Bivalve boom? Researchers work to restore Elkhorn Slough’s Olympia oysters

Jacob Harris showing off Moss Landing Marine Labs' "home-grown" oysters.
(Kevin Painchaud / Lookout Santa Cruz)

A new generation of Olympia oysters has officially taken up residence in Elkhorn Slough. Scientists hope breeding oysters in the lab and returning baby oysters to the slough will help the mollusks to rebound from the brink of extinction.

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It’s a cool November day on Elkhorn Slough, where Olympia oysters will soon be released into the wild. Researchers Luke Gardner, April Ridlon and Kierstin Thigpen diligently zip-tie clam shells dotted with small oysters to the tops of PVC pipes that will soon be plunged into the muddy basin of Hester Marsh.

Since 2018, ecologists at the Moss Landing Marine Laboratories have been collecting oysters from their briny homes in the slough, bringing them to the lab to reproduce, and returning them to the wild. They hope this project will lead to increased oyster numbers and improved habitat for all creatures in the slough.

Oysters in Elkhorn Slough have had a precipitous decline over the past century. Located 6 miles south of the Santa Cruz County line in the shadow of the smokestacks of Moss Landing’s power plant, the slough serves as the estuary for the Salinas River. But the landscape has changed dramatically as the harbor was opened up and the land was diked and drained for agriculture.

“When I started looking for them in 2007, I wasn’t even sure if they were still here,” said Kerstin Wasson, research coordinator for the Elkhorn Slough National Estuarine Research Reserve. “The record said they had gone locally extinct.”

But the oysters weren’t extinct — at least, not yet. Researchers relocated them in the slough, but in very low numbers. “At the time, I estimated there might be 5,000 to 10,000 oysters in the whole estuary,” said Wasson. “In comparison, at our sister estuary reserve in San Francisco Bay China Camp you can look at one rock and see 5,000 oysters.”

The dramatic decline of the slough’s oyster population is significant because Olympia oysters (Ostrea lurida) are known as a foundation species, meaning that they alter the ecosystem for other species by improving water quality and providing refuge, Wasson explains.

Oysters clean up the water by eating large quantities of algae and poop those nutrients onto the bottom of the seafloor, where they become available to organisms that live on the basin of the marsh. The space between oyster shells provides physical habitat for other small animals, like fish. Additionally, oysters stabilize shorelines and prevent erosion. “We put rocks and riprap and jetties to stabilize shorelines, but oysters do that just by living there,” said Jacob Harris, a master’s degree candidate at the Moss Landing research facility.

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Olympia oysters are found on the West Coast of North America, from the southern tip of Alaska all the way down to Baja California in Mexico. They are the only native oyster along these coastlines, and they face ongoing threats. In Elkhorn Slough, the hard substrate the oysters used to latch onto became buried in “thick goopy anoxic mud,” explains Harris, caused by agricultural runoff like fertilizers.

Despite habitat changes, the oysters are surviving once they’re in the ecosystem — the issue seems to be that they just aren’t reproducing. “We’re not sure if the oysters are not spawning, or if those spawned babies are getting carried out to sea, or if they’re met with other unfavorable conditions,” said marine ecologist Ridlon, lead of the Native Olympia Oyster Collaborative (NOOC).

Jacob Harris showing off Moss Landing Marine Labs' "home-grown" oysters.
(Kevin Painchaud / Lookout Santa Cruz)

While scientists aren’t sure why the mood in the slough isn’t right for these important invertebrates, the oysters are happily getting it on in the lab. So researchers have been collecting the oysters from the slough, bringing them to the lab for quality time, and then returning the oysters and their offspring back to the slough.

Oysters sexually mature after about a year after birth, and can live to be several years old. Female oysters will hold onto eggs inside their shells until larvae hatch. Oyster larvae will swim around for a couple weeks and then attach themselves to hard substrate, like shells or rocks, where they will remain for the rest of their lives.

Baby oysters grown at Moss Landing Marine Labs.
(Kevin Painchaud / Lookout Santa Cruz)

In the lab, large cylindrical tanks hold baby oysters that have successfully attached to shells. Another rectangular tank holds baby oysters that have adhered to small floating bits of sediment. The latter approach is more common in aquaculture — the aquatic equivalent of farming. The small substrate allows both sides of an oyster’s shell to curve as it grows, which creates the ideal “spoon” shape for oyster eating. Wherever an oyster settles, it is permanently bound.

Harris loads the oyster-laden shells into a cooler, and carefully scoops the sediment-bound oysters into bags. It’s go time for the oysters.

A group of students and scientists ventures into Elkhorn Slough to check on planted oysters and to place new oysters.
(Kevin Painchaud / Lookout Santa Cruz)

At Hester Marsh, Harris is met by a team of about 10 other researchers and volunteers, all enthusiastic about oysters. Some researchers, like Gardner, Ridlon and first-year graduate student Thigpen, measure and secure the baby oysters to PVC pipes. The largest oyster on each clam shell is measured, data that will help researchers see how well the oysters are growing when the team returns.

Others, waist deep in water, shove the PVC pipes laden with clamshells at the top into the basin of the marsh. The sediment-bound oysters will be placed in baskets and lowered into the murky waters of the slough.

Women plant new oysters on restoration stakes in Elkhorn Slough
Katie Nichols of the California State Coastal Conservancy (left) and Kerstin Wasson, research coordinator for the Elkhorn Slough National Estuarine Research Reserve, plant new oysters on restoration stakes.
(Kevin Painchaud / Lookout Santa Cruz)

This is the third go-round of putting baby oysters back in the marsh; baby oysters were placed in the marsh in 2018 and again in 2021. It’s too early to tell whether the numbers are rebounding, but the goal is to get the number of oysters up to one million — high enough where they can sustain themselves.

As Ridlon measures oysters and Gardner attaches the clam shells to the pipes, they explain how aquaculture could be a good long-term solution to the problem of declining oysters. NOOC, where Ridlon works, is launching aquaculture pilots in hopes that shellfish farming can revitalize Olympia oyster populations in California — including in Elkhorn Slough.

At first, aquaculture and conservation might seem like opposing ideas: How could eating oysters benefit the oyster populations? But oyster aquaculture is able to do what the researchers at Moss Landing Marine Laboratories are doing, just on a much larger scale. Commercial growers operate by breeding oysters in indoor facilities before returning them to the slough en masse to live until they are harvested; it usually takes three to four years for Olympia oysters to reach market size, Ridlon explains. Aquaculture helps stabilize Olympia oyster numbers: “If you can incentivize commercial growers to help out and they can make some money and do something good, then they’ll keep doing that — that’s the hope anyway,” said Gardner, aquaculture specialist at California Sea Grant.

Despite the many mysteries of the oyster, one thing is clear: Healthy oysters are crucial for a healthy slough environment: “If you have a healthy oyster population,” Harris said, “you can provide all these ecosystem services that help animals, they help the people that live in the community, and lots of people want to eat oysters. The more oysters the better.”

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