelers and bomb down to the South Corral, four miles from the camp, where dozens of the 800 nests from the June 3 arribada are erupting with hatchlings, about 90 per nest. The babies are three inches long and look like black rubber-toy turtles. They crawl down to the surf and, as soon as they hit the water, their angled forelimbs begin to flap wildly. Then they're pulled into the breaking waves by the undertow and are off, on their own, into the great unknown. Guided by pure instinct, fueled by the remaining yoke in their waterproof belly sacs, they will swim straight out for five days or so until they hit the mats of sargassum, a golden-brown, free-floating marine algae (these lines of sargassum are often only 20 or 30 feet wide, but can extend for miles, and offer cover and food for the hatchlings). We don't know how many hatchlings will survive to adulthood, but the most common ballpark estimate is only one in a thousand. Many will be picked off by sharks, many other species of fish, dolphins, and sea birds. Everything wants to eat them. But many more than usual will die when the clockwise currents of the Gulf carry the turtles directly up into the area contaminated by the Deepwater Horizon spill. "The internal damage from the hydrocarbons to the organs of the ridleys could make them unable to reproduce," Dr. Tavera tells me. "That would mean extinction. But nobody knows."

Her fears could be well founded. A new study of shorebirds finds that the ingestion of only a small amount of oil can cause lasting changes in brain function and behavior. The males' pheromones are inhibited so they stop doing their mating behavior.

Conservationists rallied round the ridley in 1978, when human predation left them hanging by a thread. Poaching of the eggs - rich and delicious, they had long been part of the local diet - was stopped, and in l986, when only 600 females came back to nest at Rancho Nuevo, an American law was passed requiring shrimp fishermen meeting certain criteria to equip their nets with escape holes for turtles known as TEDs (turtle excluder devices). For a time, it was working. In 2009 there were 21,000 nests. Six thousand females came ashore over a two-day period that May, the biggest arribada in the 40-year history of the conservation program at Rancho Nuevo. But this year there were only 13,115 nests, the result of a record cold winter followed by three months of red tide, a toxic algae bloom that prevented the females from being able to access the beach. Then, on June 30, the beach was slammed by Hurricane Alex, and a thousand more nests were lost.

Barbara Schroeder, of the National Oceanic and Atmospheric Association in Silver Spring, Maryland, thinks the spill is unlikely to spell the end of the ridley but it "is definitely a setback to the turtle's recovery. We are going to have to enhance our efforts to get the species back on the trajectory it was on, and we will need to re-look at the most significant human threats - bycatch from shrimp and other trawlers and gill nets, hook and line-fishing, and boat strikes."

That the four million barrels of oil seem to be dissipating more quickly than expected does not mean the turtles will no longer be affected. The oil below the surface concerns many experts. Kemp's ridleys in nearshore areas feed on the bottom, which means they have to dive through the oil. What's more, this relatively quick disappearance of the large oil pools was achieved because BP dumped nearly two million gallons of the highly toxic chemical dispersant Corexit into the Gulf - in some cases, without the necessary approval of the Environmental Protection Agency. Corexit, used to break up large pools of oil in water, is an alarmingly unknown entity. Scientists in Louisiana are just beginning to study its effects on marine life in the Gulf. They've discovered high levels of it in blue-crab larvae, which suggests the poison may have already entered the food chain, just in time for the start of Louisiana's shrimp season. Blue crabs are the ridley's favorite food.

Ed Clark, the president of the Wildlife Center of Virginia, who has been treating oiled wildlife for 28 years, tells me that the dispersant is like "putting a coat of new paint on a junk car." The official marine-life casualty numbers, Clark maintains, are grossly underestimated. "If they're saying 400 turtles were killed, I'd bet my house it's more like 4,000," he says.

"BP is responsible for the damages" - up to $50,000 per turtle, as per the Endangered Species Act - "but it is incumbent on the government to prove what [the damages] are," says Clark. He has heard rumors that the cleanup crews on Grand Isle, Louisiana, which are mainly made up of prisoners, were bagging dead turtles and birds in plastic bags marked for incineration because no one from Fish and Wildlife responded to their calls. The F.W.S. agents were mainly focused on federally owned coastline. It may go beyond unresponsive government agencies. Clark also heard rumors that BP was deliberately burning oiled sargassum, even though living sea turtles were known to be still in the floating mats.

So the crisis isn't over, as BP and the government would have you believe. It's only beginning. The biological consequences of this disaster will be felt for years, over generations, like Chernobyl. And we may never know how bad it was.