Summary of the Preliminary Study on the Ecological Impact of Barriers in the South-Western Carpathians

30 April 2025

 

The study was carried out by the Aqua Crisius Anglers Association as part of the project “Rewilding rivers in Southern Carpathians, Romania – Preparatory work for removing barriers in 5 catchments”, financed through the European Open Rivers Programme and supported by Arcadia. 

 

Fish provide powerful tools for assessing the condition of aquatic environments due to three essential attributes: high sensitivity to most forms of human-induced disturbances, applicability at all levels of biological organization, and a favorable cost-benefit ratio for monitoring programs (Harris, 1995). Moreover, fish can be used as effective indicators across wide spatial and temporal scales, covering all trophic levels of aquatic ecosystems.

 

The main objective was to assess fish diversity and the impact of transverse barriers (dams, weirs, fords, etc.) on river connectivity in five catchments of the Southern Carpathians: Cerna-Belareca, Pârâul-Rece, Feneș, Sebeș, and Bistra.   

 

Data collection was carried out by scientific fishing using the technique of electrofishing. This method involves the use of a controlled electric current to induce a temporary paralysis of individuals, facilitating their capture by fishing nets. It is considered to be the least invasive method as it does not cause fish mortality and ensures complete sampling of the fish at the working sites, thus providing a representative representation of the local ichthyofaunal composition. After identification and measurement, fish were immediately released in the vicinity of the capture site, minimizing stress. 

 

It is important to note that the use of electrofishing requires specific authorization from the competent authorities. Such activities can only be conducted by specialized personnel, in compliance with existing environmental regulations. 

 

Within the studied sectors, a total of 5845 individuals were identified, belonging to 24 fish species and one species of lampreys. Among these, 9 species are of community interest (Natura 2000), including the lampreys (Eudontomyzon sp.), the European grayling (Thymallus thymallus), the European bullhead (Cottus gobio), and the Balkan/Romanian barbel (Barbus balcanicus/petenyi), along with other species of community interest. 

 

The data collected were correlated with the presence or absence of fragmentation along the studied rivers, with barrier evaluations carried out by the Rewilding Romania field team. 

 

The presence of these species indicates the existence of relatively well-preserved aquatic habitats, providing suitable ecological conditions for the completion of their life cycles. However, the exclusive identification of larval stages in lampreys (Eudontomyzon sp.) suggests limitations in migration and normal development, likely caused by the longitudinal fragmentation of river courses. The European grayling (Thymallus thymallus) and European bullhead (Cottus gobio), both highly sensitive to hydromorphological alterations, confirm that the studied sectors are not completely degraded, although anthropogenic pressure on river integrity is evident. Clear differences in faunal composition between fragmented and unfragmented sectors highlight the negative impact of barriers on biodiversity and river connectivity (Bănăduc et al., 2018; Nagy et al., 2023). 

 

Beyond fish species data, the study also recorded other important elements of aquatic biodiversity. The presence of the Eurasian otter (Lutra lutra) was confirmed in the Cerna-Belareca basin in several study sectors, indicating the existence of habitats with sufficient food resources for the species. Additionally, the stone crayfish (Austropotamobius torrentium) was identified in the Cerna-Belareca, Sebeș, and Bistra basins, signaling the existence of favorable aquatic habitats characterized by well-oxygenated waters and natural substrates.

 

The aquatic ecosystems in the studied area have been subjected over time to intense longitudinal fragmentation, mainly through the construction of dams and water abstraction works for hydropower (Gasparatos et al., 2017; Hecht et al., 2019; Jungwirth et al., 2003; Lees et al., 2016; Reid et al., 2019; Schmutz și Sendzimir, 2018; Stendera et al., 2012; Ziv et al., 2012; van Treeck et al., 2022). These interventions have negatively affected river connectivity, species migration, and biological diversity. 

 

Starting from the hypothesis that in the absence of longitudinal barriers, fish communities would have shown higher diversity and abundance, the study shows that field data confirm the negative impact of longitudinal fragmentation on the connectivity and distribution of fish populations. For example, in the Sebeș basin, sensitive species were predominantly found in unfragmented sectors, while areas affected by barriers, despite appearing to offer favorable habitat conditions, exhibited significantly lower biodiversity. 

 

To restore river connectivity, the most effective measures are the complete removal of barriers, especially those abandoned or in advanced stages of degradation. Where complete removal is not possible, functional fish passages should be installed. These passages must comply with technical standards to allow free migration of fish species, considering water velocity, height differences, and passage slope (especially in bypass-type solutions) (FAO/DVWK, 2002). 

 

The negative impact of fragmentation can also be partially mitigated by ensuring an adequate ecological flow downstream of water abstractions. The recommended minimum flow should range between one-third and two-thirds of the natural upstream flow, ideally around 50% (Cristea, 2007). Furthermore, the ecological flow must be dynamic, with seasonal variations that replicate natural conditions, ensuring that water depth in the riverbed does not fall below 10 cm, thus supporting aquatic biodiversity (HG 148/2020). 

 

Additionally, for the protection of aquatic ecosystems, maintenance activities such as the flushing of desilting basins must be conducted exclusively during high-flow periods and for limited durations to minimize the disturbance to aquatic fauna (Nistorescu et al., 2016). Monitoring the effectiveness of fish passages is essential and can be achieved by installing automated fish tracking systems.

 

In conclusion, to ensure the conservation of biodiversity and the healthy functioning of aquatic ecosystems, an integrated approach is needed: restoring longitudinal connectivity, ensuring a dynamic ecological flow, and adapting existing infrastructure to meet the ecological requirements of fish species.

 

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