Invasive species like the Chinese mitten crab (Eriocheir sinensis), introduced roughly 100 years ago, pose a threat to native ecosystems and can do enormous ecological and economic harm. In its latest report, released just a few weeks ago, the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) once again highlighted the lack of internationally coordinated strategies to combat invasive species. The Alfred Wegener Institute (AWI) and Dresden University of Technology (TUD), together with partners from Belgium, France and Sweden, have now launched the European project “Clancy”. Its goal: to markedly reduce the mitten crab population in European rivers, improving their ecological status in the process.
Back in 1912, when the Chinese mitten crab was first observed in Northern Germany’s Aller River, few expected the oddly hairy crab species to spread so massively here. But today, more than a century later, it’s crystal clear: the species, which was most likely introduced in cargo ships’ ballast water, can now be found in the thousands in nearly every river and ditch along the coasts of the North Sea – from Northern France to Southern Scandinavia. Accordingly, the species has been added to the International Union for Conservation of Nature (IUCN) list of the world’s 100 most dangerous invasive species. Eight institutes from Belgium, France, Sweden and Germany have now joined forces to develop strategies for effectively reducing the mitten crab population through the EU-funded Interreg North Sea project “Clancy”.
New trap design virtually eliminates bycatch
As part of the project, a trap design developed in Belgium, and which has proven to catch the bulk of the crabs, is to be tested internationally. A channel is installed at the bottom of the respective body of water, which the migrating crabs fall into. The channel is designed so that, once they fall in, they can no longer climb back out. As a result, the crabs automatically crawl along pipes that lead to cages on the shore, which can be easily checked and emptied. “Thanks to this design, the trap works with virtually no bycatch, since all animals capable of swimming can escape the channel and only the mitten crab – one of the most invasive river crab species – and frogs are capable of traversing the pipes to escape the water,” explains project staff member Oliver Hauck from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI). “As a result, in the span of four years, roughly two million crabs were caught in just one trap in Belgium – and a total of three frogs, which were released.”
The trap design capitalises on the mitten crab’s unique biology. “Every autumn, the adult crabs, which can measure up to 30 cm from claw to claw, begin migrating toward the coast, sometimes covering several hundred kilometres in the process," Hauck adds. “They cross streets, climb over levees, and sometimes even appear in people’s yards and basements, until they finally reach the sea. Once there, they mate and die from exhaustion soon after. The next spring, the young mitten crabs begin their migration in the opposite direction. When the new trap is placed in areas with heavy migration traffic, like fish ladders or the mouths of rivers and ditches, many of the crabs can be caught.”
Fewer mitten crabs = healthier waters?
But why go to so much trouble? “The sheer mass of crabs that migrate along the rivers is a huge problem,” says Sengdavanh Thepphachanh, a researcher at the Institute of Hydraulic Engineering and Technical Hydromechanics, Dresden University of Technology (TUD). “Although no-one knows exactly how many there are, various sources indicate that we’re talking about several million. And we can safely assume that these massive numbers also negatively impact the aquatic ecosystem.” For example, studies have shown that the appearance of large numbers of mitten crabs corresponds to a decline in soil biota (worms, bivalves and insects) and water plants. In addition, they damage riverbank structures and clog up water intake points – like those of power plants located near major rivers. The crabs also pose a problem for fishers, as they not only eat their bait, but also damage their nets and traps and gnaw on the fish they’re trying to catch. “Therefore, if we can manage to substantially reduce the mitten crab population, we’ll also be making an important contribution to reaching the targets of the EU’s Water Framework Directive, since it will mean improving the ecological status of our rivers,” says Torsten Heyer, the project’s coordinator at the TUD.
Testing the trap at a variety of locations represents an important first step in that direction. At the same time, valuable data will be gathered on the size of the mitten crab population and interrelatedness within it. All results will then contribute to a Europe-wide strategy for effectively combating the mitten crab. “The project is the first step towards combating this invasive species across borders, and ideally, to restoring native waters and waterways to their original state. Needless to say, the mitten crabs caught in the process will have to be killed humanely. There’s also the question of finding a sensible use for the meat,” explains AWI researcher Björn Suckow, who coordinates public relations for the project. “Consequently, the project will be accompanied by public campaigns, along with information on the steps planned.”
Eight institutes, one goal
In this regard, the project’s collaborative international approach will be critical to achieving a lasting success. Along with Dresden University of Technology and the Alfred Wegener Institute, Flanders Environment Agency, the Province of East Flanders and the University of Antwerp (Belgium), GEMEL and the Cellule de Suivi du Littoral Normand (France), and the University of Skövde (Sweden) will contribute essential expertise in a range of areas.
In this context, the TUD’s research focus lies in particular on investigating the mitten crab’s rheotactic behaviour, i.e., on the question of which flow conditions it prefers and under which hydraulic conditions it can no longer migrate. Armed with this knowledge, it will then be possible to identify potential, optimal and water-body-specific locations for the crab traps. In addition, the TUD will install and maintain a trapping site on the Elbe near Dresden to determine whether, and if so, in what numbers mitten crabs from the North Sea are capable of migrating hundreds of kilometres upriver.
Besides the international coordination of public relations, the AWI will be responsible for maintaining, and assessing the catches for, four traps in the catchment area of the Weser River. It will also collaborate with partners from industry to find sensible use concepts for the crabs trapped. Here, the focus will be on using them for feeds in aquaculture and on harvesting chitin from their shells for use e.g. in the pharmaceutical industry.
This project receives funding under the Grant Agreement Number 41-2-51-22 from the Interreg North Sea Programme co-funded by the European Union. You can find more information about the project here: https://www.interregnorthsea.eu/clancy
