Fish-Kills in the Urban Stretch of the Tiber River After a Flash-Storm: Investigative Monitoring with Effect-Based Methods, Targeted Chemical Analyses, and Fish Assemblage Examinations

In 2020 and 2021, fish-kills events occurred in the Tiber river in the city of Rome. These events, which caused the death of thousands of fish of different species (e.g., Barbus spp., Cyprinus carpio, Squalius spp.), were preceded the days before by severe flash-storms. Heavy rains in urban areas in recent years are linked to climate change and fish-kills events. With the aim to investigate the causes of these events, effect-based methods (EBMs) and targeted chemical analyses have been performed on a specific site of river Tiber in the center of the city. Additionally, examination and classification of the floating dead fishes have been performed. The chemical analysis performed on several groups of contaminants showed the presence of some pharmaceuticals, insecticides, and PFAS at up to ng/L. Results with the Fish Embryo Toxicity (FET) test confirmed lethal and sub-lethal effects, while acute effects were not detected with the Daphnia magna acute test. The fish-kills events probably can be generated by several factors as a consequence of an increasing human anthropization of the area with the contribution of different stressors together with chemical releases and emissions. This study shows that in multiple stressor scenarios characterized by heavy rainfall, droughts, and strong anthropogenic pressures, the application of EBMs, chemical analysis, and fish assemblage examinations can represent a useful support in the investigation of the causes of extensive fish-kills events.