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Researchers identify shellfish-killing phytoplankton behind massive summer die-offs in Puget Sound

In July of 2018 and 2019, large numbers of oysters, cockles and clams died on beaches all around Puget Sound. No one knew why. It was a particularly bad couple of years, but summer mortality events with mass die-offs of shellfish happen regularly. They’ve been recorded by researchers in western Washington as far back as the 1930s. The source has remained a mystery.

Now, scientistshave pinpointed the cause: two species of toxic algae that don’t threaten people much — but can wreak havoc on the ecosystem. Beginning to figure out what’s killing so many shellfish is a breakthrough for growers and communities who live near the beaches.  

People who monitor water quality all around Puget Sound helped make the discovery. Their organization is calledSoundToxins.

...which is like a neighborhood block watch of the sea that caught a microscopic bad guy,says Vera Trainer, a research oceanographer with NOAA’s Northwest Fisheries Science Center in Seattle. 

She says SoundToxins includes private citizens, shellfish growers, environmental learning centers and tribes. Trainer started the organization 15 years ago to improve monitoring of algae blooms that can harm people and many forms of wildlife.

Protoceratium reticulatum, one of two species of toxic algae that have been causing mass summer mortality of shellfish in Puget Sound, seen through a microscope in 2019.
Credit courtesy of Teri King / Washington Sea Grant
Washington Sea Grant
Protoceratium reticulatum, one of two species of toxic algae that have been causing mass summer mortality of shellfish in Puget Sound, seen through a microscope in 2019.

“It's not scientists working in isolation, but scientists working with community members who are the eyes on their backyard and are able to tell us what is there at multiple sites,” says Trainer.

She says the "microscopic bad guys" that are causing mass shellfish die-offsevery summer are two species of tiny phytoplankton called dinoflagellates. Akashiwo sanguinea and Protoceratium reticulatum both produce toxins that kill most bivalves.  

“They look amazing in the microscope. They're quite beautiful. But these, you know, these organisms that you can't even see with your eye are causing millions of dollars of damage,” Trainer says.

Dinoflagellates have two whip-like tails that let them swim up and down the water column and find what they need to survive. Instead of dying off the way most algae do, Trainer says, they can drop seed-like cysts that keep springing back to life.  

Akashiwo sanguinea as seen through a microscope.
Credit Teri King / Washington Sea Grant
Washington Sea Grant
Akashiwo sanguinea as seen through a microscope.

“And we do think that these dinoflagellates, which are more highly resilient to stressful environmental conditions, are going to be some of the winners under climate change,” she says.

She says it’s a threat we have to learn to live with. So SoundToxins wants to develop a warning system, similar to the ones in place for toxic algae that affects human health, that would alert growers when blooms reach dangerous levels.

Impact on shellfish growers

At Taylor Shellfish in Shelton, crews unload the day’s harvest for processing. Oysters, clams and mussels run through conveyor belts where workers wash and shuck them.  

Taylor is the largest shellfish grower in the United States with about 10,000 actively farmed acres.

Farm manager Justin Lanman and fifth-generation farmer Nyle Taylor point to crates full of footlong geoducks that have just arrived. More than 600 pounds are from North Bay, one of the beaches where the mass die-off happened two summers back.   

"These geoducks were planted by our crews in the summertime about six years ago,” says Lanman.

“Yeah, these geoducks would have survived that event. They've been in the ground longer than that. They've lived through it all,” says Taylor with a laugh.  

The geoducks hunkered down and became less active. Unlike everything else on the beach, they didn’t die. Taylor and Lanman still remember how it smelled when other species did.  

Healthy geoducks arrive at Taylor Shellfish for processing.
Credit Bellamy Pailthorp / KNKX
Healthy geoducks arrive at Taylor Shellfish for processing.

“Hundreds of thousands of pounds of clams, tens of thousands of dozens of Pacific oysters – in 10 days basically,” says Taylor.

He says as a shellfish grower, regular annual losses are part of the deal. But this was an extreme event.  

Clams were popping up out of the ground, which is what they do when they're, unfortunately, about to die. And I started noticing them surfacing, which was really atypical for that time of year,” says Lanman. “And then two or three days later, it was just a blanket of dead clams everywhere. And then a couple more days and a bunch of oysters started dying. And then it just turned into just a graveyard out there. It was awful.”

