Marennes-Oleron Oyster devastation - warmer Atlantic waters to blame

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Researchers have been scrambling to respond to a crisis that has rocked France's ostreiculture industry since last month: the discovery that 40% to 100% of oysters aged 12 and 18 months being raised in France's Atlantic cultivation beds had died. The reason, officials at the French Institute for Research Into Use of the Sea (Iframer) say, is Oyster Herpes Virus type 1 (OsHV-1). Scientists believe that the virus has proliferated along France's Atlantic coast due to a mild winter and abundant rains that allowed ocean water to remain warm.

 

These conditions created an abundance of plankton which the oysters have gorged on.  Fully fed and assured of more food whenever they wanted it, the youthful oysters turned their energies and attention to their sexual organs, leaving the rest of their system vulnerable to herpes infection. "Once sufficiently nourished to ensure for survival, oysters focus on development of sexual organs - creating sperm and ovaries mostly - and ignore their defense systems," explains Tristan Renault, the director of Iframer's genetic and pathology laboratory. "Oysters are capable of only one function at once, especially in springtime. So if a threat is posed during this period of concentrated sexual development, they're very susceptible."

 

To make matters worse, the same climatic changes that caused the abundance of herpes and plankton on the Atlantic coast - and which contributed to an explosion of jellyfish in Mediterranean waters have also caused a proliferation of Vibrio splendidus bacterium. The effects of that bacteria left younger oysters both more vulnerable to herpes infection.

 

To avoid that, Renault and his Iframer colleagues are looking for ways to battle the OsHV-1 threat. They want to know why cultivations in the Arcachon basin southwest of Bordeaux have been spared while virtually all others in France have seen most or all of their young oysters decimate. Experts will also look for reasons why a few individuals in those otherwise devastated populations survived. "Since we can't vaccinate shellfish, one way of protecting future generations is to use oysters that are resistant to OsHV-1 and generally more robust in reproduction [to produce] future oyster generations that may better withstand the virus," Renault explains.

 

The good news for oyster farmers and consumers alike is that OsHv-1 has no impact on older mollusks, nor on humans who consume them. With China producing over 80% of the world's oyster supply-Japan and South Korea are the next biggest growers - there's no great risk of a global shortage.