The objective of this study is to demonstrate the impact of pollution on the fish population in the Ain Zada Dam, where the quantity of fish has experienced a dramatic decline, dropping from 118,139 kg in 2010 to only 7,578 kg in 2022, with a concerning low of 6,335 kg recorded in 2018. The main causes of this pollution are contamination by organic materials, including nitrates, nitrites, phosphates, and ammonium, which serve as nutrients for plankton and plants. This leads to their dense growth and proliferation in the reservoir and on the water surface. They consume oxygen and block light from reaching the reservoir’s waters, reducing oxygen production by the plants within the reservoir. This results in an accumulation of organic matter, a phenomenon known as eutrophication. The COD (Chemical Oxygen Demand) of 84.6 mg/l and the BOD5 (Biochemical Oxygen Demand over 5 days) of 6 mg/l recorded in 2022 confirm a drastic reduction in oxygen levels, with a recorded value of 7.6 mg/l in 2022. These levels fall below the permissible thresholds of 7 mg/l in summer and 10 mg/l in winter, highlighting the urgent need for intervention to address this situation..