Oysters are a must on French dinner tables during the festive season, but shellfish farmers say climate change has caused production to plummet by up to 30 per cent.

Many French producers fear warming temperatures may force oyster farms to move north in search of cooler weather, possibly outside France.

Oyster production depends on cold winters when the shellfish “can rest and mature,” according to Yoann Thomas of the French Research Institute for Development.

Philippe Le Gal, head of the French shellfish producers’ association, said: “The warmer climate is beginning to have an impact. Oyster farmers are seeing volumes down by 20 to 30 per cent for 2018.”

Mr Le Gal told the AFP news agency that just a 10-gram decrease in the weight of each oyster brings down the price. France’s 4,500 oyster farmers sold about 100,000 tons in 2017 at an average price of £4,500 per ton.

Warmer sea temperatures also favour the spread of viruses that attack oysters. Over the past decade, up to three-quarters of young oysters have been lost to disease in some years, according to Fabrice Pernet of the Ifremer marine research institute. He warned that fish moving north to escape rising sea temperatures could bring new diseases to French oyster beds.

Another French festive delicacy, foie gras, has also suffered a decline in recent years as outbreaks of bird flu have forced massive culls of ducks and geese. But farmers say production has returned to normal after two lean years.