Recovery Dynamics of Fish Assemblages Two Years after a Major Chemical Spill in the Bečva River, Czech Republic

In September 2020, the Bečva River (Danube basin) was affected by a large-scale chemical spill suspected to contain cyanide. This incident resulted in the death of approximately 47 500 kg of fish along a 40 km stretch of the river. The aim of this study was to evaluate the recovery of the fish assemblage at four sites along the affected stretch over a two-year period. Electrofishing surveys were carried out in 2020, 2021, and 2022, and the results were compared with pre-disturbance data. Two months after the chemical spill, there was a significant reduction in fish abundance, biomass, and diversity. At site 2, no fish were present, while the other sites displayed very low abundance and species richness compared to historical data. In the subsequent years, abundance, biomass, species richness, and diversity indices increased at all sites, except at the most upstream location (site 1), where recovery was limited. By 2022, the species composition at sites 2–4 was similar to pre-disturbance conditions, with rheophilic species such as Chondrostoma nasus and Squalius cephalus being dominant. The variability in recovery between the sites was influenced by factors such as distance from the chemical spill source, presence of migration barriers, and habitat characteristics. This study demonstrates the capacity of fish assemblages in the Bečva River to recover following a severe chemical spill, while also highlighting the importance of river connectivity and habitat heterogeneity in the recolonization process.