Shell-shock: Study finds bad news for local oyster farming

Last Updated on May 1, 2025

If oysters are among your favorite bivalves to consume, read on.

The warming temperatures of our local waters may be responsible for oyster die-offs, according to a new study by researchers at UC San Diego’s Scripps Institution of Oceanography.

Oyster deaths threaten San Diego aquaculture, said Emily Kunselman, lead author of the study and a recent SIO graduate, which means fewer oysters at your next seafood outing.

The study, published recently in the journal Sustainable Microbiology, found rising water temperatures in San Diego Bay accelerate a deadly herpes virus in juvenile Pacific oysters.

Although the ostreid herpesvirus (OsHV-1) doesn’t harm humans, it can cause significant oyster deaths.

Researchers discovered the mortalities after exposing oysters to the virus “via natural routes, … from the water itself,” Kunselman said, at temperatures from 59 to 75.2 degrees Fahrenheit representing various local water temperatures throughout the year.

As water temperature increased, the oysters “were more likely to get infected and die,” she said, noting that in 59-degree water, though the oysters contracted viral copies, “none of the oysters actually died.”

Natural oyster habitat on the western side of San Diego Bay. Photo: E. Kunselman.

The main impact this has will be on the farmed oysters in San Diego Bay, Kunselman said, as there’s

currently only one farm, a nursery stage farm where oysters are brought in from a different hatchery elsewhere.

The warmer waters in San Diego Bay accelerate the growth of the oysters, which are then normally sent to other locations to finish maturing.

The so-called “naive oysters” coming into the local nursery are the ones most susceptible to this virus, Kunselman said.

“As we see temperatures increase and … be present for longer portions of each year, it limits the growing season for this nursery,” she said.

The uptick in virus numbers also limits the potential for that movement to other farms out of concern for spreading the virus.

As “California is really pushing for the aquaculture industry,” Kunselman, also an advocate of seafood farming, is increasingly worried.

“A lot of the seafood that we eat in San Diego is imported,” she said. “If we want to see the success of aquaculture in Southern California,” we need to note the rate of growth of the virus and that “it’s already taking out one of the fastest and best-growing oyster species.”

The study is “a warning sign that we could grow other species and … have the same problem just because our [water] temperatures are so high,” Kunselman said.

The good news so far is the wild oyster populations in San Diego Bay and along our coast haven’t shown signs of the virus, she said.

“It’s possible they’ve already built up some type of resistance or resilience against it,” Kunselman said. “The goal is to continue monitoring those wild oyster populations.”

In addition to fewer oysters at the raw bar, a decline in oyster numbers has environmental impacts.

“Oysters are great at filtering water, “ Kunselman said, from algae to pollutants.

Having healthy wild oysters in our ecosystem “is really important for water quality and also for maintaining the shorelines,” she said, as oysters will build reefs that help prevent erosion.

Wild oysters working to keep the bay clean also aid the growth of the farmed oysters, Kunselman added.

“Having oysters in the environment is [a] positive benefit to the environment,” she said.