Expansion of aquaculture throughout the world has caused a real concern over farm effluent discharge into rivers and streams, among both environmental activists and scientists. Similar concerns also exist in Iran as aquaculture sector showing a speedy growth. Thus, to investigate the environmental impact of fish farm discharge on Zayandeh-Rud River, three trout farms namely; Dimeh, Hojat Abad, and Takab with three different production capacities were studied. Five sampling stations were assigned to each farm as follows: inflow and outflow of the farm plus three stations in the river, above the farm (as control); approximately 50-100 meters below the outfall; and 1 km downstream of the outfall. Impacts of farm effluent on the river ecosystem were investigated by utilizing water physico-chemical analyses and macrobenthic invertebrate biomonitoring. Biotic indices are suitable to measure changes in benthic communities affected by trout farm effluent and are reliable in assessing benthic responses to organic pollution. Water and benthos samples were collected once every season; autumn, winter, and spring. Three water samples from each station were collected and analyzed (using Standard methods) to determine parameters such as temperature, DO,pH , BOD 5 , COD, TSS, TDS, Ammonium, Nitrate and phosphate. Benthic samples were collected using a quantitative technique with three replicates at each station. Metrics calculated for the macroinvertebrate community were taxa richness, EPT taxa richness, EPT abundance, mean benthic density (no/m 2 ), Hilsenhoff index, Shannon and Simpson index. Water quality index (NSFWQI m) was also measured to evaluate water receiving farm effluent. Results showed a significant increase in dissolved nutrients (ammonium, nitrate and phosphate) and BOD 5 , COD, TSS, TDS. Results also indicated a significant decline in dissolved oxygen concentrations (DO) and pH in outflow in comparison with the inflow. NSFWQI m values decreased at outflow station indicating water quality deterioration. However, in general, concentrations of measured pollutants at each farm were within acceptable ranges. Overall, values for criteria measured were significantly higher (P 0.05) in Dimeh farm effluent than other farms. This is mainly due to higher biomass of fish raised in this farm. Significant correlation was found at 0.01 level between fish production rates and orthophosphate, BOD 5 , COD, TSS and nitrate produced in trout farms.
