Eight miles off the coast of Virginia Key, Martin Grosell and his crew spot the telltale sign they had been looking for—a thick patch of sargassum seaweed floating on the surface. “That’s where we’ll find them,” said the University of Miami marine biologist, steering the 36-foot twin-engine Yellowfin boat toward the bushy brown algae.

It is early March, and Grosell and two other UM scientists are plying the waters of the Atlantic, fishing for mahi-mahi.

Swift and acrobatic, the ray-finned pelagic fish are among the fastest swimmers in the ocean, using their spectacular speed—they’ve been clocked at 57 miles per hour—to migrate, hunt, and escape predators.

But what would happen if something made these commercially and ecologically important species sick? “They’d have all sorts of problems,” said Grosell. “It’s a sure thing, like death and taxes.”

Grosell, a professor of marine biology and ecology at the Rosenstiel School of Marine and Atmospheric Science, speaks from firsthand knowledge. He and his team have exposed mahi-mahi in the lab to oil from the Deepwater Horizon spill, finding that the fish, when placed in a swim tunnel, didn’t swim nearly as fast as those that hadn’t been exposed to the crude—in fact, as much as 37 percent slower in one test group.

Slower swimming speed can spell disaster for mahi-mahi in the open ocean. Suddenly, they’d be vulnerable to attack from predators.

Grosell and his team knew the oil was slowing them down somehow. But was it affecting their gills or their hearts? “We looked at the gills, and they looked fine,” he explained. “So, we suspected the problem had to be in their cardiovascular system.”

To test their theory, postdoctoral associate Rachael Heuer extracted heart cells from mahi-mahi held in captivity at the Rosenstiel School’s Experimental Hatchery, exposing a control group of cells to a nutrient-rich fluid that is always found in the fish’s blood and then exposing an experimental group of cells to the same fluid, but with oil mixed in.

What they discovered is that the two groups of cells behaved quite differently, with the cells exposed to oil contracting much less.

“We could see the marked reduction in swim performance, but we wanted to know why it was happening,” Heuer said. “By looking at isolated heart cells, we were able to zero in on the mechanism.”

Long before the Macondo blowout, scientists knew that oil could cause heart defects in certain marine species, specifically fish in the early stages of embryonic development. And that’s one of the reasons the Deepwater Horizon accident was so alarming to marine biologists. Much of the spilled oil rose from the bottom to the surface at a time when many of the open-water species in the northern Gulf of Mexico were spawning, Grosell said.

“But we’ve subsequently shown that the heart defects also occur to fish in latter-life stages—that very large, robust fish are also sensitive to even low levels of brief oil exposure,” Grosell said.

The sensory abilities and central nervous systems of the mahi-mahi also diminished, his research found. And the fish tended to make poor decisions, which could spell disaster in the wild where they would be vulnerable to bigger predators.

His results are lab-based, culled from observations and findings in a controlled setting. “The big question now is what does it mean for populations in the natural environment?” he said.

Expeditions like his recent mahi-mahi capture trip in the Atlantic will help find out. During the early March outing, Grosell and two fellow scientists—John Stieglitz and Carlos Tudela—went after broodstock in the Atlantic for breeding purposes. But this June, they will sail to the Gulf of Mexico aboard the Rosenstiel School’s 96-foot-long F.G. Walton Smith to capture 50 mahi-mahi in all, half of which will be exposed to oil, outfitted with satellite archival tags, and released back into the Gulf. The other 25 will be tagged and released without being exposed to oil.

The satellite tags will detach after 90 days and rise to the surface, transmitting data on everything from how far the fish swim each day to how deep they dive.

“I think the effects we see in the lab will be magnified in the wild,” Grosell said.

He’s been studying the impact of oil on pelagic fish like mahi-mahi ever since the Deepwater Horizon disaster nine years ago.

“At first we worked with NOAA (the National Oceanic and Atmospheric Administration) as part of the Natural Resource Damage Assessment process, the purpose of which was to build litigation against BP.”

Now that the case has settled out of court, Grosell has conducted the bulk of his research with funding from the Gulf of Mexico Research Initiative, helping the scientific community gain further insight into what happened during and after the accident, with the far-reaching goal of better preparing for future spills.

“The GoMRI funding has been a welcome opportunity,” said Grosell. “It’s money provided by BP, but it is managed completely independent of BP. It’s managed as research grants, not contracts. And what that means is we study what we’ve identified as important scientific questions with the understanding that these questions may evolve as we discover new things.”

During his ongoing research on how oil affects the health of mahi-mahi, he’s learned previously unknown facts about the species, discovering their migratory routes and that they sometimes swim up to 62 miles a day.

“In the wake of an environmental disaster, is there a silver lining? The answer is yes,” said Grosell, noting new information he’s learned about mahi-mahi. “Of course, Deepwater Horizon was a disaster. Whether we like it or not, it happened. But a lot has come out of it after the fact—a huge increase in our understanding of the ecosystem and the organisms in that environment.”