Hundreds of French oyster producers have equipped their parks with fake molluscs containing high-tech anti-theft devices after an upsurge of crime.

The spy oysters look like the real thing, but send a warning message to the producer’s smartphone if they are subjected to a sudden change of temperature or unexpected movement. They also contain a tracking device that enables police to follow a batch of stolen molluscs.

The development of the electronic surveillance system follows a rise in the quantity of oysters being stolen around the French coastline. About 20 tonnes were taken in the first nine months of this year, compared with an average of about 15 tonnes over the same period in recent years.

They also contain a tracking device that enables police to follow a batch of stolen molluscs.

The run-up to the festive season, which represents about 40 per cent of the annual oyster sales in France, has led to further acceleration in criminal activity. Last week, a total of 4.5 tonnes of oysters and 200kg of clams worth a total of about €20,000 were stolen from the Port Blanc oyster farm in Baden, Brittany.

France is Europe’s biggest producer of oysters, with overall sales worth about €1 billion a year.

A false oyster emits a signal that allows the owners to be informed in case of theft.

Emmanuel Parlier, the founder of Flex Sense, a start-up that specialises in oyster protection systems, said that thefts were nothing new in the sector, but with producers merging to form bigger and more professional farms, the size of the hauls was increasing.

“The thefts often occur between professionals,” he said. “Mechanisation means that they have boats, lorries and forklift trucks, which makes it easy to take their neighbours’ oysters.”

“The thefts often occur between professionals.”

His start-up put the spy oysters on the market last year after three years of research and development and sold a few dozen, but sales have grown by 20 times this winter compared with last year. Mr Parlier, 41, said that he now had 400 or so customers in France and was registering interest in other European countries and from oyster farmers in the US.

Producers take batches of six or twelve spy oysters, which they hide among the real ones in their parks. The fake molluscs are rented at a cost of €10 each per month.

“If there is a suspect movement that is characteristic of a [theft], they send a message to the smartphone,” Mr Parlier said. He added that he had been working with police in rural areas, who have their work cut out to prevent oyster thefts.

Even the Republican Guard, the elite police unit whose members are more used to protecting President Macron and visiting heads of state and government, has been drafted in. Councils in Normandy have agreed to let the unit keep its horses in local stables for free. In return, it has agreed to patrol oyster beds on horseback.