Using microphones submerged 3,000 feet below the surface of the Gulf of Mexico, scientists have learned that whale and dolphin populations were still declining 10 years after the Deepwater Horizon oil spill.

Some of these species were initially expected to fully recover within a decade.

The researchers analyzed deep-sea audio recordings collected between 2010 to 2020 at five sites across the Gulf of Mexico. They used an algorithm to help identify the vocalizations of toothed whales, their echolocation “clicks,” which can be attributed to specific whale species. Based on the number of clicks they captured on their recordings, researchers estimated whale populations.

In a paper published last month in the scientific journal, Nature Communications Earth and Environment, the team of scientists, led by researchers at UC San Diego’s Scripps Institution of Oceanography, report that sperm whale populations may have declined by as much as 31%. Certain dolphin populations may have dropped by 43%.

Beaked whales — a reclusive, deep-sea class of animals with snouts that look like dolphins — dropped in numbers by up to 83%. They were expected to have fully recovered within 10 years of the spill.

“The sperm whales, dolphins, beaked whales — we saw declines across the board,” said study’s lead author, Kaitlin Frasier, an oceanographer at Scripps.

“The preponderance of evidence … suggests that this is a large phenomenon that probably has a connection to the oil spill impacts,” Frasier said.

Lingering impacts

The Deepwater Horizon oil spill in 2010 was the largest ever, spewing some 134 million gallons of oil into the Gulf and creating an oil slick over 43,300 square miles, roughly the size of the state of Virginia. The rig explosion killed 11 people.

A post-spill scientific assessment estimated the disaster’s impacts on marine mammals, which was used to determine BP’s liability. In the 10 years following the spill, the oil company spent about $69 billion on cleanup. Some species of whales weren’t expected to fully recover in 10 years, but most whale populations were expected to at least start rebounding by then.

Frasier said that her team’s study shows that “there is no evidence” that the initial assessments were correct.

Her team provided “direct measurements indicating that we’ve had significant and widespread declines in these animals’ populations” since the spill, said Andrew Read, a professor of marine biology at Duke University who was not involved in the research.

Tina Yack, a research scientist with Duke’s Marine Geospatial Ecology Lab, who was also not involved with the study, noted that the study would only count whales that were vocalizing. 

“The authors were careful to note that relying on echolocation clicks means that silent or non-foraging animals are not accounted for, potentially underestimating the local population size,” she said.

‘Massive quieting’

Over the 10-year study period, Frasier said that funding for the project was at times difficult to maintain. They hitched rides out to research sites on shrimping boats, she said, and pieced together funding through various research grants.

It takes a “giant leap of faith” to place expensive microphones on the seafloor, according to Read. Each device costs about $20,000 to build, and takes $1,500 to maintain annually, Frasier said, and her team maintained five mics at the bottom of the Gulf for a decade.

When it comes time to recover the recording devices, the researchers use an underwater speaker to play a sound in the water — a code, which Frasier described as a series of beeping tones — that tells the submerged recording device to drop its iron weight and float up to the surface.

“Imagine walking through a tract of woods you’re familiar with the birdsongs,” Read said. “Then something happens. There’s a big storm, or a fire, and when you walk through the woods again, it’s quiet.”

“That’s what [Frasier’s] team has found,” he added. “There has been this massive quieting in parts of the Gulf of Mexico that were affected by the Deepwater Horizon spill, either because the animals have left or they’ve died.”