USF researchers sampled more than 2,000 fish in the Gulf of Mexico. They found oil pollution in every one.

In the decade since the Deepwater Horizon oil spill, researchers from the University of South Florida have circled the Gulf of Mexico, catching fish and cutting them open in search of toxic pollution.

They found that the gulf is a “greasy place,” said Steve Murawski, a USF fisheries biologist. All of the 2,503 fish they studied showed traces of oil exposure, not to the level of being unsafe to eat but enough to raise questions about species’ long-term health, according to the study just published in Nature Scientific Reports.

The scientists looked for evidence of toxic hydrocarbons — compounds found in crude oil — and did not connect their results to the disaster specifically. Oil leaks into the gulf for many reasons, from natural seeps to river runoff and boating discharges. That makes it nearly impossible to track pollution in fish to a specific cause. But some findings suggest the spill had an effect, Murawski said.

Concentrations of oil pollution were highest in fish around the north central gulf, the researchers found, and levels there were elevated soon after the Deepwater Horizon disaster. Murawski estimated the discharge of oil after the rig sunk was seven times greater than the yearly leakage from natural seeps — in just 87 days — providing a possible explanation for the higher levels.

“What we saw was from that event and not natural chronic exposure,” Murawski said.

The Deepwater Horizon oil rig exploded April 20, 2010, killing 11 people, and then sank about 50 miles offshore from Louisiana. Oil began gushing nearly a mile down in the gulf. It continued spewing for months, until as many as 210 million gallons had been spilled. The disaster devastated the tourism and fishing industries, pillars of the gulf economy. Tar, floating in mats and balls, washed up in Florida, where officials received a $3.25 billion settlement for the economic and natural losses from oil operator BP.

A decade of research followed, unprecedented in the gulf, where funding had lagged before the catastrophe raised inescapable questions about oil’s long-term effects on the ecosystem. At USF, Murawski has led an international research consortium called the Center for Integrated Analysis and Modeling of Gulf Ecosystems, C-IMAGE.

The research extended well beyond fish, stretching to Mexico and near Cuba, covering sediment samples and other wildlife and historic trends. The latest effort, unveiled in a paper Wednesday, honed in on signs of pollution in 91 species of fish, from red snapper to leopard toadfish. The authors found a hot spot for oil pollution in fish off Tampa Bay, they wrote, which they attributed not to seeps or a spill but runoff from land and boat traffic.

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Scientists searched specifically for evidence of toxic compounds from oil called polycyclic aromatic hydrocarbons, or PAHs. Bile is part of the digestive process that helps remove waste, later released through urine or fecal matter, said Erin Pulster, lead author of the study. It can show recent exposure to oil.

Before the USF project, Pulster said, there was almost no research on oil’s effect on fish. The thinking, she said, had always been that the animals were adept at metabolizing toxins so there was little worry about compounds accumulating in their bodies.

“Fish were largely ignored,” she said.

The paper raises concerns about repeated exposure, and the long-term effects of bile that lingers in a fish’s system. Murawski drew a parallel with humans who metabolize alcohol upon a night of drinking. A few cocktails will be cleared fairly quickly, but alcoholics might suffer cirrhosis and a failing liver over the long term.

“If there’s repeated pollution events, the system in the liver breaks down,” he said. “At what point do chronic exposures really undermine the systems of the animal?”

Over the course of an entire population, he said, that could mean diminished productivity, including for fish like red snapper, crucial to the gulf’s economic engine.

Fish are exposed to hydrocarbons in a few ways, according to the scientists. Eating is the primary method, with bottom-feeders sucking up crabs and other food where oil has settled.

After Deepwater Horizon, settling happened in a process the researchers relate to snow, with oil droplets adhering to dead plankton and sediment from the Mississippi River that sinks to the seafloor. Like a snowglobe, they said, this layer gets stirred up by storms and severe weather and distributed back into the water column — a process they think might have happened and caused a surprising uptick in pollution in fish they observed in 2017.

Fish can also draw toxins through their gills by swimming through suspended oil, a method behind one working theory for the researchers’ biggest surprise: that the most contaminated species was yellowfin tuna. Tuna are known to swim in the open sea rather than burrowing along the gulf floor like golden tilefish, another species with more expected high contamination. Murawaski thinks a second reason is the likely culprit — that the tuna are found closer to rigs, where oil operators discharge water that may have moderate levels of oil pollution.

Murawski and Pulster say their findings point to the need for more monitoring and research. The biggest obstacle, they said, was a lack of data before Deepwater Horizon. The funding of the last decade enabled them to set baselines that didn’t previously exist, but the funding ends this year, and it remains uncertain whether they’ll receive future support. Their hope, they said, is the industry will pay for research or regulators will demand more reporting and testing from oil companies.

“There’s more of a culture of safety in the oil industry because nobody wants to be the next Deepwater Horizon,” Murawski said. “I don’t think we’re going to see an 87-day spill again.”

But he views another deep-water blowout as inevitable. Oil companies, Murawski said, are pushing into deeper and deeper wells.

A longstanding moratorium on drilling off Florida is set to expire in 2022, but Sens. Rick Scott and Marco Rubio have said they want a 10-year extension of the policy. Murawski said operators are signaling their own desire by securing leases closer to the moratorium line, to the south and east of the Deepwater Horizon site.

Correction: This story previously provided an incorrect distance for how far the Deepwater Horizon rig was from the Louisiana coast.