Evaluating the Aquaculture Effluent Impact on Macroinvertebrate Community and Water Quality Using BMWP index

We investigated the environmental impact of trout farm discharge on Zayandeh-Rud River. Three farms with different production capacities; 250, 25 and 70 tones were assigned to this study. Five sampling sites were designated to each farm. Benthic macroinvertebrates were collected in autumn, winter, and spring using the quantitative techniques. A total of 11 classes, 16 orders and 53 familie's of macroinvertebrates were identified. Information on macroinvertebrates of each site was used to calculate the biological monitoring working party (BMWP). BMWP index was significantly (p < 0.01) low at farm effluents especially when the production capacity was high. At these locations, abundance of tolerant taxa (e.g. Chironomidae, Simulidae, Oligochaeta families) increased, but sensitive taxa (Ephemeroptera and Trichoptera) declined in number. The BMWP index values were closely related to production rate and the lowest values (6.03±1.17) was recorded at the farm with the highest production. Although, BMWP index at site 50 m after the outfall showed low or no recovery of water quality and community structure, however, at site 1 km after the outfall BMWP values exhibited a partial downstream spatial recovery of community structure so that no significant difference with upstream site was observed. We concluded that self-purification capacity and the high water flow of the river dilutes effluents so that biotic index showed no alarming condition at the farthest downstream station.