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Named after the dense mats of hair on their front claws, Chinese mitten crabs are intimidating creatures. Their dark brown bodies can grow as big as three inches (eight centimeters) and with claws outstretched, they can span 10 inches (25 centimeters) – roughly the size of a dinner plate.
Classified as one of the world’s 100 worst invasive alien species, the crabs are near indestructible: they feed on almost anything, can survive on land as well as in fresh and saltwater, and they have a rapid reproductive rate – with females producing between 250,000 and 1 million eggs per spawning.
They were first spotted in Europe over a century ago, in a river close to Bremen, Germany. It’s likely they made the journey from their native Asia in the ballast water of ships. Since then, populations across the continent have exploded. Today, 18 of the European Union’s 27 member states have established populations of the crab, and it’s included on the body’s list of invasive alien species of concern, which notes its negative impacts include disrupting the aquatic food chain, transmitting crayfish plague, and increasing the erosion of dikes and banks through its burrowing.
Scientists across the continent are looking for ways to reduce the mitten crab population, and in doing so protect the native ecosystems being destroyed by the invasive species. In 2023, they united under an EU-funded project named “Clancy.”
So far, the most successful strategy has been a trap developed by the University of Antwerp and the Flanders Environment Agency in Belgium. Three such traps have been installed in the country since 2018, catching an estimated 3 million crabs, according to Jonas Schoelynck, professor of aquatic ecology at the University of Antwerp. A metal channel is installed across a river, which the crabs fall into as they sidle along the riverbed. Unable to swim out like other aquatic species, they are forced to crawl along pipes leading to cages on either shore, where they can be collected in their masses and disposed of.
Another trap of the same design, independent of the EU project, was installed in the UK last year, and the group plans to roll out more across Belgium, Germany, France and Sweden in the coming years.
Crabs invading cities
The traps will target areas through which large numbers of crabs migrate. During autumn, the adult crabs migrate from freshwater to the sea to release their eggs, and in spring, the young crabs travel from the sea up rivers. They cover astonishing distances, as far as six miles (10 kilometers) a day, says Schoelynck, adding that some have been sighted more than 600 miles (1,000 kilometers) inland.
It’s during these month-long periods that they are at their most visible, gathering round sluice gates or moving overland to avoid weirs and other obstructions.
“Sometimes they take a wrong turn and end up in the city center,” says Schoelynck, who recalls hundreds of crabs crawling through the streets of Lier, a small Belgian town that lies on the confluence of two rivers. He has even heard testimonies of them climbing up walls and sneaking into the moist bathrooms of people’s homes.
The crabs are thriving in European environments for a combination of reasons, he explains. Like most invasive species, they have no natural enemies, are opportunistic feeders and are very resilient. But climate change could also be playing a part, as warmer waters could be helping to make the crabs feel more at home.
There is also the species’ strong genetic diversity. Schoelynck says that scientists in Europe have reported crabs that are a hybrid of both the Chinese and Japanese species of mitten crabs. “When you become a hybrid, all of a sudden you are stronger than your competitors and you are better able to survive the challenges. This, along with the better water quality (in recent decades), along with maybe climate change, (is contributing) to the new boom that we see occurring today,” he says.
The EU estimates that invasive alien species cost its members €12 billion ($13 billion) a year in economic damage, and its Biodiversity Strategy for 2030 commits to managing these species and preventing their spread.
Scientists believe the traps offer a cost-effective solution. “The system is really neat, really simple,” says Björn Suckow, a scientist at the Alfred Wegener Institute, which is based in Germany, and involved in the Clancy project. He explains that if rolled out more widely, they would be placed in small, accessible tributaries rather than deep, wide rivers, and during the busy migration season, the boxes on either side of the bank would have to be checked and emptied regularly to avoid a build-up of crabs.
Other trapping methods, such as the use of nets, are more labor intensive and costly, says Schoelynck. “On a good day, in Belgium, you might catch 80 to 100 crabs in a day (using a net), whereas with the new trap, you catch 8,000 to 10,000 a day.”
It also has far less bycatch, he adds. “If you use nets or fykes (a tubular net with hoops), you will catch fish, even endangered fish that you’re trying to protect. With this trap, there is – up to now – no side effects seen on any of the aquatic organisms.”
A humane end?
One of the major unanswered questions is what to do with the crabs once they have been collected. Suckow explains that different countries in the EU have different animal welfare rules. In Germany, scientists are only allowed to kill crabs by boiling or using an electric current. Whereas in Belgium, they have been freezing the crabs in buckets.
“We expect that the project will bring feedback on (the most) humane way to kill them,” adds Suckow, as well as potential uses for the dead crabs.
While mitten crabs are a seasonal delicacy in China, Schoelynck says that they have little meat and therefore aren’t popular in the European market. He adds that many of the crabs caught in the traps will be juveniles, whereas those eaten in China are the larger adults. However, the crabs collected in one of the Belgium traps go to a local zoo to be used as animal feed, he says.
The scientists recognize that it is a work in progress, but they hope that the coordinated European effort will help to gather knowledge and data on the invasive species and its impact, helping to form an international strategy for controlling its population.
“If we know where the problem is, how big it is, and how these populations are connected to each other, we can pinpoint certain locations on the continent,” says Schoelynck. “We need to have a coordinated action to help each other out, because there are no political boundaries for these animals.”
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