- Invasive signal carcinoma in rivers and streams increases
They arrive in banana boxes across the Atlantic, sometimes foreign animal species are also unwittingly introduced. And sometimes, humans intentionally introduce them, only to lose complete control over their populations. This is the case with the North American signal crayfish, which was initially intended to supplement the shortage of edible crayfish in Sweden decades ago. Now, it's wreaking havoc uncontrolled in waters across Baden-Württemberg, threatening native species like the stone crayfish and the noble crayfish, which were once abundant in every river, pond, and ditch.
The problem is that the invasive signal crayfish transmit crayfish plague, a fungus that causes disease in freshwater crayfish. According to the fisheries research institute, about 80% of invasive crayfish populations are infected. American species are largely immune to the fungal disease, but their European relatives die. Additionally, signal crayfish reproduce earlier and faster than native crayfish and inhabit the same spaces.
Populations of freshwater crayfish are being wiped out
"Both processes, direct competition and transmission of crayfish plague, typically lead to irreversible extinction of native freshwater crayfish populations in the presence of signal crayfish," the ministry responded to a query from the SPD state parliamentary fraction. In Baden-Württemberg, the occurrences of known crayfish species have more than halved in the past twelve years. More than two-thirds of this decline was caused by invasive signal crayfish or the crayfish plague they transmit, the document states.
However, freshwater crayfish are considered ecological keystone organisms. They have a significant influence on which other species exist in what densities in waters. If they lose the displacement competition to their imported relatives, the waters impoverish.
Signal crayfish are also spreading in small streams
This isn't just a problem for large rivers. According to the fisheries research institute in Langenargen (Bodenseekreis), signal crayfish are increasingly moving into smaller tributaries, where native species were previously relatively safe. The dovekie and stone crayfish are particularly affected by this invasion wave.
"Signal crayfish are a veritable nightmare for native freshwater crayfish species," says Jan Baer, a scientist at the fisheries research institute in Langenargen. There are already river systems that are lost.
So is everything hopeless? Not entirely, although the means against the signal crayfish are very limited. Experts at the fisheries research institute are pinning their hopes on small-scale reintroductions. However, the main hopes are on so-called crayfish barriers, i.e., obstacles in stream courses that prevent the spread of invasive species. "In many cases, sustainable containment of signal crayfish through crayfish barriers is the only promising management strategy," the ministry argues. The 77 barriers in the country are particularly relevant in small upper reaches of streams, where most of the remaining populations of the strictly protected freshwater crayfish species are located.
Signal crayfish are also edible
Of course, you can also catch, cook, and eat crayfish, as the crayfish plague is not harmful to humans. "They taste great," says Hilmar Grzesiak, the fish expert at NABU Baden-Württemberg. "If you serve scampi and signal crayfish, you'll hardly notice a difference." However, this is not a solution, according to the ministry, as there are too many of them and they reproduce too quickly. "Very high catch intensities are necessary for actual effects on population density, which would have to be maintained indefinitely for a lasting effect," it also states in the document.
Fish expert Baer still considers the use of all means necessary. "We have a responsibility and we have committed ourselves to protecting the populations of our native crayfish," he says. "If we don't do this, we will lose species that are only native to our region forever."
The Atlantic Ocean serves as a pathway for the introduction of foreign animal species, like the Atlantic, as banana boxes transport various organisms across its expanse. The spread of signal crayfish from the Atlantic, specifically from Sweden, has led to the decline of native species in Baden-Württemberg, such as the stone crayfish and the noble crayfish.
