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'Massive numbers': New Gulf oil spill study finds even deadlier impact on one of Florida's most popular fish

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National Oceanic and Atmospheric Administration
A new study has found mahi mahi, or dolphinfish, can have their survival rate cut in half up to eight days after being exposed to low levels of oil.

More than a decade after BP’s Deepwater Horizon drilling rig exploded into a lethal inferno that killed 11 and spilled more than 3 million barrels of oil in the Gulf of Mexico, researchers piecing together its lasting impacts have found more profound damage than previously known — to one of the Gulf’s most important fish.

Testing wild mahi mahi, the team found for the first time that even low amounts of oil can cut survival rates in half within a week of exposure. The fish also stopped spawning for at least a month.

“Those are massive numbers,” said Martin Grossell, lead principal investigator for one of 12 research groups funded by the BP’s Gulf of Mexico Research Initiative and a professor at the University of Miami Rosenstiel School.

The findings were first published in September in the journal Environmental Science and Technology.

In previous experiments, Grossell’s lab confirmed low levels of oil can damage the hearts, hearing and vision of young lab-bred mahi, impairing their fitness. The field work, done over three weeks in the northern Gulf of Mexico, now confirms that damage can be deadly, he said.

“It will lead to mortality in the wild where fish have to compete for resources and avoid predation,” he said. “So it's a tougher life out there than it is in the lab.”

For drilling opponents, these findings and others provide more evidence to end Gulf oil exploration. The Biden administration has proposed expanding it and is now taking public comment on a new lease.

“These findings about the mahi are really no surprise,” said Catherine Uden, the South Florida field representative for the nonprofit conservation group Oceana. “It just goes to show that there are so many long lasting effects from fish to sea turtles to dolphins, and all of the impacts to the environment.”

The group has joined a broad coalition of environmental groups and scientists calling for the administration to end Gulf drilling under the proposal.

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Martin Grossell
University of Miami Rosenstiel School of Marine, Atmospheric and Earth Science
These mahi were exposed to oil. Satellite trackers were then attached to their backs to record their location and speed, along with water temperature and depth every 75 seconds.

When BP’s Macondo well blew, it became the largest marine spill in U.S. history, spewing oil across more than 1,300 miles of coastline and a gulf prized for its bountiful fishing and stunningly beautiful mangrove forests, barrier islands and estuaries. Along with whales, dolphins, sea turtles and sea birds, scientists estimate between 2 and 5 trillion fish were killed.

In the aftermath, BP poured billions into settlements to stop legal battles. That included a $17 billion payout to the government that has helped fund research work like the mahi project.

Knowing the conditions are likely becoming more dire with climate change warming waters, scientists have been working to connect dots and better describe damage. In the northern Gulf, waters are expected to become about 3.6 degrees Fahrenheit warmer than in the 1990s.

“With environmental disasters like this, what we ultimately care about is ecosystem function,” Grossell said. “We want to know whether the fisheries in the Gulf of Mexico can still sustain communities that rely on fishing. We want to know if oyster harvesting, for example, is going to be sustainable. We want to know if the marine mammals are going to be okay for the decades to come after the spill. Those are the things we care about.”

A complicated research project

But the work can be complicated and expensive.

For this project, Lela Schlenker, the study’s lead author and a postdoctoral student working in Grossell’s lab, suggested catching 50 wild mahi, then testing the mahi aboard UM’s FG Walton Smith research vessel. Half the fish would be held in tanks with oil-tainted water that mimicked the Deepwater spill. The other half would go into untainted ocean water. They’d then be tagged with satellite trackers and released.

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University of Miami Rosenstiel School of Marine, Atmospheric and Earth Science
For the experiment, researchers exposed mahi to oil in tanks aboard a research vessel in the Gulf of Mexico.

That required heading into the northern Gulf and fishing for three weeks straight.

To catch the fish, Grossell piloted a faster fishing boat, scouted out the fish and then radioed the location to the lumbering 96-foot research vessel.

The mahi all needed to be a similar size. And because mahi do so poorly out of the water, they needed to be transferred quickly to the tanks. So the team rigged up a hand-off: once the mahi were hooked on the fishing boat, the line would be transferred to rods on the research vessel to be reeled in.

To do that, the team on the research vessel would attach their line to a tennis ball and toss it to the fishing boat. The fishing crew would then detach the leader on the line with the hooked mahi and clip it to the line to be reeled in to the research vessel, where the mahi would be loaded into a slings and moved into the tank.

“It sounds easy when I describe it, but it is not,” Grossell said. “It is not.”

Fighting winds and waves, anglers would get tangled in the line, he said. The leader would break or the clip would come undone and the fish would swim off. There were a lot of misses with the tennis ball.

“We got very good at it at the end, but it was a very steep learning curve,” he said.

The satellite trackers that recorded location, water depth and temperature and fish speed allowed the researchers to see what happened once fish were back in the wild.

While survivability and spawning topped the findings, the team also also documented new information about movement, with oil-exposed mahi behaving differently. The findings suggest impacts could extend to generations.

Research has also found that impacts from oil are greater in warmer water, a growing risk from climate change. “So a spill 20 years from now is going to be much more impactful than the one we saw in 2010,” Grossell said. “The bottom line is those consequences are resulting from the oil spill. That's the bottom line.”

Jenny Staletovich has been a journalist working in Florida for nearly 20 years.