Under a sky glowing orange from the dawn sun, Martin Grosell gunned his twin-engine sportfishing boat toward the Gulf Stream one morning in August, in search of one of his and South Florida’s favorite fish: mahi.
On board, sprawled on a beanbag, was one of his best anglers, his youngest daughter Camilla, 12.
“She's born and raised doing this and she's caught a lot more fish than most in South Florida,” Grosell said, then admits: ”Most of the time, it's actually her telling me what to do.”
Grosell is an ichthyologist — a marine biologist who studies fish — at the University of Miami Rosenstiel School where he specializes in mahi. As one of the principal investigators for a research project studying lasting impacts from the 2010 Deepwater Horizon oil explosion in the Gulf of Mexico, his lab looked at how the massive spill harmed two of the state’s most popular trophy fish - bluefin tuna and mahi.
READ MORE: As seas get hotter, South Florida gets slammed by an ocean heat wave
As part of that work, the team found that warmer water can slow mahi, a fish known for its speed. And that suggests a bigger threat may be looming: warming oceans heated by climate change.
Already, anglers are seeing changes. During last summer’s heat wave, Grosell found mahi hiding far below the surface in cooler waters. During their August outing, the Gulf Stream hovered just above a steamy 88 degrees.
“This is like when you get a heat advisory, and we’ve had a few of those this summer. That's what this is like for mahi,” he said. “And we know this because we've done experiments on them at these temperatures. And they swim very poorly. They're not great at acquiring the oxygen they need. It's right up against the wall.”
Researchers have also documented other fish moving poleward — away from the steamy equator — and that could have cascading implications. An that looked at 150 species fleeing to cooler waters left dramatic population declines in their wake.
“This is a shift away from an equilibrium that's been established over the millennia and anything like that is always cause for concern,” Grosell said.
A by the National Oceanic and Atmospheric Administration echoes those concerns. Based on more than 35 years of catch data, the study for the first time looked at population counts in the western Atlantic by location and season. After decades of relative stability, the study found the mahi population in decline for the first time beginning in 2019.
A closer look revealed what might be fingerprints from climate change.
“We saw that in many places like Florida and the Caribbean, where we've heard anecdotal reports of there being a lot fewer dolphin fish in the last several years, there was a declining trend overall,” said Matt Damiano, a NOAA fisheries biologist and the study’s lead author. But as he looked further north along the Atlantic coast, the declines disappeared.
“What we suspect is happening is this interaction between the effects of fishing and the changing ocean environment,” he said.
But teasing out that interaction is difficult for a fish that covers such a broad territory, he said. Pressure from overfishing can exacerbate harm from warmer waters, and vice versa, but more data is needed to untangle how they play off one another.
“Are [mahi] moving up or down the coastline to get to waters that are more preferable?” Damiano said. “All we can really say about that is we know they like to stay within that thermal band, that narrow Goldilocks zone.”
For “In Hot Water,’ an episode of the podcast Sea Change produced by WWNO New Orleans Public Radio and WRKF Baton Rouge Public Radio and distributed by NPR, WLRN took a closer look at what these impacts could mean: to the fish themselves and to the people who rely on them. While reefs and sea rise deservedly get a lot of attention, scientists are just beginning to untangle what warming seas could mean for life in these deeper waters.
“Not all animals are going to take these geographical shifts, and they're not going to take them at the same rate,” Grosell said. “So it’s introducing new species into existing ecosystems, or you're removing species from existing ecosystems. Those imbalances can have severe impacts and it’s going to be very difficult to predict what’s going to happen.”
Rising, warmer waters in the Gulf
In the Gulf of Mexico in particular, where waters are rising at of the global average, impacts could be dramatic.
Today, the Gulf of Mexico is nearly 2 degrees warmer than it was a half century ago. Mahi in the Gulf prefer water between 77 degrees and about 84 degrees. But summer temperature now average just over 85 degrees. Just a degree higher and it’s hot enough to be dangerous, even lethal, for mahi. So this has scientists, and not just Grosell, alarmed.
“We saw that firsthand as anglers that the mahi behavior was clearly different because of this high temperature. That would definitely, definitely change behavior in these animals,” he said.
And while they could hide in deeper water, like they did during the heat wave, if that becomes a regular shift, that could have consequences, Grosell said.
“The problem is now they are entering a different environment where there are different predatory pressures,” he said. “So it is going to change their circumstances and change their foundation for life.”
While Grosell’s research focused on the Gulf of Mexico, mahi on the Atlantic coast that hunt in the Gulf Stream are also experiencing the hotter water. Over the last two decades, than the global average, rising 2 degrees Fahrenheit.
If mahi flee waters around South Florida, that could be a big hit to a saltwater fishing industry that generates up to $16 billion a year. In a state where sport fishing is followed with the fervor of a religion, mahi are one of its patron saints, renowned for both their speed and delicious, flaky meat.
“If you got in your car and drove from Key West to Arizona right now, you would find mahi on every single menu, at Costco, BJ's, everywhere,” said Jon Reynolds, a lanky charter boat captain based in the Upper Keys. Reynolds sits on the Dolphin Wahoo Advisory Panel for the South Atlantic Fishery Management Council that sets the nation’s fishing rules from Florida to North Carolina. Mahi are also known as dolphinfish.
But fishing regulations have failed to keep up with the climbing demand or recognize the peril faced by the fish from climate change, he said. Not only are there fewer mahi, but Reynolds said it's becoming hard to find big mahi.
“A year-old fish is 20 to 40 pounds and you only catch a couple of those a year now, maybe,” he said. “Every single fisherman is screaming at the top of their lungs that something needs to be done. Yet management is still acting on un-accordingly.”
With mounting research, Reynolds said fishery managers need to act fast.
“Everyone knows that we have a serious problem and the data shows it,” he said. “They used to say it was anecdotal. But the data shows it.”
During their day fishing, Grosell and Camilla caught just two fish. One, a schoolie, was too small to keep. But a second landed in the ice chest, bound for the dinner table.
You can listen to “In Hot Water,” at or visit NPR’s . Or, click on the 'listen' button at the top of the page.
*An earlier version of this story misspelled Martin Grosell. We regret the error.
