The Chinese mitten crab: An underwater invasion
The Chinese mitten crab is taking over Flemish rivers. Why is this a problem? Discover the impact on waterways, biodiversity and people — and how science is seeking answers.
One Chinese mitten crab seems like a harmless little creature. Together, however, thousands of these intruders with ‘woolly’ pincers are posing a threat to our river system. Native to East Asia, these crabs first appeared in Europe more than a century ago, presumably as stowaways in the ballast water of ships. The species was first spotted in Antwerp in 1933. Although it remained relatively rare for a long time, the mitten crab has become widespread in the past 10 years.
Cleaner and warmer
One key explanation is the improved water quality. ‘Chinese mitten crabs live in fresh water but migrate to the brackish water area between the sea and the river to reproduce,’ explains Jonas Schoelynck, a researcher and aquatic ecologist at the University of Antwerp (Faculty of Science).
‘These treks can be hundreds of kilometres long. Pollution used to be an obstacle, but these crabs can now be found in all rivers.’ Other factors include climate change and the import of genetically modified crab species: warmer water temperatures increase the survival rates of crab larvae.
Disrupted ecosystem
The impact of the mitten crab is primarily ecological. Schoelynck continues, ‘As omnivores, they are displacing native species. They also damage aquatic plants, such as Elodea and Potamogeton. They do not just eat them up in one go. Instead they snip and tear them apart. As a result, plants are disappearing faster than they can grow back.’
This is a problem, as plants produce oxygen, filter out pollution, trap soil particles and provide shelter for other species. Without plants, the water becomes cloudy, and the ecosystem loses its balance.
In caves and cellars
Chinese mitten crabs also burrow into riverbanks. Schoelynck explains, ‘If you look closely, you’ll regularly spot oval-shaped holes along the Scheldt — alongside the classic round mouse holes. You can see the flattened shapes of crabs in them. Thus far, they seem harmless, but millions of burrows can also pose a risk to dykes. That is certainly being monitored closely.’
The crabs sometimes cause a nuisance in village centres situated along waterways. ‘Near locks, they turn up in cellars or on the street — where they get run over and start to rot,’ says Schoelynck. Recent studies by colleagues at UAntwerp have indicated that anglers are also being affected: more than 90% say that the crabs are having an impact on their hobby.
We used to catch 300 crabs in a day. With the new trap, the current daily record in Grobbendonk stands at 17,000 crabs.
Trap on the riverbed
Schoelynck and his team are conducting a wide-ranging study of how the species behaves in the wild. ‘For example, what they eat, how they reproduce, how they compete with other species — as well as the impact that this has on biodiversity.’ Together with the Flanders Environment Agency, he developed a crab trap that was tested in the Mesodrome, an artificial river on Campus Drie Eiken.
The trap resembles a half-open tube that is fixed to the riverbed like a letterbox. Fish swim over it, but crabs wander into it and cannot get out again. As a result, we have recently come to realise just how big the problem is. ‘We used to catch 300 crabs in a day with a net. The current daily record in Grobbendonk stands at 17,000 crabs.’
Monitoring for PFAS
The crabs that are caught are not simply destroyed. As part of the Interreg CLANCY project, Schoelynck is involved in the search for applications, including fermentation for biogas production. The researcher has also demonstrated that crabs caught in the area provide useful information about the presence of PFAS and other pollutants in waterways.
Finally, in another interesting project, the research group is focusing on how we can control the population. ‘One promising approach appears to be capturing the animals in the breeding area, but further research is needed to determine how feasible this is.’
Jeroen Olyslaegers, French music and Chinese mitten crabs
Discover the musical performance about endangered native species at the STROOM festival: ‘The penultimate Carnival of the Animals’. To mark the occasion, festival curator Jeroen Olyslaegers has written a new piece in which he introduces various animals to the audience. At the concert venue, there will be several aquariums containing Chinese mitten crabs.