On April 20, 2010, an explosion at the BP Deepwater Horizon oil rig released over 130 million gallons of crude oil into the Gulf of Mexico. It was the biggest oil spill ever in U.S. waters and remains one of the worst environmental disasters in world history.

Eleven rig workers lost their lives. So did untold millions of marine mammals, sea turtles, birds, and fish. While the world watched, helpless, oil gushed into one of the planet’s most biodiverse marine habitats for 87 long days.

View Images Boats use absorbent booms to corral the Deepwater Horizon oil spill in May 2010. Photograph by Tyrone Turner, Nat Geo Image Collection

A decade later, many species, such as deep-sea coral, common loons, and spotted sea trout, are still struggling, their populations lower than before. By contrast, a few Gulf inhabitants have shown a robust recovery—among them, menhaden fish and the brown pelican, Louisiana’s state bird. (Read how the Gulf oil spill has harmed dolphins and turtles.)

Scientists say it’s still too early to tell definitively what the impact has been for longer-lived species such as dolphins, whales, and sea turtles.

View Images A loggerhead sea turtle feeds on an oil-contaminated Portuguese man-of-war in the Gulf of Mexico on May 5, 2010. Photograph by Carol Guzy, The Washington Post/Getty Images

Even so, “based on our science to date, if you were a marine mammal alive in the Gulf at the time of the spill, it doesn’t look good for you,” says Cynthia Smith, a veterinarian at the National Marine Mammal Foundation. “Animals that weren’t born yet, those are the hope,” says Smith, a marine mammal expert who traveled to the spill.

Smith is one of many scientists whose careers pivoted after this event. Funds from the Gulf of Mexico Energy Security Act, the Gulf of Mexico Research Initiative—and more recently, the $16 billion settlement between BP and the U.S. federal and state governments—have enabled a legion of researchers to undertake long-term projects investigating how the spill affected Gulf wildlife.

Many species have been difficult to study. But after a decade of close monitoring, Smith feels that she and colleagues have a clear picture of what is going on with that most gregarious of cetaceans, the bottlenose dolphin—and it’s grim.

About a thousand dolphins died in the months following the spill, after they ingested toxins from the oil. Many others apparently have been sick ever since. (Read about a die-off of baby dolphins in the Gulf.)

Recent research, not yet published in a peer-reviewed journal, has revealed that only about 20 percent of pregnancies among the dolphins in Louisiana’s heavily oiled Barataria Bay are successful, compared with 83 percent in unoiled regions. This number remains unchanged from 2015 findings.

Ten years out, Smith is also seeing higher rates of reproductive failure, lung disease, heart issues, impaired stress response, and death in bottlenose dolphins.

Interestingly, says Smith, these symptoms mirror the most common health issues faced by another large mammal exposed to the oil spill: humans. Two recent studies, both published in 2018, found impaired lung and heart function and strained breathing, respectively, among cleanup workers and U.S. Coast Guard personnel who had been in contact with the oil.

“You don't necessarily think of a dolphin as being representative of yourself or a human being representative of a dolphin, but our lives overlap,” Smith says. “We're in this space together, and there's a lot to learn from that.”

Listening for life

Kaitlin Frasier remembers the day in 2010 that her Ph.D. adviser told her he thought she should focus her career on the recent Deepwater Horizon spill.

At the time, Frasier, couldn’t have imagined where that journey would take her. Today, she’s an assistant project scientist at the Scripps Institution of Oceanography at the University of California, San Diego, and has spent the past decade listening for signs of life in the Gulf—namely, the clicks and clacks of echolocating marine mammals.

“We can’t really see the seafloor, so we don’t really know how [the oil] has affected whales,” Frasier says. It’s hard to tell, she says, whether or not oil from sediments is getting resuspended into the water and affecting cetaceans’ food. (Here’s why “shocking” amounts of oil fell to the seafloor.)

View Images Deepwater corals in the Gulf of Mexico, such as these bubblegum and bamboo corals, were well studied before the spill, giving scientists a better idea of how the oil harmed them. Photograph courtesy of the NOAA Office of Ocean Exploration and Research, Gulf of Mexico 2014.

The Gulf of Mexico is home to 21 species of marine mammal, most of which humans rarely see—so scientists have to listen. The sounds these animals emit can reveal which species are still active many years after the spill, and which have declined.

One species Frasier hears less and less these days is the pantropical spotted dolphin.

“It is a surprise in some ways,” Frasier says, “because they used to be so commonplace. The visual observers called them rats because they were crawling all over the Gulf. And now, we just get way fewer encounters on our acoustic data.”

For many species, results are not this clear. In part, that’s because scientists knew little about the habits of many deepwater marine mammals before the spill, so have trouble detecting changes from current data.

Take the little-studied dwarf sperm whale: It’s unclear how to interpret the short, high-pitched clicking sounds Frasier can associate with them now. Likewise, sperm whales, which emit longer, lower-frequency clicks, haven’t been detected recently near the spill site, but this may just mean they have moved.

Marine mammals are important indicators of the overall health of the ocean, so studying them can tell scientists a great deal about their environment.

