Threatened Species: the Stone Crayfish
The alteration of water bodies and the spread of invasive species and their pathogens are the main threatening factors causing a significant decline in the stone crayfish population.
Stone crayfish by Ildikó Varga.
One target of EU Biodiversity Strategy 2030 addressing #IAS aims to reduce the number of Red List species threatened by invasive alien species by 50%. Nevertheless, due to the continuously connected running water system, the protection of freshwater species is a major challenge. Thus, let us give you some interesting examples of protection possibilities and constraints in our Threatened Species series.
The stone cray is the smallest size native species in Europe and is considered a keystone species in freshwater ecosystems. It lives in small, fast-flowing, clean, stony-bottomed watercourses in mountainous areas, though, in these waters, it prefers slower-flowing, deeper outfalls. Nocturnally active, it hides in sheltered places during the day, under rocks, among roots and in burrows. It breeds in mid-autumn; the female carries the eggs and then the young under her abdomen until the first moult. She lays fewer eggs than competing species.
The stone crayfish is listed as a priority species in the Annex II of the Habitats Directive. The species was classified as "DD" (data-deficient species) in the IUCN Red List in 2010. Most of their range — in Middle- and South Europe, in the Danube River Basin and the Balkans — has a significantly decreasing population size. Besides water pollution, the alteration of water bodies and the spread of invasive species and their pathogens — namely, crayfish plague — are the main threatening factors which have caused a significant decline in the stone crayfish population.
In the latest Article 17 report for the 2013-2018 period, the conservation status of species was assessed in 19 cases — covering 12 Member States. Only in two cases was the overall assessment favourable (FV), in 14 cases unfavourable inadequate (U1), and in two cases unfavourable bad (U2). In 10 of the 19 assessments (for Austria, Germany, Croatia, Hungary, Czech Republic and Greece), invasive alien species or their diseases were considered to be of a high (9) or medium (1) importance threat.
Three alien invasive crayfish species — namely, Orconectes limosus, Pacifastacus leniusculus, Procambarus clarkii — are known to be expanding in Europe; they carry the crayfish plague vector but are themselves immune to it. In the water bodies where they occur, the native decapod species is significantly declining and, in the longer term, will be completely replaced.
What is the best method to protect against them?
Electro-fishing used to sample fish is suitable for catching and collecting invasive crayfish, but only in smaller waterbodies and in combination with other methods which prevent the accession of invasive crayfish into new watercourses.
The accession can be prevented with chutes built-in into the bottom. In the case where Orconectes limosus has been shown, they did not use the fish pass in an upstream direction against the water flow (Kozák et al. 2004).
Finally, research is also needed to understand the resistance to the pathogen of crayfish plague to ground for a possible biological control.