To investigate the environmental impact of fish farm discharge on the Zayandeh-Roud River in Iran, three trout farms (Dimeh, Hojat Abad, and Takab) with three different production capacities were studied by examining water physicochemical criteria and the macrobenthic invertebrate community. Six sampling sites were assigned to each farm: the inflow and outflow of the farm, the outfall into the river, 50 m upstream from the outfall (control), 50-100 m downstream from the outfall, and 1 km downstream from the outfall. Water and benthos samples were collected once during autumn, winter, and spring. Benthic samples were collected by using a quantitative technique (3 replicates/site). Results showed a significant (P < 0.05) increase in 5-d biochemical oxygen demand, chemical oxygen demand, and total suspended solids and a decline in dissolved oxygen concentration and pH in the outflow. However, concentrations of measured variables at each farm were generally within acceptable limits. Total taxonomic richness, abundance of Ephemeroptera, Plecoptera, and Trichoptera (EPT), and richness of EPT taxa were considerably lower at outflow stations than at inflow stations, especially for the farm with a higher production rate. In contrast to physicochemical results, the benthic community indices were significantly different between upstream and downstream samples. Sensitive taxa declined at downstream sites in comparison with upstream sites, while abundance of tolerant taxa (e. g., Chironomidae and Oligochaeta) increased at downstream sites. Although species richness and diversity improved farther downstream, the recovery was not complete within our study range. However, effluent effects were greatest during periods of low river flow. Thus, to reduce environmental impacts of aquaculture, production rates should be based on the lowest anticipated flow rate of rivers.
