A new oyster invades

The world’s most widely consumed, widely farmed bivalve – the delectable Pacific oyster – has taken up residence in San Diego’s bays and lagoons and may be in the early stages of a full-fledged, non-native species invasion.

“They are all over the place now,” says Jeff Crooks, a former California Sea Grant graduate trainee, who is now the research coordinator for the Tijuana River National Estuarine Research Reserve.

The oyster, which is from Japan and cultured locally in northern San Diego County, can be found on rip-rap in Tuna Harbor in San Diego Bay, on pylons in Mission Bay, in Los Penasquitos Lagoon, Agua Hedionda Lagoon and at the Tijuana River Estuary. “They have definitely become established and conspicuous within the last few years,” he said. “It’s a pretty big deal.”

A big deal because the Pacific oyster is a reef builder. It re-engineers its environment.

“We could end up with oyster reefs like they have in Japan,” said Crooks, who discussed local shellfish at the recent Headwaters to Oceans (H20) conference in San Diego.

The Pacific oyster is a $3.7-billion-a-year industry globally in part because of the bivalve’s large size, growth rate and tolerance to a range of marine conditions. The traits that make it so suited to culture could also make it a formidable invader.

“Our worry is that native oyster restoration efforts may backfire and we will end up creating habitat for the invasive oyster,” said Danielle Zacherl, a professor at Cal State Fullerton, who has been documenting the Pacific oyster’s spread in San Diego and Orange counties and is involved in native oyster bed restoration in Southern California.

She and students have already witnessed native and non-native oysters recruiting on shells spread on mudflats in Alamitos Bay in Long Beach to create hard substrate for oyster larvae. Substrate placed lower in the intertidal zone in Newport Bay resulted in high recruitment of native but not non-native oysters. “We are gathering this type of information so we can proceed with native oyster recovery without simultaneously boosting the non-native’s spread,” she said.

The native Olympia oyster still outnumbers the Pacific oyster 10 to 1 in San Diego Bay, but it’s harder to see because it’s smaller and lives at deeper depths, she said. “We don’t want to lose our momentum and opportunity to restore our native ecosystems.”

California Sea Grant Aquaculture Specialist Paul Olin has a slightly different take on what may be going on, in terms of the suitability of local habitats to the native Olympia oyster.

“If Pacific oysters were to become highly abundant, they would be harvested, and they would not pose a constraint to native oyster restoration,” he said.

“Olympia oysters have had 100 years to recover from overfishing,” he said. “The fact that they have not done so leads me to believe that during that time we have altered their historic habitat such that it is no longer that.”

Ironically, people in the late 1800s and early 1900s tried but failed to introduce Pacific oysters to California and at the time blamed water temperatures as the culprit, Crooks said.

“The stars have aligned for it now, and we don’t know why, but it’s worth tracking because of its ability to massively modify habitats,” he said.

Note: Zacherl does not recommend eating locally growing Pacific oysters because of concerns about the pathogens and contaminants that might be concentrated in the animals’ tissue. Local farmed shellfish, she says, are depurated (cleansed) in purified seawater before they are sold for human consumption to protect human health.

Written by Christina S. Johnson