Research on the distribution and state of populations of target species of freshwater fish in Krka NP

Research on the distribution and state of populations of target species of freshwater fish in Krka NP
During 2021 and 2022, the Faculty of Science, University of Zagreb conducted research on the distribution and state of the population of target species of freshwater fish in Krka National Park. The lead investigator was Professor Dr. Davor Zanella.

The research was conducted with the aim of collecting new and additional data on the populations of freshwater fish focusing the habitat and distribution of target species. These data will be used to draft the foundation for zoning areas of the Natura 2000 ecological network. The research results will serve in developing specific measures to conserve target species and determine indicators of success of the implementation of those measures.

The Krka River is the watercourse of the Adriatic Basin with the highest number of freshwater fish species, and this diversity puts it side by side with much larger rivers, such as the Neretva or Cetina. Due to the large number of recorded species, the Krka is one of the most interesting ichthyological areas in Croatia, and also at the European scale. Despite numerous studies, the actual catchment area of the Krka River has only been partially mapped, since the entire watershed lies in the karst landscape and therefore the boundaries of underground watercourses are not fully known. The programme to research the distribution and state of populations of target species of freshwater fish in Krka National Park encompassed research of the freshwater ichthyofauna of the Krka River and its tributaries within the park boundaries in Šibenik-Knin County. Sampling the Krka River and its tributaries was performed to obtain a sample sufficiently representative to give a realistic overview of the fish population at every station. Eight field surveys were conducted on 16 sampling days in 2021 and 2022 to sample the ichthyofauna. Standard electrofishing procedures were used in the survey from the banks or from a boat (active method) and sampling in the lakes was performed using standardised single-layer Nordic-type nylon nets (passive method). Additional methods included angling and setting traps. The survey recorded a total of 24 fish species, with 12,279 individuals belonging to 16 families. Of the total number of recorded fish species, six are alien species: black bullhead, gibel carp, Eastern mosquito fish, pumpkinseed, topmouth gudgeon and grass carp. Eight species recorded were native, endemic species: Dalmatian barbelgudgeon, barbel, Visovac goby, Dalmatian minnow, Dalmatian rudd, Illyrian chub and Turkskyi dace. In the total catch, the Eastern mosquito fish was most abundant (43.7%), followed by the topmouth gudgeon (24.1%), Dalmatian rudd (15.4%) and pumpkinseed (6.4%) while all other recorded species accounted for less than 5% of the total abundance. During the research, the largest number of fish were recorded, as expected, in the main course of the Krka River (9253 individuals or 75.4 %), the on Visovac Lake (1690 individuals or 13.8%) and on Torak Lake (1088 individuals or 8.9%). A smaller number of individuals was sampled in the Čikola River and Šareni Lakes in relation to the total number of fish (230 or 1.9% and 18 or 0.1%). Two target species for the EU Natura 2000 ecological network were found in the Čikola and Krka River: the Dalmatian minnow and Dalmatian barbelgudgeon in the Čikola River, and the Visovac goby and barbel in the Krka River. Finally, the number of recorded native species was somewhat higher than the number of alien and translocated species, though the number of individuals of alien and translocated species in the areas of the Krka River and its tributaries within and outside the boundaries of Krka National Park was far higher than the number of individuals of native species.

Monitoring the state of the fish community, native and introduced fish will continue to enable timely reactions to changes in the state of populations, and to restore suitable habitats. Additional research is planned, including research of key habitats and preserved habitats on the Krka River, and molecular research of environmental DNA (eDNA) to determine the possible presence of target species and rare endemic and strictly protected species in different habitats and different parts of the Krka River catchment.