Determining the status of invasive fish species in the Krka National Park area

Determining the status of invasive fish species in the Krka National Park area
In 2022 and 2023, research of the ichthyofauna of the Krka River continued, with an emphasis on establishing the status of invasive fish species in Krka National Park

Invasive fish species are an increasing problem in watercourses around the world. The impacts of these invasive species can be reduced by preventing their spread and removing individuals wherever possible, and this is a priority in biodiversity conservation in the European Union, including the Republic of Croatia. This most recent project aimed at combatting invasive species has established guidelines and proposed measures to reduce the abundance of invasive species, with recommendations for implementing these measures.

In order to establish the status of invasive fish species in the area of Krka National Park, research was conducted on the allochthonous (alien) invasive and introduced freshwater ichthyofauna in the Krka River and its tributaries.

In 2022 and 2023, five field surveys over a total of 10 days were conducted to sample the ichthyofauna. Standard collection methods were used to research the fish fauna, including electrofishing from shore or boat (active method) and sampling in the lake-like sections of the river using standardised single Nordic type fish nets (passive method). Fish traps were also set as an additional method. In the research, 20 fish species were sampled, with a total of 10,546 individuals belonging to 16 families. The most common families were the Leuciscidae and Cyprinidae, each represented by three species, while all other families were represented by a single species. Of the total 20 species recorded, six were alien: black bullhead, Prussian carp, eastern mosquitofish, pumpkinseed, topmouth gudgeon, and grass carp. Ten species are considered native fauna (autochthonous): Dalmatian rudd, Italian chub, freshwater blenny, Zrmanja chub, barbel, eel, Visovac goby, threespine stickleback, Adriatic trout, and flathead mullet. The remaining species (carp, pike, tench and perch) have been translocated from the Danube Basin.

Of the recorded species, the most abundant was the eastern mosquitofish (3749 individuals, 35.5%) followed by the topmouth gudgeon (2728 individuals, 25.8%), pumpkinseed (2023 individuals, 19.1%) and Dalmatian rudd (1223 individuals, 11.6%). These four species together accounted for 92.1% of all fish individuals caught, while other species together accounted for less than 2% of the total sample. Of all locations where sampling was performed, the highest number of alien and translocated species by far was recorded at Carigradska Draga – the lake-like expansion of the Krka River near the Krka Monastery, where 7660 alien and 48 translocated individuals were caught, followed by the location “Krka under Roški Slap”, where 1295 alien and 54 translocated individuals were caught. Taking into account the total number of individuals recorded during the research in the Krka River and its tributaries, the alien species were far more abundant, with 8705 individuals caught, compared to 1573 native individuals and 268 translocated individuals.

Invasive species are one of the greatest threats to the native fish community in the Krka River system. Today, both alien and introduced fish species are present in the Krka, and some have established stable and very dense populations where they dominate the fish community, in turn endangering the populations of native species. The densest populations are those of the pumpkinseed, pike, perch and black bullhead. Pumpkinseed, black bullhead and eastern mosquitofish are all included on the list of invasive alien species of concern in the European Union, which was updated in 2022. Their impact on the native fish fauna is many-fold. Adult individuals of these alien species, especially pike and perch, feed on young individuals of the native species, thereby reducing their reproductive success. Further, alien species create competition for food resources and suitable shelter, while also changing the habitat conditions. In that sense, the pike is a particularly grave threat due to the high density of its population.