Patrick Cassidy has reeled in all sorts of fish off Cape Cod over the last 35 years. But even the saltwater fly-fishing guide was surprised on a trip this summer, when he saw a flying fish — usually seen only in Southeastern and tropical waters  — pacing his boat through Nantucket Sound. 

“It was clear as day, and just kind of scooting along with the boat,” Cassidy said. To escape predators, flying fish accelerate while swimming, launch themselves above the surface, and fan their fins to glide over the water. “We’re running at a good speed, and this thing is flying along beside us.” 

Other fishermen have also caught sight of tropical fish off the South Coast this year. Todd MacGregor, a sport fishing guide and commercial rod-and-reel fishing captain out of Fairhaven, said his crew got tangled up with a school of marlin a few weeks ago while fishing for tuna off West Atlantis Canyon, roughly 100 miles south of Martha’s Vineyard. 

Marlins are among the fish making recent appearances in South Coast waters. Credit: National Oceanic and Atmospheric Administration

“I’m talking a group of 200- to 400-pound blue marlin,” he said. “We weren’t even prepared for those things. In 75-degree water.”

The appearance of these marine animals in local waters — alongside reports of tarpon and other species — has many residents wondering what’s bringing tropical fish to the South Coast. Some speculate that the appearances of these mid-Atlantic marine species and warmer area waters are the results of large-scale ocean warming, a product of the growing amount of greenhouse gases in the atmosphere.

In addition to marlins, flying fish have also been observed in local waters. Credit: National Oceanic and Atmospheric Administration

For instance, a marine heat wave in the Atlantic Ocean is currently producing hot-tub-level water temperatures off the coast of Florida. And scientists say the Northwest Atlantic has been heating faster than almost any body of water on Earth over the last 25 years.  

But local fishermen say incremental warming in ocean waters is not driving the appearance of tropical fish off coastal Massachusetts, which has happened in decades past. And scientists say the flying fish and marlin are not here because of ocean warming, so much as they are getting swept up in a rare seasonal phenomenon called “warm core rings” — rotating blobs of tropical water that are forming more frequently as the Northeastern climate changes. 

Glen Gawarkiewicz, an oceanographer at Woods Hole Oceanographic Institution in Falmouth, said an increased number of warm core rings are hitting the continental shelf in the warm months of the year. He sees the increase as a warning of the accelerating impacts of climate change.

He added that major wind currents are slowing in response to atmospheric warming, which may cause these pockets of tropical water and extreme weather events to persist longer across the Northeast. 

While water temperatures are also warming in Buzzards Bay, local experts say this gradual change is less connected to the ocean, and more to seasonal weather conditions. Still, they say rising spring and summer temperatures are leading to declines in water quality, and altering the ranges of fish and shellfish.

“We have to work together in the very near future to address these issues,” Gawarkiewicz said. “It’s really evident we’re going into a different set of climatic conditions.”

Warm core rings 

Warm core rings break off of the Gulf Stream, which is a major deepwater Atlantic Ocean current. The Gulf Stream, driven mostly by wind patterns and fluctuations in water temperature, originates in the Gulf of Mexico. It pushes warm water up the eastern coastline of the United States, accelerating before pivoting toward Europe roughly halfway up. 

The Gulf Stream normally forms warm core rings as a way to release energy when it gets too narrow or too fast as it moves up the coastline. These swirling eddies then drift toward the edge of the continental shelf, moving clockwise and westward toward coastal waters, where they can persist for months. 

Graphic: Kellen Riell / The New Bedford Light, OpenStreetMap, Datawrapper

UMass-Dartmouth and Woods Hole Oceanographic Institution research shows that prior to 2000, the East Coast experienced an average of 18 warm core rings annually. Yet from 2000 on, an average of 33 warm core rings formed. 

In June 2021, eight warm core rings circulated in northeastern U.S. waters. “I had never seen that many before,” Gawarkiewicz said. He said that at least four warm core rings have appeared in these waters so far this year. One got stuck in the underwater canyons 100 miles south of Martha’s Vineyard for months. 

What makes the growth of this phenomenon so perplexing to scientists is that while the Gulf Stream seems to be slowing over time, the current is forming more warm core rings. Gawarkiewicz explained that the question of why we are seeing more warm core rings is still open and debated in the scientific community.

Regardless, the researcher said Woods Hole scientists believe the increasing quantity of warm core rings may be what is pulling tropical fish into coastal Massachusetts waters more often, and for longer periods of time.  

“This is all just a hypothesis, but one that we are researching very closely,” he said. 

