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Oyster farms in Virginia and beyond are suffering unexplained mass die-offs

Bruce Vogt at his Big Island Aquaculture farm in Gloucester County.
Courtesy of John Wallace/VIMS
Bruce Vogt at his Big Island Aquaculture farm in Gloucester County.

The Virginia Institute of Marine Science is spearheading an effort to understand what’s causing the ongoing issue known as Sudden Unusual Mortality Syndrome.

When Bruce Vogt and his team at Big Island Aquaculture in Gloucester pull up their oyster cages from the water, they can instantly tell when some are dead.

“We call them clunkers,” Vogt said. “Because it’s kind of a hollow sound the shell makes when they’re knocked together.”

Since he opened Big Island about a decade ago, Vogt said they're seeing more and more clunkers. The company estimates they lost about four million oysters total – about four times the amount they plant each year.

“You pull the bag up and you’ve got over 50% dead ones in it,” he said. “It’s real discouraging. And we said, ‘What's going on?’”

Vogt is far from alone. Oyster farmers up and down the East Coast and the Gulf of Mexico in recent years have been struggling with mysterious mass die-offs of their crop. Researchers call the issue Sudden Unusual Mortality Syndrome, or SUMS.

The Virginia Institute of Marine Science is now spearheading an effort to pinpoint the syndrome’s causes – and find solutions for the industry.

“I have had heartbreaking letters and phone calls with farmers who clearly were at the brink of giving up because of how much they had lost,” said Bill Walton, a marine science professor and coordinator of VIMS’ shellfish aquaculture program. “We need to get the answers.”

Oyster aquaculture starts with seed – pinky nail-sized baby oysters – raised in hatcheries and sold to farms. Then they’re placed in floating cages or baskets in a natural water body and tended for up to two years before they reach market size.

“You’re basically doing what nature does,” Walton said. “You’re just trying to do it in a way that’s maximizing survival and growth.”

The unexplained mortality syndrome can affect oysters anytime they’re growing in that water. And when it does, the oyster meat dissipates quickly, making it difficult to collect samples.

Scientists became aware of the oyster die-offs in the early 2010s, when they started getting calls from farmers like Vogt asking for help, Walton said.

The problem hasn’t appeared in wild oysters, though there could be issues going unreported because those creatures are less monitored, he said.

The issue isn’t isolated to one area, or even places with the same water conditions.

“We’ve seen suggestions that it’s low salinity, but then we’ve had other states where it’s associated with high salinity,” Walton said.

He said researchers believe the deaths are likely due to a combination of factors, such as warming waters and less oxygen in the water. Think of oysters as a “cup that can hold a certain amount of stress.”

“So as you add different stressors, you might overflow (the cup) and cause mortality,” Walton said.

The goal is to find a way to measure that stress so growers can see when that cup is getting full and take steps to protect their harvest.

Bill Walton, second from left, and other VIMS researchers work on a research farm.
Courtesy of John Wallace/VIMS
Bill Walton, second from left, and other VIMS researchers work on a research farm.

Walton said VIMS is using some recent funding from the National Marine Fisheries Service to document the magnitude of the issue.

The institute also recently convened a workshop with academics from up and down the East Coast to discuss how to tackle the problem.

In their resulting report, the group outlined a path forward including polling growers about their experiences and collecting genetic samples.

Walton said traditional academic research can be gradual while developing hypotheses and seeking funding.

“But what we've found is that that progress can be a little slow when you have a problem like this where we have oyster growers telling us that this is an existential threat to their business,” he said. “So we wanted to see if we could accelerate the process.”

Katherine is WHRO’s climate and environment reporter. She came to WHRO from the Virginian-Pilot in 2022. Katherine is a California native who now lives in Norfolk and welcomes book recommendations, fun science facts and of course interesting environmental news.

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