Oysters have been mysteriously dying in the worst crisis to hit France's shellfish industry in 40 years.

In the last few days farmers have lost between 40 and 100 per cent of their oysters aged one to two years old.

Such a death toll has not been seen in France since 1970, when virtually an entire harvest was wiped out. Experts are unsure if the shellfish are succumbing to a viral epidemic or to poor water conditions caused by global warming.

Mortality rates are always higher in summer but not on this scale, and never simultaneously throughout France, experts said.