The project partner Public Institution “Sea and Karst” systematically implements monitoring and catch of the signal crayfish (Pacifastacus leniusculus) in the Jadro River, thus actively contributing to the preservation of its sensitive ecosystem. Particular attention is paid to protecting the endemic softmouth trout (Salmo obtusirostris salonitana), one of the most important species of the Jadro River. The signal crayfish, an invasive species originating from North America, presents a significant threat because it aggressively disturbs the natural balance and carries crayfish plague, fatal for native species.
The signal crayfish and a threat to the Jadro River biodiversity
Over last several decades, the signal crayfish has been causing serious problems in European water ecosystems, including Croatia. Its presence in the Jadro River is particularly worrying since it directly threatens the softmouth trout, a unique species living only in this region. This invasive species destroys habitats by burrowing tunnels at the river bottom, destroys sedimentation and degrades key spawning locations of the softmouth trout. Besides, it competes with trout fry for food, thus additionally reducing their chance for survival.
Rare/endemic white-clawed crayfish discovered
During catching actions, the white-clawed crayfish (Austropotamobius pallipes), a rare and protected native species, was found at the source of the Jadro River. Its presence is a remarkable find, since the white-clawed crayfish is highly threatened due to the spread of invasive species such as the signal crayfish, pollution and changes in the environment. Its presence additionally highlights the importance of implementing such measures as it points to water of high quality and a potential for the recovery of the natural ecosystem.
Integrated approach to the catch of invasive species
Our project field team uses a combination of fish traps and electro-fishers to successfully catch the signal crayfish. Fish traps placed at key points enable selective catch and monitoring of the population, while the electro-fisher emits low-frequency electric impulses that help draw crayfish from their shelters at the river bottom.
In addition to the signal crayfish, during monitoring and catching actions other invasive species that threaten the Jadro River ecosystem are removed as well, such as the carp (Cyprinus carpio) and the rainbow trout (Oncorhynchus mykiss). These species, imported for the needs of aquaculture and recreational fishing, often adversely affect the native species, including the softmouth trout, due to predation, competition, and habitat degradation. Combined catch of these species is part of a wider strategy to preserve native flora and fauna of the Jadro River.
Purpose and relevance of protective activities
The purpose of these activities is to reduce the number of invasive species and preserve natural habitats of the Jadro River, with special emphasis on the softmouth trout as the key endemic species. The removal of invasive organisms creates preconditions for the restoration of the natural ecosystem and preservation of biodiversity.
Discovering the white-clawed crayfish during catch actions confirms that such activities not only control invasive species, but also enable the recovery of sensitive native populations, which additionally stresses the importance of monitoring and active management of river ecosystems.
Through continuous work and cooperation with scientists, local communities and other institutions, the Jadro River can remain an example of how to successfully protect Croatia’s natural heritage, preserving its exceptional biodiversity.