Oysters in peril as climate warms
Science in race to restore shrinking habitat
Humancaused climate change is increasingly harming oysters in Tomales and San Francisco bays and could soon devastate shellfish across California, as the chemistry of the water in estuaries morphs and livable habitat shrinks, a UC Davis study has found.
Even moderate changes in water temperature, acidity and dissolved oxygen make it harder for native and commercial oysters to grow their calciumbased shells, a situation that does not bode well for the future, concluded the paper published this week in the journal Limnology and Oceanography.
It means the severe climatic changes predicted as the Earth warms over the next few decades could dramatically shrink the habitat for both farmed oysters and the native species that scientists have been trying desperately to restore, said Ted Grosholz, a professor of environmental science who led the study with funding from a California Sea Grant.
“The issue here is that we are going to see these estuarine areas get smaller and smaller over time,” said Grosholz, who works out of the university’s Bodega Marine Lab in Bodega Bay. “This zone of estuarine influence will be increasingly pinched on both the ocean and river sides and there will be less habitat because of this.”
The study focused on native Olympia oysters, which once blanketed subtidal regions from
Southern California to southeastern Alaska before they were nearly wiped out, and commercially grown Pacific oysters, a $25 million industry that supplies many restaurants in the Bay Area.
Both Olympias and Pacifics grow in Tomales Bay, where Grosholz and his colleagues planted juvenile oysters from both species in test beds in nine locations from 2014 to 2017. They monitored the growth, health and mortality of the oysters during a variety of seasonal conditions and measured factors including water chemistry, temperature, salinity and oxygen levels.
The researchers also captured and studied tiny oyster larvae, which normally float in the water column before settling on rocks and other hard surfaces.
The study found the lowest growth and survival rates in areas impacted by freshwater runoff and pollution during winter rains. The oysters also suffered when there were upwellings of warm, acidic sea water low in dissolved oxygen during the spring and summer.
All of these factors — higher water temperatures, ocean acidification, lack of oxygen and intensive runoff during winter rains — are expected to become bigger problems in estuaries as more carbon dioxide from car exhaust and factories wafts into the atmosphere.
Grosholz said acidification combined with the other chemical changes related to climate change that occur in the brackish water of bays and estuaries will lead to greater stress on oysters. Those effects will be exacerbated in areas where the water chemistry fluctuates the most — near the ocean and where rivers and creeks dump out.
It is important because Tomales Bay is home to a thriving trade in Pacific oysters, natives of Japan that are incapable of reproducing naturally in California’s Mediterranean climate and must be replanted after every harvest. Somewhere around 6 million oysters a year are grown in Tomales Bay. About 10 million a year are grown in Humboldt Bay, in Humboldt County in far Northern California, where Marinbased Hog Island Oyster Co. recently established a hatchery.
John Finger, president and coowner of Hog Island, in Marshall, said oysters started showing signs of distress 10 years ago when hatcheries in Oregon began having trouble producing oysters and larvae. He said adequate shell formation appeared to be the problem.
“Yes, we’re concerned,” said Finger, whose oyster farm is one of seven companies on the West and East coasts that founded the Shellfish Growers Climate Coalition “We’re trying to let people know that we are already seeing effects, and we think it’s going to get worse. This isn’t theoretical. This is happening and we’ve got to be conscious of it.”
It is a huge issue for biologists who have spent the past two decades trying to reestablish native Olympia oysters in San Francisco Bay. The mollusks, once a crucial source of food for native Americans and among 49ers during the Gold Rush, are considered a vital cog in the Bay Area marine ecosystem because they filter and clean water and form shell beds that provide habitat for a wide variety of aquatic life.
In 1893, Olympia oyster beds covered 8,033 acres in Newport Bay in Southern California, Elkhorn Slough on Monterey Bay, San Francisco Bay and Humboldt Bay, according to a study published a few years ago in the scientific journal Proceedings of the Royal Society B. Almost 500,000 oysters per acre once crowded together along the bay floors, according to the report.
Because they were so small — about the size of a 50cent piece — it took 1,600 to 2,000 shucked Olympias to make a gallon. So it didn't take long for the oyster beds in San Francisco Bay to be depleted. Although some harvesting continued in some areas of California until the 1930s, wild oysters in the Bay Area were pretty much wiped out by 1911.
Researchers have been placing shellmound reefs in numerous locations in San Francisco Bay in an attempt to catch larval oysters as they drift over the silty floor in search of places to settle and grow. Scientists hope to restore the species at Quartermaster Reach, a vacant lot the Presidio of San Francisco is planning to turn into a 7acre expansion of Crissy Field Marsh within the next year.
The restoration efforts are already facing serious obstacles, even without climate change. Huge numbers of the Olympias in Tomales Bay and San Francisco Bay are being devoured by voracious alien whelk snails, known as Atlantic oyster drills. In some places, 80% to 90% have been eaten by these gluttonous predators.
There is still hope, said Grosholz and his colleagues. It may be too late to prevent the climate from changing, he said, but people can reduce sediment runoff and contaminants from dairies and ranches. More new habitat and infrastructure, like the project at Quartermaster Reach, can also be built in San Francisco Bay and other estuaries.
“The Goldilocks zones within California estuaries, where conditions are just right for maximum growth and survival, are incredibly important for the economic success of farmed oysters and the ecological success of native oysters,” said UC Davis graduate student Jordan Hollarsmith, who coauthored the study. “Our findings help us to understand the environmental factors that make these zones so favorable, which will aid in restoration and aquaculture efforts.”