From reuters:

“A herpes virus is killing young oysters in France because they have spent too much energy developing their sexual organs rather than their natural defenses, an oyster crisis team has found.

Scientists have spent weeks investigating a mysterious surge in mortality among the mollusks that the French love to devour with lemon and white wine.

France’s main marine research institute, Ifremer, set up the crisis team on July 3 and its members have been working flat out to understand why 40 to 100 percent of oysters aged 12 to 18 months were dying in all but one of France’s breeding areas.

An Ifremer spokeswoman said on Monday the team had established that a virus called Oyster Herpesvirus type 1, or OsHV-1, was killing young oysters, helped by unfavorable weather conditions that had weakened the mollusks.

“We had a warm winter followed by a rainy spring, which caused high levels of planktonic plant life to develop,” spokeswoman Johanna Martin said.

“This meant that the oysters were particularly well fed and spent a lot of energy developing their sexual organs to the detriment of their natural reserves, leaving them vulnerable to OsHV-1,” she said.

There is no cure for OsHV-1.

Ifremer is continuing its investigations and admits that other factors could be contributing, such as toxic seaweed or Vibrio Splendidus, another virus present in France this year.

France produces about 110,000 tonnes of oysters a year, according to Ifremer data. It is the world’s fourth biggest producer after China, which alone accounts for 83 percent of world production, followed by Korea and Japan.

All of France’s oyster breeding areas, of which 90 percent are on the western coast, are affected by high mortality rates except one area at Arcachon in the southwest. Scientists do not know why Arcachon oysters have been spared.”