Research in: Agricultural & Veterinary Sciences

Artificial reservoirs in urban ecosystems are considered vulnerable to pollution from poor oxygen circulation and wastewater discharge. In this paper, we investigated the mass mortality of fish in the artificial reservoir Baharshor (Kichik Gyzylnohur), located near the Mahammedli settlement in Azerbaijan. From June 1 to 4, 2025, we observed a mass mortality of European perch (Perca fluviatilis), with mortality rates reaching 100%. A complete parasitological dissection of the dead fish was conducted. Also, water samples were analysed to assess physicochemical and ecotoxicological conditions. The parasitological examination did not reveal any dangerous species. The toxicological study results revealed a significant deviation from generally accepted water quality standards. The first sign of hypoxia in the water was turbidity, reaching 57.9 FTU (limit: <5); dissolved oxygen levels were depleted to 0.7 mg/L (limit: >4.0). The ammonium level was 8.45 mg/L (over 17 times the permissible concentration). These data indicate severe organic pollution. A strong sewage odour confirmed a likely domestic wastewater input. Most heavy metals were within limits, but manganese exceeded the threshold. Although acute toxicity tests did not classify the water as toxic, the combination of low oxygen and high ammonium caused the mass fish mortality. The findings highlight the ecological risks posed by untreated wastewater in urban water bodies. This case highlights the importance of continuous monitoring of wastewater pollution. Preventive measures are essential to protect aquatic ecosystems and public health in rapidly urbanising environments.