Mussel farmers in northern France are grappling with a deepening crisis as spider crabs devastate their crops. A prized marine resource themselves, the crustaceans have become relentless predators of mussels and other shellfish – threatening the future of the industry.
In the gulf between Brittany and Normandy, the spider crab population has exploded fourfold over the past decade. They’re fished year-round these days, no longer just from early autumn to early spring.
This season, farmers in the bays of Saint-Brieuc and Fresnaye, on Brittany’s picturesque northern coast, were alarmed to discover the spider crabs had eaten most of their crop.
It’s a problem that has persisted for years but which the regional mussel farmers union says is now untenable. Local businesses are reporting losses of millions of euros and calling on the authorities for urgent help.
Sought-after molluscs
The shallow bays, fed by nutrient-rich waters from the Atlantic, have for generations supported France’s treasured bouchot mussel industry: mussels grown on vertical wooden stakes that are submerged during high tide and exposed at low tide.
Their unique cultivation method provides a distinct tender texture and sweet taste that distinguishes bouchots, introduced to Brittany in the 1960s, from other varieties of mussels.
As well as losing this year’s harvest, producers were dismayed to discover that future yields had also been destroyed when juvenile spider crabs devoured the baby mussels, known as seed, in June.
"When the tides rose, we realised that 80 percent of our baby mussels had been eaten in just four days," farmer Cédric Serrandour told RFI – adding this episode alone had cost him 50,000 euros.
Serrandour's family business, Les Merveilles du Cap, has been growing bouchot mussels and hollow oysters in the Bay of Fresnaye for four generations.
"If we don't have baby mussels for the 2025 season, it will be catastrophic. We're talking about a 'blank season'," he said.
A video shared by Les Merveilles Du Cap shows empty mussels shells lying in the water.
Serrandour estimates that over the past five years he’s lost about 500 tonnes of mussels. With the ongoing surge in spider crab numbers, he expects that at least 200 tonnes of next year's harvest will be destroyed.
Frustration is growing over what farmers say is a slow and inadequate response from the government.
To draw attention to the crisis, industry workers in July turned out to protest, blocking Brittany's famous Rance Dam station – a key landmark known for its pioneering role in tidal energy.
Mussel farmers say a lack of immediate financial support has left many of them on the brink of collapse. "We are asking for direct assistance," Serrandour said, warning that many farmers won't survive if they don't get help soon.
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Crabs a resource too
Although a menace to mussel farmers, Atlantic spider crabs now provide a steady income for local fishermen who are able to catch them outside of their traditional fishing season.
Known for their sweet if somewhat chewy meat, the crustaceans – protected under the Bern Convention – are an increasingly profitable resource for seafood markets across Europe.
They’re sold either whole or packaged up as pulp for use in recipes such as pies, spring rolls and salads.
The French Research Institute for Exploitation of the Sea (Ifremer) has been tasked with studying the behaviour, biology and evolution of spider crabs, with the aim of developing strategies to mitigate their impact.
So far crab traps and nets have done little to stem the problem.
Figuring out why the crabs have managed to become so abundant so quickly is a challenge, Ifremer biologist Martial Laurans told RFI. One obvious factor, though, is warmer waters.
"Spider crabs need temperatures of around 15 or 16 degrees to begin reproducing," Laurans said, adding that the period when temperatures are favourable has become longer.
"We're also working to understand their movements ... how the spider crabs have been able to arrive at the mussel concessions."
Spider crabs are sensitive to noise, so last year Ifremer tested a method using sound to deter them along the Channel coast. Chains attached to trawlers were used to generate vibrations in the hope of scaring them away from mussel farms.
While this approach drove down the number of crabs, it hasn’t eliminated the problem.
Protected species
Because the spider crabs, which are native to Breton waters, are protected by law, mussel farmers are not allowed to destroy them. They do, however, have permission to catch the crustaceans and release them offshore.
What’s needed, Laurans says, is a long-term solution that balances the needs of both the fishermen and the mussel farmers.
"Spider crabs are a resource for fishermen. Mussel farmers don't have a licence to fish them, so they have to work with fisherman to try to reduce the numbers close to their concessions," he said.
However, producers are pushing for spider crabs near farming zones to be classified as pests – a move that would permit more aggressive control measures.
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