Warming in Buzzards Bay

The South Coast is seeing some impacts from more warm core rings forming in the Northeast. Men o’war — a venomous tropical organism related to jellyfish —  have twice washed up on beaches in Westport in the past two years, and recently in Marion. 

But aside from these unusual events, oceanic weather patterns are less impactful on the Buzzards Bay ecosystem than elsewhere on the Atlantic Coast.

Local scientists say warming patterns in Buzzards Bay are increasing, mostly in the spring and summer. They add that these changes seem to be driven more by warmer air temperatures and solar radiation, given that the bay’s waters are shallow and less exposed to open ocean.  

Rachel Jakuba, the vice-president for bay science at the Buzzards Bay Coalition, said she hasn’t heard of any marlin or flying fish being spotted in the watershed. However, she said changes in summer water temperatures have been “noticeable,” with increases of 4 degrees Fahrenheit in some embayments from 1992 to 2012. 

The Division of Marine Fisheries has also tracked intensifying heat patterns in Buzzards Bay waters. An agency official noted that in 1986, the bottom temperature of the bay only reached above 68 degrees Fahrenheit 40 days per year. In 2021, there were 100 days of temperatures above that level.  

While these warming patterns may be less extreme and frequent than those elsewhere on the Atlantic coastline, scientists are still eyeing the anticipated consequences with caution. 

Jakuba says that she is worried about the growing amount of algae in Buzzards Bay as temperatures rise. Too much algae hurts fish, eelgrass and shellfish. She noted a troubling increase in chlorophyll concentrations — an indicator for algal growth – throughout bay waters over the past two decades. 

Algal blooms in New Bedford Harbor. Credit: Buzzards Bay Coalition

Why algae blooms are harmful

New Bedford, Cape Cod and the South Coast area have been dealing with the problem of harmful algal blooms for decades. These algae use nutrients — such as nitrogen — and sunlight to grow, coloring the water green or brown and making it less transparent. The presence of these algae can block out sunlight needed for marine plants like eelgrass to thrive. Reduced growth in eelgrass is leading to population declines in Buzzards Bay shellfish and bay scallops, which use the plant beds as habitat. 

When the algae die, the bacteria that break them down consume oxygen, lowering the amount of oxygen in the water available for fish and shellfish, in a phenomenon known as hypoxia. 

Jakuba noted that heating waters in the bay will require local environmental managers to tighten their daily nitrogen pollution limits to slow down algae growth, since algae grows more prolifically in warmer conditions.

The marine fisheries official said these above-average summer temperatures have led to increases in the number of scup and black sea bass in Buzzards Bay waters over the past two decades. Meanwhile, local populations of coldwater fish, such as winter flounder, have declined significantly. 

The spokesperson added that warmer waters earlier in the year are altering the timing of lobster molting patterns, leaving them vulnerable to disease and regional population decline. They noted that lobster landings on the southern coast of New England have declined from a peak of roughly 2.5 million pounds in 1997 to just under 750,000 pounds in 2021. 

Scientists are observing warming waters across many ecosystems in the region. One reason is the jet stream, Gawarkiewicz said. Over the past decade-plus, Woods Hole researchers have seen that in some years, the massive wind current is moving south to the East Coast from Canada later in the fall than usual. 

The jet stream is a fast, meandering current of cool air flowing from west to east around the globe. The heart of the weather pattern — centered around North Atlantic waters — flows 6 to 8 miles above the ocean, but its cooling effects extend down to surface waters. 

Gawarkiewicz said this delay in the jet stream is why we have experienced more mild winters, and is one reason local waters are warming. He said these trends, combined with more frequent storms, point to the “8,000-pound elephant in the room,” the possibility of a devastating summer storm event. 

The scientist noted that the Cape Cod region hasn’t experienced a major storm since Hurricane Bob in 1991. He added that he lived through the Category 5 Hurricane Camille that made landfall in Mississippi in 1969, and saw it rip houses off of their foundations. 

“It’s just not a good experience,” he said. “Be ready. That’s my number-one message.”

Email The Light’s environment and climate reporter Adam Goldstein at

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    1. Flying fish are not Sea robins. Sea robins are ruffle finned fish who make croaking sounds when held out of the water. Flying fish are sleek fish with long pectoral fins that look and act like wings. Flying fish glide through the air when escaping predators. The difference in the fins and body are significant, both are worth googling.

  1. There is no evidence that increased water temperatures in the north will increase the incidents of strong hurricanes here, just speculation. I really wish that people would stop talking about warming using such hyperbole because it just fuels all the yahoos that say the science is all fake.

  2. We have plenty of sea robins in this area. But, the photo in this story is definitely a flying fish. As noted above, they are two distinctly different species.

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