California's native oysters are unusually well adapted for climate change

San Francisco Chronicle​

30-05-2025

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The tiny native oysters of San Francisco Bay managed to outlive the Gold Rush, bay-shore development and decades of punishing pollution. New research shows they have a fighting chance to survive global warming as well.
A different species than the farmed Pacific oysters slurped up in restaurants, Olympia oysters are the West Coast's only native oyster species, once forming huge reefs along thousands of miles of coastline from Baja California to British Columbia. Though delicious, they're not as commercially viable and can't be safely harvested from San Francisco Bay because of pollution. But efforts are underway to restore the native oyster in the bay and along the West Coast for its important role in the ecosystem, including providing habitat for baby salmon and crab.
'If we had been here 300 years ago, it would have been this striking, essential part of San Francisco Bay,' said Kerstin Wasson, research coordinator at Elkhorn Slough National Estuarine Research Reserve on Monterey Bay. 'Now it's so rare that most Californians have never seen a native oyster, have never touched one, have never eaten one.'
Now, Wasson is the lead author on a study of the native oysters from Mexico to Canada that shows them to be surprisingly well adapted to warmer air temperatures caused by climate change.
In recent years, extreme heat waves killed thousands of shellfish that inhabit the same type of intertidal zone, including mussels that were baked in their shells in both the Pacific Northwest and Northern California during low tide. Scientists involved with oyster restoration were really concerned when that happened, said Chela Zabin, ecologist at the Smithsonian Environmental Research Center and one of two dozen co-authors on the study.
'Into the future, are these big heat wave events going through going to affect our ability to restore oysters?' she said.
The study, which was published last month and involved significant team effort in 26 locations up and down the West Coast, with funding from the nonprofit organization the Nature Conservancy, showed that native oysters thrive in a wide range of habitats.
The researchers assumed that oyster populations, which occupy only a narrow band of the intertidal zone, would cluster closer to the low tide waterline to seek relief from hot air in warmer climates such as Baja California and Southern California. Instead they found that the oysters actually inhabited a larger range, demonstrating that they're more adaptable than expected. That bodes well for oysters farther north, including in the Bay Area, when climate change continues to increase air temperatures there, the authors said.
'What is happening in Baja today is what will happen in San Francisco Bay tomorrow,' Wasson said.
On a tour of oyster restoration sites at Point San Pablo in Richmond on Wednesday morning, State Coastal Conservancy Project Manager Marilyn Latta demonstrated how finding native oysters during an extremely low tide was as easy as overturning rocks near the shore — like looking for pill bugs in the garden.
The Coastal Conservancy, a state agency, provided funding for native oyster restoration projects at several locations along the Point San Pablo bay shore. That includes at a site called Terminal Four where contractors recently removed a derelict wharf and added new native plants as well as concrete structures, including ones that resemble sand castles, specifically designed to provide habitat for oysters.
Zabin held a rock with a dime-size native oyster attached, most likely a baby; adults in San Francisco Bay are only slightly larger than an inch in diameter, making them much smaller than Pacific oysters. In addition to restoration efforts underway in the bay, aquaculture may also be necessary in the future as a backup plan to protect the species, she said.
Oysters are known as filter feeders for their ability to clean the water and provide habitat that supports salmon migration back and forth to the sea, Wasson noted. When there are enough of them, they create reefs that provide shoreline protection from waves, she said. However, in California, not enough oysters have been brought back to serve this role.
Up and down the West Coast, the native oysters were enjoyed by Indigenous people as well as European settlers, including during the Gold Rush, when they were overharvested and later subjected to pollution and habitat loss, especially as the bay was filled for development.
Some oyster farmers, including Hog Island Oyster Co. in Tomales Bay, are experimenting with growing native Olympia oysters — though they're more difficult to produce commercially because they're small and slow-growing, Wasson said.
However, growing native oysters may have other advantages, because they are known to be less vulnerable than Pacific oysters to ocean acidification that comes with climate change and inhibits the formation of shells.
'Our poor oysters have suffered a whole bunch of things in the past century,' Wasson said. 'But at least this particular way humans are messing with them is probably going to be OK, at least for the near future.'