Oysters in the Anthropocene

This story was first published on Bracing For Impact, a reporting project featured on the crowdfunded journalism platform Beacon Reader.

All along the coasts of France, oyster farmers are listening for death. As they haul mesh bags of young oysters from brackish waters and give them a shake, they hope they won’t hear the rattle of empty shells.

Annie Castaldo farms oysters in the Étang de Thau, the largest of a chain of lagoons that stretch along the southern coast of France, near Montpellier, adjacent to the Mediterranean sea. Though she didn’t lose oysters this spring, her neighbors suffered losses of 60 percent, she says. “I’ve seen all these little oysters dead. It was upsetting.”

A virus has sent French oyster stocks tumbling since 2008. The disease mainly strikes young Pacific cupped oysters (Crassostrea gigas) less than a year old. When it hits, it can kill most of a farm’s young stock within a few days. Over the years, farmers have seen, on average, 70 percent of their crop wiped out, but mortalities can be as high as 100 percent.

Oyster farms on Utah Beach, in Sainte-Marie-du-Mont, France.

Before the outbreak, France produced around 126,000 tons of oysters annually. By 2011 its output had plunged to 84,000 tons, according to the UN Food and Agriculture Organization. Although France is one of the larger oyster producing countries affected by the virus, it isn’t the only one. The virus has also attacked oysters in Ireland, the United Kingdom, Australia, New Zealand, and the United States, a global industry worth an estimated $4 billion.

The virus threatens livelihoods—many French oyster producers are small family-run affairs. Castaldo says she’s watched businesses shut down and families fall apart. “People separate, they divorce, they have suffered major depressions,” she says. The virus may be key to the deaths, but science suggests that warming ocean temperatures may be coaxing the virus along—or weakening the oysters’ defenses.

Small Changes, Big Impact

Tristan Renault, a scientist who studies diseases in marine animals at Ifremer, the French Research Institute for the Exploitation of the Sea, and his colleagues first identified the virus in the 1990s, when it was little more than a nuisance. At that time, hatcheries had been reporting high death rates among their oyster larvae, animals less than 15 days old. Within a couple of years, there were reports of spat—animals under a year old—dying in large numbers in the open water, but the outbreaks remained localized, says Renault, who pinned the deaths on a new virus they named oestrid herpesvirus-1 (OsHV-1).

It takes roughly three years to grow an oyster to market size.
The full force of the disease wasn’t felt until 2008. That summer, oyster producers reported mass mortalities of their juveniles, first in the south and then up along the Atlantic coasts. “It was in all the breeding grounds, from the Mediterranean to Normandy, from the south to the north of France,” says Renault. Estimates put the death count in the billions. When Renault and his colleagues took a closer look, they discovered a new variant of the virus (OsHV-1 μvar) was associated with the swift attack and widespread mortality events.

Renault was beginning to suspect that water temperatures influenced the timing of OsHV-1 outbreaks. When asked, oyster farmers often mentioned increasing sea water temperatures before the deaths took hold, and scientists were detecting OsHV-1 when temperatures increased rapidly. During the summer of 2009, Renault and his colleagues positioned bags of wild-caught juveniles in the Marennes-Oléron basin in the southwest of France. Death arrived for the young oysters as spring transitioned to summer and water temperatures rose quickly, passing a threshold of 61 F.

In Hot Water

Climate change and warmer ocean temperatures could be one of the factors contributing to the appearance of the disease. But Renault says they need more data to link oyster deaths to climate change. “But that doesn’t mean the virus doesn’t do better in current conditions,” says Colleen Burge, a specialist in aquatic animal health at the Institute of Marine and Environmental Technology at the University of Maryland Baltimore County, who has studied OsHV-1 infections in Pacific oysters in Tomales Bay, California. “With increased water temperatures and an increased length of stay of permissive temperature, it could be more of a problem.”

Summer time ocean temperatures have been on the rise in the Northern Hemisphere since the mid-1970s. For the past decade, France has experienced warmer than average temperatures during the summer months. Marine researchers are just beginning to understand how climate change might influence marine infectious diseases. Disease outbreaks among sea fans in the Caribbean have been linked to warmer temperatures and black abalone in California waters are hovering near extinction due to a rickettsial disease, which occurs at elevated water temperatures. Elsewhere, oyster farmers are struggling with the early effects of ocean acidification.

But not all oyster farmers are convinced the industry’s problem lies solely with rising water temperature. The transition to factory-produced oyster larvae may also be contributing to the high levels of mortality, says Castaldo.

Oyster farmers use white washed tiles like these to capture wild oyster larvae from the ocean. The larvae cement themselves to the tiles and are later removed and transferred to bags.

According to Castaldo, the farmers who have been hit the hardest in her region are those who buy sterile oyster larvae that have been bred in laboratories and raised in hatcheries. The genetic modification allows the farmers to harvest oysters in the summer months and supply restaurants during the tourist season.

Castaldo is a natural oyster farmer. She buys wild oyster larvae from farmers on the Atlantic Ocean, in the Charente or Arcachon, who capture floating juveniles on submerged whitewashed tiles and plastic discs. Castaldo says oyster larvae became expensive for a couple of years, when there were less of them. The juvenile oysters she buys were likely exposed to the virus, but survived and remain resistant to its effects.

Scientists are now searching for a way to curb the epidemic, by identifying oyster lines that are resistant or resilient to the infection. In California, Burge is selectively planting oysters to find those that are genetically resistant to the virus, as is Renault, in France. “The oysters and this virus are an old couple. They have evolved together, and known each other for a long time. Oysters have the capacity to fight it off,” says Renault. “But it is the human activities that we have to question.”

Crassostrea gigas.

Header image by jacme31, licensed under CC BY-SA 2.0

Edited by Sarah Webb.

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