For years now, if a commuter were to glance to the north side of the San Mateo Bridge, they might see a lonely barge, painted with the words “Lind Marine,” floating a few hundred yards from the shoreline.

A stray vessel in the San Francisco Bay is not an uncommon sight. But this particular barge is the last sign of one of California’s oldest mining industries, which trades in what might be the Bay Area’s most unusual non-renewable natural resource.

Not gold. Not oil. Oyster shells.

For thousands of years, the San Francisco Bay was home to hundreds of millions of Olympia oysters. Native to the West Coast, they were one of the defining species of the bay’s ecosystem. They were also engineers — ubiquitous creatures that formed enormous reef structures, cleaned the water and sheltered other organisms.

The 1,000-acre stretch of water just north of the San Mateo Bridge was an especially appealing location for the 3-inch-long mollusks. Over the millennia, as generations of oysters lived and died, their shells accumulated into an enormous, underwater shell mountain. Although now hidden under 30 feet of silt, the oyster shell deposit is considered one of the largest of its kind on the west coast.

“It’s this massive, natural and totally irreplaceable resource,” said Matthew Booker, an environmental historian who has researched and studied the bay’s natural history.

Oysters are a bigger part of that history than most locals know. They were so plentiful that they were used to build walkways in the late 1800s and have been commercially mined in the bay since 1924, mainly for use in cement. In recent years, the ancient deposit of shells has found new purpose as a calcium-rich dietary additive in chicken feed.

For the last 40 years, the massive deposit has been mined by Lind Tug and Barge. The Vallejo-based company has mined at least 22,000 tons of oyster shells every year since 2006, using a hydraulic suction dredge to remove shells from the silt and transfer them a processing plant in Petaluma.

Lind currently leases a 1,560-acre plot of the bay adjacent to the bridge from the California Lands Commission, and maintains a “take permit” from the California Department of Fish and Wildlife. Lind Tug and Barge declined to comment on their mining activities.

For decades, the company has had nearly unfettered access to the underwater mountain of oyster shells. But that access is now coming under scrutiny as a groundswell of agencies and volunteer organizations look to restore oysters in the San Francisco Bay, and rethink the relationship with the body of water that defines the region.

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