San Francisco Bay oyster research fumbled 

Conservationists have toiled for more than a decade to restore a native San Francisco Bay oyster population thought to have collapsed since the early 19th century.

But an emerging theory suggests those efforts might have been misguided.

Native Olympia oysters once helped filter the waters of the Bay and their shells were found in large numbers in mounds left behind by Muwekma Ohlone Indian tribes that inhabited the Bay Area prior to European settlements.

But the mollusks, which remain a scientific mystery due to an overall lack of research, are no longer prevalent in the Bay.

Theories for their decline include overharvesting and an accumulation of silt from gold mining. Invasive oyster populations and pollution are considered factors too.

To restore their numbers, volunteers and nonprofit workers since the 1990s have planted concrete blocks and artificial reefs to provide the bivalves with a habitat. The work is funded with donations and grants.

Yet, no evidence exists that the Bay has been home to high numbers of the mollusks in recent millennia, according to an emerging theory championed by Andrew Cohen, director of the San Francisco Estuary Institute’s Biological Invasions Program.

“As far as we know, there is no evidence that oysters were ever abundant in the Bay,” Cohen said during a talk earlier this year at the UC Museum of Paleontology in Berkeley. “The absence of types of evidence that we would think we would find suggests that they probably weren’t.”

Analysis of shells found in mounds indicates that most are thousands of years old, with few younger specimens discovered, according to Cohen.

Cohen’s theory makes oyster restoration a “dicier proposition” that would require overhauling the Bay environment.

“Even if that was something that we could do, it’s very doubtful that we would want to,” he said.

California recently published a draft subtidal habitat plan for the Bay that calls for acres of habitat to be set aside for oysters and seeded with their young.

Cohen’s idea, which is being treated skeptically by other oyster scientists, will be discussed next week during a San Francisco Bay Conservation and Development Commission meeting.

“The policy implications are fascinating,” Executive Director Will Travis said. “Should we be devoting a lot of energy and hope to restoring something that may not have been here in abundance for the past 2,000 years?”

jupton@sfexaminer.com


Olympia oysters

Range: Southeast Alaska to Baja California

Shape: Lower valve is concave and upper valve clicks shut

Color: Ranges from white to purplish black, sometimes striped

Gender: Begins as male then fluctuates seasonally

Larvae brood: 250,000 to 300,000 per spawn

Larval actions: Move with tiny foot to find hard surface

Grows on: Rocks, shells, wood, metal, boats, piers

Scientific name: Ostrea lurida

Other names: Native oyster, California oyster

Source: U.S. Geological Survey

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