“We have all these different pieces of the puzzle, but it’s hard to know how they fit together,” Frasier says.

View Images A Kemp's ridley sea turtle digs a nest on a beach in Rancho Nuevo, Mexico. This spill seems to have thwarted the critically endangered species' recovery. Photograph by Sandesh Kadur, Nature Picture Library

The silent behemoths

Some of the longest-lived animals of all sit silent and sessile at the bottom of the sea.

Peter Etnoyer, a marine biologist at the National Oceanic and Atmospheric Administration’s (NOAA) Hollings Lab, studies deep-sea corals. Some were thriving very near Deepwater Horizon’s wellhead before the spill, according to seafloor surveys. After the spill, scientists found that half of those coral colonies—colorful, fan-shaped creatures called gorgonian octocoral—surveyed had been injured to some extent.

“We don’t know how long it’s going to take these coral colonies to recover,” Etnoyer says. “They grow very, very slowly. The ones we found to be injured are on the order of decades to hundreds of years old.” (Learn how the Gulf oil spill was even bigger than thought.)

Corals are important habitat for species such as shrimp, crabs, grouper, and snapper. And because they exhibit growth rings like those of trees, corals act as “little environmental monitors, recording conditions over time,” Etnoyer says.

Now, his team is preparing for future disasters, mapping deep-sea corals and developing a coral database with more than 750,000 records so far. The team also has a seven-year plan to help coral rebound, which includes traveling to the seafloor using divers or a remotely operated vehicle and cloning or transplanting a few hundred coral from one spot to another.

“It’ll be the first time it has ever been attempted to transplant these specific corals at an industrial scale,” he says.

A setback for endangered turtles

The Gulf of Mexico is home to five species of sea turtle, all of which are protected under the Endangered Species Act.

Leatherbacks and Atlantic hawksbills roam offshore waters, while loggerhead, green, and Kemp’s ridley sea turtles frequent near-shore habitats. A 2017 study estimated that of at least 402,000 sea turtles exposed to oil during the spill, 51 percent were Kemp’s ridleys, the smallest and most critically endangered species.

Before the spill, the Kemp’s ridley population had been projected to grow at a rate of 19 percent per year. Instead, the number of nests on Gulf beaches—the species’ main nesting location—dropped 35 percent between 2009 and 2010, and plummeted again in 2013, according to a 2016 study. That research also suggested that Kemp’s ridley females have struggled to maintain the weight and health necessary to reproduce.

A new version of a federal recovery plan for the Kemp’s ridley was signed in 2014 in response to the spill. The move resulted in new protections for nesting beaches in Texas and Mexico, and requirements that shrimp fisheries in the Gulf use excluder devices to prevent the reptiles from being captured in trawls.

A bright spot for birds

Birds were among the hardest-hit animals immediately after the spill, says Erik Johnson, director of bird conservation for Audubon Louisiana.

“We know the number of birds affected was somewhere between 100,000 and one million. Unfortunately, we’ll never know the true number,” he says. (See photos of birds and other wildlife coated with oil.)

That statistic includes common loons, northern gannets, double-crested cormorants, royal terns, Wilson’s plovers, black skimmers, and seaside sparrows, to name a few. Also affected: Up to 32 percent of laughing gulls and up to a quarter of all brown pelicans. (Learn how nature can bounce back from an oil spill.)

Many birds that weren’t killed outright by the oil coating their feathers have since shown higher rates of oil-related cancers, reproductive issues, and a reduced ability to regulate their body temperatures due to feather damage, according to a 2020 study.

But just as birds overall were most devastated, in some cases they seem to be showing some of the strongest recovery. Settlement money was put to use restoring Louisiana’s Queen Bess Island as bird habitat. The project was completed this past February and is being hailed as a success for brown pelicans, with up to 20 percent of the state’s population already nesting there, along with great egrets, roseate spoonbills, royal terns, and tri-colored herons.

Oily fish

What was a bust for birds turned into a temporary boon for some fish: Scientists think that the lack of birds in the skies over the Gulf of Mexico is one reason some populations of fish exploded after the spill.

There were twice as many Gulf menhaden, for example, in the years following the spill as in four decades before, likely because so many fish-eating birds were absent.

Other fish species have shown evidence of having been harmed by oil, including nearly two thirds of all Gulf sturgeon, a threatened species. Studies of the economically valuable spotted seatrout and red drum found that fish in oiled areas showed reduced reproduction, and that even years after the spill, oil remaining in the environment is still toxic to fish larvae. (Read how some fish deformities have been linked to the spill.)

Recent research that tested 2,500 different fish across the Gulf found evidence of oil exposure in all 91 species sampled, suggesting that the impacts of the spill are widespread and ongoing.

Looking ahead

It could take decades to understand how oil affects the next generation of whales, coral, sea turtles, birds, fish, and more.

For Smith, Frasier, Etnoyer, and others involved in spill research, this event has become career-encompassing. Their research will be devoted to monitoring and understanding the Gulf for many years to come—particularly if these ecosystems remain vulnerable.

Meanwhile, Kaitlin Frasier will remain at her desk, listening for the chirping sounds of Risso’s dolphins and the long, low vocalizations of sperm whales.