They push their native conspecifics further and further back: signal crabs that come from the USA.
You can find them in the Isar, among other places.
Geretsried – He caught his eye while walking in the Isar meadows: A crayfish was crawling on the bottom of the small and clear stream.
"It could be a signal crab," said discoverer Kai Braunias afterwards.
"That's right," confirms Hans Buxbaum.
The Wolfratshauser is familiar with the animals that live in our local waters.
Buxbaum keeps an eye on the population of signal crayfish in the region for a very specific reason.
Signal spot on the scissors joint gives cancer its name
Kai and Tanja Braunias live very close to the Isar in Geretsried-Gartenberg.
The couple often goes for walks there.
Recently they wanted to cool off in the Isar.
They crossed a creek on their way to the river.
There in the water something moved - it was a crayfish.
"We were actually surprised," says the Geretsrieder by choice about this chance find.
After looking at a photo of the animal, expert Buxbaum immediately realized that it was a signal crab.
It has a smooth carapace surface and a conspicuous bluish-white signal patch in the scissor joint, which gives it its name.
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Isarfund: This animal caught Kai Braunias' eye.
© Kai Braunias
The signal crayfish is native to North America between the Pacific Coast and the Rocky Mountains.
In the 1960s, it was widely promoted and disseminated as a plague-resistant alternative to crayfish, reports the State Fisheries Association in a brochure.
The animal has a similar way of life and similar demands on its environment as the native crayfish.
Signal cancer threatens native conspecifics
"The problem is that the signal crayfish puts a lot of pressure on the native crayfish populations," says Buxbaum.
The Wolfratshauser is the official mussel supervisor and is also familiar with crabs.
Its territory is the Mooshamer Weiherbach, one of the largest occurrences of the river mussel in Bavaria.
In addition to the river mussels, there are also crabs there, which usually do not live in large rivers but in side streams that do not have the character of a torrent - such as the Mooshamer Weiherbach.
But according to the district office, crabs are also settled in lakes - such as the Walchensee.
I put the noble crabs back, the signal crabs go into the cooking pot.
Hans Buxbaum
“Noble crayfish and stone crayfish are native to Germany,” explains Buxbaum in an interview with our newspaper.
Although there are no stone crabs in the district.
But the native species are being pushed back more and more by the signal crayfish that has been introduced, for one simple reason: it produces many more offspring.
"They drive out the native crabs through turf wars," says the expert.
Worse still, the animal can transmit crayfish plague, a fungal disease.
"The signal crabs are immune, the native crabs die from it."
TUM plans to examine the crabs
There are crayfish and signal crayfish in the Mooshamer Weiherbach.
"In the lower reaches at the entrance to Ascholding, there's also a mixed stock."
The Technical University of Munich (TUM) wants to find out in the fall whether this is really the case.
"It is to be investigated whether the signal crabs carry the fungus," announces Buxbaum.
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Hans Buxbaum is also familiar with crabs.
© Sabine Hermsdorf-Hiss
And how is it that the signal shrimps at the Mooshamer Weiherbach have not yet spread further?
"We're lucky that the Siegertshofer Weiher has a very high dam at the southern end," explains Wolfratshauser.
There the water runs vertically down a pipe.
The stream then continues to Ascholding.
Buxbaum: "The signal crabs have not yet come over this dam and the pipe." That is a stroke of luck, because the animals can migrate over land.
Animals are subject to the fish law
Since the spread of the signal cancer can hardly be controlled, one can only try to keep the population within limits.
The animals are subject to the fishing law and may be removed.
Buxbaum catches them with crab traps.
"I put the crayfish back, the signal crabs go into the cooking pot," he says.
On average, one or two copies go online every month.
"So fighting this invasive species has at least a culinary purpose."
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