SETH BORENSTEIN WASHINGTON — In a deep, dark part of the Gulf of Mexico, the number of marine animals plummeted by about 60% in the years following the 2010 Deepwater Horizon explosion , a new study found.

In a crucial part of the Gulf between the light surface and the darkest depths, where the BP-operated well failed and dumped 134 million gallons of oil , the number of fish found dropped by 63.5% from the year after the spill to 2015. Crustaceans, including shrimp and krill, a key food for whales, went down 62%, and larger animals like squid and octopuses fell 59.7%, according to a study published Monday in the journal PNAS. And those populations haven’t come close to recovering, the study found. Researchers focused on the ocean’s “twilight zone,” a vast area above the seafloor, where much of the surface marine life, including dolphins and whales, feast on creatures.

Yet little is known about this gigantic ecosystem, and this region of the Gulf had not been studied before the 2010 spill, said study lead author Tracey Sutton, a biological oceanographer at Nova Southeastern University.

A 2011 post-spill exploration cited in the research, which used six sets of nets at different depths and acoustic sonar, gave scientists a good sense of how much sea life was in the area between the surface and 5,000 feet deep (1,500 meters). They returned to the same place with the same equipment three-and-a-half years later and found sea life was between one-third and one-quarter of what it was in 2011.

“We saw this big decline,” Sutton said. “The question was then why? So we looked at all of the natural variables we thought could account for it, namely temperature, oxygen supply and food. None of those vary with the decline. The event that did was the oil spill. So we think that’s the most likely cause of the decline.”Researchers kept on sampling since 2015 and the low populations remained, with a slight rise in 2022 and 2023 that did not prove statistically significant or lasting, Sutton said.

Sutton’s team documented the population declines with three different lines of evidence: what they caught in the nets, what their acoustic sonar found and genetic studies showing the species losing genetic diversity because of inbreeding.

A lot of marine life died instantly after the explosion, which killed 11 workers, and from nearly five months of leaking oil and gas. Those pollutants got stored in the eggs of survivors and subsequent generations of critters got more compromised and weaker, Sutton said.

BP spokesperson Paul Takahashi said the accident “forever changed” the company: “We have made many changes intended to help prevent an event like this from ever happening again.”Both the oil and the dispersants used to clean the spill hurt fish, but in different ways, said study co-author Isabel Romero, a chemical oceanographer at the University of South Carolina. Oil is more toxic and an overall threat, but the dispersant breaks the oil into smaller droplets, increasing the exposure of animals that live in the deeper water column, she said.

Dispersants caused many problems, and new studies show that in future accidents, it would be better to avoid those chemicals and instead add agents that increase oil-munching microbes, said former National Science Foundation Director Rita Colwell, a University of Maryland environmental microbiologist who headed a 10-year, $500 million BP-funded program to study spill effects.

Sutton said his team couldn’t prove an exact causal link to the BP oil spill, but by eliminating the other possibilities and looking at the timing, it’s logically clear that the spill caused the problem. The krill population dropped 72%.

Lanternfish — odd fish with blue-green lights embedded over their bodies — are a third of what they used to be, Sutton said. The stunning but scary dragonfish — with sharp teeth, light organs and a lure attached to its head — also had big losses, he said.

“These are foundational species,” Sutton said. “They’re the things most things eat. So it’s no wonder that we had wholesale changes across the community.”Scientists learned much about this unexplored area of the Gulf.

“One of our biggest findings from the whole thing is how connected the whole oceanic Gulf is from top to bottom because the animals move up and down,” Sutton said. “There are predators at the top that dive deep. Lots of things that are deep that come up at night. So the system is one very large pulsing system, rather than just a layered system like people might have thought beforehand.”Outside scientists were especially bothered by where the spill damaged.

“We know more about space in some ways than we do about the inner ocean. Because we can’t see these regions, there is a tendency to assume everything is fine, but here we have a clear result showing that something went very wrong,” Scripps Institution of Oceanography marine scientist Kaitlin Frasier said in an Frasier, who wasn’t part of the study released Monday, published a post-spill study in 2024 looking at Gulf of Mexico whales and dolphins, finding population drops as high as 83%.

“It’s shocking how a single failure in a drilling rig can have this vast and sustained impact over thousands of square kilometers, with little visible recovery over 13 years after the spill,” said Dalhousie University marine scientist Boris Worm, who wasn’t part of the study. “It is important that we see this, because for the longest time these consequences were completely out of sight and out of mind.”Worm and Colwell said this damage should be kept in mind as the world talks about deep-sea mining and increased oil drilling in deep waters.