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Green fish with boat behind

The Deepwater Horizon Oil Spill’s Hidden Impacts on Mahi-Mahi      

Mahi-mahi were more likely to be eaten and less likely to spawn after being exposed to sublethal concentrations of oil, raising concerns about the risks oceanic drilling pose to life in the ocean.

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Natalia Mesa

Natalia Mesa was previously an intern at The Scientist and now freelances. She has a PhD in neuroscience from the University of Washington and a bachelor’s in biological sciences from Cornell University.

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ABOVE: © iStock.com, FtLaudGirl

On April 20, 2010, the Deepwater Horizon oil rig exploded, dumping 134 million gallons of crude oil into the Gulf of Mexico over the course of 87 days. The spill has been called one of the worst environmental disasters in history, and is estimated to be responsible for the deaths of thousands of marine mammals and sea turtles, just under 1 million coastal birds, and probably millions of fish—though even now, more than a decade later, scientists are still unpacking the spill’s catastrophic impacts. 

“It’s important to understand the true costs of an oil spill in terms of ecological damage so that we can appropriately weigh the decision of drilling in new locations,” says Lela Schlenker, a marine biologist at the Coastal Studies Institute at East Carolina University. At the time of the spill, oil and gas company BP was leasing the Deepwater Horizon oil rig from Transocean. 

To that end, Schlenker and colleagues’ research, published earlier this month (September 2) in Environmental Science and Technology, finds that even fish exposed to sublethal concentrations of oil are more likely to die within eight days of exposure than unexposed fish, and may be less likely to spawn for weeks after—results which suggest the spill’s effects on the Gulf ecosystem could have been greater than previously appreciated. 

Boat 
The research vessel used to perform experiments on mahi-mahi
RECOVER Consortium

In previous laboratory work, scientists found that oil exposure impacts the development and health of fish. But scientists aren’t sure how these observed effects translate to fitness in the wild, since such studies haven’t had measures of how fish were faring before oil exposure. The new study, which involved catching mahi-mahi (Coryphaena hippurus), briefly exposing them to oil, and monitoring their behavior after releasing them back into the wild, helps to fill that knowledge gap, says Victoria McGruer, an environmental scientist at the University of California, Riverside who was not involved in the work but knows some of the authors professionally.

Schlenker says that the team was interested in understanding how exposure to low concentrations of oil impacts fitness in wild populations of mahi-mahi, an apex predator in the Gulf of Mexico and a key ecological species. Not only are mahi-mahi a popular commercial fish fished for consumption, they’re also a popular research species due to the relative ease at which researchers can raise them in the lab.            

The study was more than several years in the making, says Schlenker. “The only way we were able to pull this off was years of planning. . . . [And] it wouldn’t have been possible without the collaboration of a lot of different scientists.” 

After a lot of planning and an initial trial mission, in 2019, Schlenker and a team of scientists and anglers set off on a research boat for 16 days to do their experiment. Every day, from dawn until dusk, they fished for mahi-mahi with the goal of catching at least four per day. Once caught, the fish spent fourteen hours in a tank, which was filled with seawater. For about half of the fish, the researchers also added a low concentration of oil to the tank. Then, the scientists attached a satellite tag called a Pop-Up Satellite Archival Tag (PSAT) to the back fin of each fish and released them back into the Gulf of Mexico within 40 km of the Deepwater Horizon site. Over the course of the whole experiment, the team caught 50 fish and exposed 24 to oil.  

The PSATs recorded temperature and depth information every 75 seconds as well as light levels twice per day to estimate the migrations of the tagged fish. The scientists also modified the tags to record acceleration data, which allowed them estimate how often the fish spawned (mahi-mahi alter their swimming patterns when they spawn). The tags stored all of this information, then, when they detached from the fish, beamed the data from wherever in the world they ended up to satellites they encountered. The tags were designed to pop off after 96 days, but all popped off before 37 days due to unknown reasons or because the tagged animal was eaten. (Schlenker says when the tag recorded total darkness for a time, the tag and the mahi-mahi it was attached to were likely stuck inside a predator’s stomach). 

Overall, the data revealed that in the first eight days, oil-exposed fish were more likely to die than control fish, most often by being consumed by a predator. After eight days, the oil-exposed group bounced back to their regular survival rate, and the scientists didn’t see any other decreases in survival. But based on the acceleration data, the oil-exposed fish were less likely to spawn throughout the rest of time the researchers tracked the group. The control fish altogether as a group spawned six times total over the course of the whole experiment. The oil-exposed fish, on the other hand, didn’t spawn at all. 

two green fish in a blue tank
Two tagged mahi-mahi in a research tank
RECOVER Consortium

The authors also observed that oil-exposed fish behaved oddly. “They [were] not behaving like normal fish,” says Schlenker. For instance, the oil-exposed fish performed higher numbers of rapid acceleration bursts than their unexposed counterparts, indicating that they may have become hyperactive for a brief period of time. Schlenker says this may be due to damage to their central nervous systems, which she says she and other scientists have observed after oil exposure in a laboratory setting. One of the oil-exposed fish also migrated all the way to the Atlantic Ocean, indicating that oil spills could even impact the fitness of fish in faraway regions. 

”We don’t really know how long it takes for fish to begin spawning normally again,” says Schlenker. “Understanding that is another component of assessing the damage from both this past oil spill and future oil spills. Obviously, it’s going to have impacts on the whole population.”

“I was really excited to see this paper,” says McGruer. “My background in this area is more in the lab doing controlled experiments,” she continues, “and our goal is always to bring those findings as closely as possible to what is happening to wild populations. This paper really takes a lot of that lab-based research and tries to understand how it applies in the field.”

“These companies are continuing to push for deeper and continually more risky drilling operations. It’s important to understand the consequences of a spill like this and appropriately fine these companies when they make mistakes and hopefully prevent these things from happening in the future,” says Schlenker, because “until we completely switch away from oil drilling, this will happen again.” Offshore drilling efforts have recently increased and energy companies are pouring billions into such projects.

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