The far-flung journeys of juvenile sea turtles could mean that the impact of 2010's Deepwater Horizon oil spill was global.

More than 300,000 sea turtles were likely in the region of the Gulf of Mexico affected by the oil spill, according to a new computer simulation. About three-quarters of these marine animals probably came from Mexican nesting populations, the research found. Others hailed from South America, Costa Rica and as far away as western Africa.

As a result, efforts to rehabilitate the environment after the spill should likely reach far beyond the Gulf Coast of the United States, said study researcher Nathan Putman, a biologist at the University of Miami. [See Images of the Aftermath of the Deepwater Horizon Oil Spill]

A serious spill

On April 20, 2010, an explosion rocked the Deepwater Horizon oil rig, which was operating on a BP-owned well in the Gulf of Mexico. Oil gushed from the well bore at the bottom of the gulf until July 15.

Research in the gulf has found possible long-term impacts on wildlife, including a high mortality rate and a low number of bottlenose dolphin calves in the region. But it was difficult to measure the wildlife impacts, Putman told Live Science, because of the challenge of determining how many animals were passing through at the time of the spill.

Oil can affect sea turtles by coating them with irritating petrochemicals, which can cause inflammation and even organ damage. Oil can also indirectly impact turtles by affecting animals lower in the food chain, making it harder for turtles to find food. Finally, oil slicks can kill the seaweed that tiny baby turtles use to camouflage themselves from predators. According to the National Wildlife Foundation, five times as many sea turtles strandings as usual occurred after the Deepwater Horizon spill. Still, strandings only hint at impacts that might be occurring far from shore, away from easy observation by humans.

"It was largely thought or accepted that there is no real good way to bracket the scope of the potential problem," Putman said.

He and his colleagues tackled the issue with a simple computer simulation based on ocean currents. They virtually "released" particles, representing turtles, into the Deepwater Horizon-affected region and then backtracked through five years of ocean-current data to see where the turtles would have come from. Depending on species, juvenile turtles spend between two and 10 years or so living in the open ocean, traveling largely with ocean currents. The researchers also took into account potential mortality rates among these traveling turtles.

Turtle impacts

The resulting simulation estimated that there were 175,064 green turtles (Chelonia mydas), 21,363 loggerhead turtles (Caretta caretta) and 3,693 Kemp's ridley turtles (Lepidochelys kempii) in the spill-affected area between April 2010 and August 2010. Serendipitously, Putman said, another research group has since released findings based on in-water estimates of turtles in that area in the years after the spill. The in-water estimates pegged the number of green turtles in the area at 154,000 and the number of loggerheads at 30,800, very close to the simulation's estimates. 

The real-world estimates, however, suggested there are typically around 217,000 Kemp's ridley turtles in that area, a big difference from the simulation's prediction of 3,693. Putman and his team adjusted their model to reflect the notion that Kemp's ridley turtles might swim against prevailing currents to get to and stay in the area of the gulf affected by the spill. A few simple tweaks brought the model and the real-world estimates in line.

What real-world estimates can't do is reveal where the turtles came from. That's where Putman's model comes in handy. The ocean-current data suggest that turtles in that area in the summer of 2010 likely came from Mexico: Between 43 and 63 percent of greens, 60 and 66 percent of loggerheads, and more than 99 percent of Kemp's ridleys were from Mexico populations, the researchers report today (Dec. 22) in the journal Biology Letters.

A third of the green turtles in the area likely hailed from Costa Rica, and as many as 16 percent may have come from Suriname in South America, the researchers found. About a third of the loggerheads probably came from the United States. Up to 4 percent of green turtles in the region may have come all the way from Guinea Bissau in West Africa.

To describe these different populations, Putman used the analogy of a bank account. If a bank loses $100,000, he said, it's important to know which accounts the money was withdrawn from. "It doesn't matter just that $100,000 got lost," he said.

There are limitations to the study, the researchers wrote, particularly in that sea turtles don't have to go with the flow when migrating. But scientists don't know the extent to which such deliberate swimming affects sea turtles' routes.

Despite uncertainties, the new simulations could help to inform policy, Putman said. Fishermen, for example, might need to lower their acceptable rate of accidental bycatch of turtles if the impacts of the spill turn out to be great. And the results show that efforts to monitor and repair turtle habitats should reach beyond the gulf, Putman said.

"Turtles aren't the only dispersive and migratory taxa," Putman said. "Hopefully, this will push people to consider other animals that might be transient through the gulf."

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