Investigation on the impacts of the Rainbow trout (Oncorhynchus mykiss) farms effluents on eutrophic and saproby conditions of the Haraz River, Mazandaran Province

Aquaculture is a production activity whose uncontrolled development would cause negative effects on water resources. This study was conducted on Haraz River to investe the sewage impacts of Oncorhynchus mykiss farms and assess the self-purification capability of the river. Sampling was carried out in 3 pilot farms and 12 selected stations containing pre-farm (control) and post-farms stations in distances of 100, 1000 and 2000 meters downstream from the sewage entry point by random method. Due to the self- purification, the impacts of upstream farms did not reach the next and lower-stream ones. Analyzes of physicochemical factors of water including: Temperature, DO, pH, EC, BOD, COD, TDS, TOC, NO3, PO4 were performed. Saproby classification was done based on biological value index. According to the results, in all downstream stations, a significant increase in concentrations of pollutants was obsereved compared to the control station. BOD was signifantly higher in the post-farm stations too. Despite the proper self-purification of river based on zonation, most sections were completely in eutrophic and polysaprob conditions due to the volume of sewage imported. The first studied farm from upstream, imported more pollution to the river and and the second and third farms are in the next ranks. Accoding to Z index, the best water quality belonged to control station of the first farm and the worst, was about the third station of the mentioned farm. Self-purification assessment demonstrated the reliable distance at 2000 m for complete purification. Despite the suitable self-purification of the Haraz river, the effluents from other smaller farms at non-standard distances, has increased the eutrophy of this river. Failure to monitor fish farms, may cause more problems in the future and the river may be in the state of dystrophy.