Invasive snails killing off native oysters

October 20, 2009 12:00:00 AM PDT
Tomales Bay in western Marin County has been the source of fresh oysters for the Bay Area for generations, but lurking beneath these tides are opportunists bent on keeping native species from surviving.

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The history of oyster harvesting in Marin dates back to the gold rush, when hungry 49ers looked for the tasty Olympia oyster in Tomales Bay. Because the oysters were so small, it took 1,600 to 2,000 shucked Olympias to make a gallon.

"Once the native oysters were fished out, very quickly other oyster species were introduced," explains Ted Grosholz. "So, we had oysters introduced from the Chesapeake, oysters introduced from Japan and other places."

Olympia oysters have not been farmed since, but their numbers still continue to dwindle. Scientists blame invasive species that came attached to the oysters that replaced them.

"The species that we have studied are predatory snails," Grosholz says.

Grosholz is a marine ecology specialist at UC Davis. He led a study that found despite decades without harvesting, the native Olympia oysters have not returned in significant numbers. The reason is that those invasive snails are devouring them.

"They're known as whelks or oyster drills, and they rasp a hole in the oyster shell and consume the oyster," Grolsholz explains.

The invasive whelk snail thrives in silt and freshwater, the same habitat as the native oysters. Grosholz says that in some parts of the bay, the snail is responsible for destroying 80 to 90 percent of the native oyster population. The only predator, the rock crab, is being squeezed out by another invasive species, the European green crab that arrived in the 90's.

Unless something is done soon, there is a chance the Olympic oyster and the ecosystem it supports could vanish.

"When you replace these native species with introduced species, the food web breaks down," Grosholz says. "The controls that normally supported oysters disappear and you lose the oysters."

But, it may not be too late. Grosholz says, "We can put effort into establishing and boosting the native oyster populations."

The very industry responsible for bringing the invasive species in more than a century ago is now hoping to help.

Hog Island Oyster Company owner John Finger says, "We became part of this project to see if we could increase the number of Olympias in the bay."

He is part of a campaign to bring Olympia oysters back to Tomales Bay.

"We have a crop right now on the south lease, right now, that hopefully we'll see some starting this holiday season. And, we'll do small amounts of them," he says. "They are not a great candidate for culturing on a big scale, which is why people brought other oysters in 100 years ago, because they take three years to get to market size."

Those three years make the recovery effort a very slow process. Still, farmers and scientists alike are hoping they can return the Olympia oyster population to healthier levels.

"The danger is not having enough filter feeders to handle the productivity of the bay," Finger says. "The oysters that we culture on essentually five percent of the area of this bay, aren't near enough in my mind, to handle the productivity of this Bay."

Grosholz says, "It's important that we do everything that we can to try and keep additional species from coming into the system. Many of these species, once established, are difficult if not impossible to get out."

Olympia oysters were once abundant from Southern California to southeastern Alaska. Now, they are only farmed in significant numbers in Washington State.

Written and produced by Ken Miguel

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