For some reason, Taylor says, seabirds that would normally scavenge and pick at dead things on the beach stayed away, causing more rotting and stink than usual.   

“It was not a fun place to walk on the beach or live in that area for two weeks, just as all of that meat was you know, starting to decay on the beach,” he says.  

Taylor says these killer phytoplankton have cost his company millions of dollars. He and others hope the recent die-offs were just an especially bad couple of years. Now that they know the species involved, they’ll research different ways to protect their crops, perhaps through genetics.

But, Taylor points out, it wasn't only farmed species that perished – everything did, including lots of native clams and other natural species. That hurt more than shellfish farmers.   

Role of shellfish in tribal cultures

Shelbi Madera says it's affected members of her tribe, the Nooksack. She's a lab assistant at the Salish Sea Research Center at Northwest Indian College in Bellingham. Her lab ran shellfish samples through a mass spectrometer to help confirm the findings about which toxins killed them. That work was her capstone project for a bachelor’s degree in Native environmental science.

Shellfish is a subsistence food for her relatives, she says.  

“I have a relative who goes to Lummi and goes out and collects shellfish. And in that form, they get extra money to pay for bills or whatever they need, as well as collecting those shells to make things such as rattles for ceremonial purposes,” Madera says.

She says the summer shellfish harvest is a cultural event that binds families and generations.

“They camp out near the beaches for days and they wait for the tide. They stay out until the tides are out. They get up early mornings, late at night, whenever the tide is, and they go out and harvest,” she says. “And I think that's just a sense of like having that time of being together as a family. It's really important and sharing that meal and those words.”  

Madera says she’s amazed to have seen first-hand how these microscopic plants – phytoplankton – can be so deadly to clams and oysters.

“They're really hard to kill; they're filter feeders, so if they're dying off … something really intense is happening.”  

Her lab will grow samples of the toxic phytoplankton so they can learn more about how they work.  

In the meantime, the scientists behind SoundToxins will continue to monitor the water.

'Hiding in plain sight'

Teri King, an aquaculture and marine water quality specialist with Washington SeaGrant, is the lead author of the paper that outlines the new findings. She says being able to provide a warning when these deadly phytoplankton are about to bloom could give some growers time to get their shellfish out of harm’s way.   

“So they might harvest that crop more quickly, if it's ready to go, rather than holding on to it for a while and hoping that it survives the area,” she says.

Teri King uses a net to collect water samples and, perhaps, phytoplankton.
Credit Bellamy Pailthorp / KNKX
Teri King uses a net to collect water samples and phytoplankton.

King is standing at the end of a long pier in Port of Allyn, near Belfair in south Puget Sound – very close to where the recent mass die-offs happened, in North Bay. She unfurls a special net and attaches a metal tip at the end.

"This is what's going to capture all of the plankton in it,” she says as she demonstrates how she samples the water here every week.

She tows the net through the water three times before transferring its contents into a special jar. She also measures the water temperature and makes note of other conditions.

King will take the samples back to a lab where she can examine the organisms they contain under a microscope. This was how she got the first major clue about the cause of the recent die-off. She found Protoceratium reticulatum in water samples from North Bay and later in samples from dead animals.

“It was the only organism in the entire bay. There was nothing else that could have caused the problem. There was no dissolved oxygen problem. There was no temperature problem. There was no nothing else. It had to be this,” King says.  

King says in other areas, it was clearly Akashiwo sanguinea.

The new paper explaining all of this is called “Hiding in plain sight.” 

That’s because when her team of researchers really dove into the available data, they found a correlation between mass summer mortality events and these two species of phytoplankton going back as far as the 1930s.

King says the clues were there all along. But the names of the phytoplankton species were changed over time as scientists learned more about them. So it took some sleuthing to figure that out.  

So amazing – this has been happening for 90 years and we just figured out what it is. That's just amazing,” King says.  

Scientists will need time and more research to fully understand the findings. But they're relieved to have found the cause of summer die-offs that have led to heartbreak for Washington’s multi-million-dollar shellfish industry and for its coastal tribal communities for nearly a century.  

Bellamy Pailthorp covers the environment for KNKX with an emphasis on climate justice, human health and food sovereignty. She enjoys reporting about how we will power our future while maintaining healthy cultures and livable cities. Story tips can be sent to