Abstract
Mass fish kills caused by wastewater treatment plant (WWTP) failure are uncommon. In this study, we analyzed the recovery of the fish community in the Czarna Hańcza River after a fish kill caused by a WWTP incident in Suwałki in 2018. The research focused on brown trout, the dominant fish species in this river. The incident lasted about six hours and accounted for 7% of the mean river’s flow during the accident. The fish population recovered quickly, and ten months after the fish kill, the number of fish species and their abundance were similar to the pre-incident state, but the species structure was different. In subsequent years, fish species richness decreased conspicuously, while their numbers remained at a high, relatively constant level. Species that are more resistant to difficult environmental conditions, such as white bream and European perch, returned to the river first. Concurrently with the improvement of environmental conditions, more demanding species, including brown trout, began to dominate in the river. Fulton’s condition factors decreased significantly as the river self-purified and the density of the brown trout population increased (P < 0.05). Our results indicated that the brown trout population was able to recover relatively quick, presumably because of stocking. Other species that were not stocked required more time to recover.