Evaluation of the effect of rainbow trout (Oncorhynchus mykiss) fish farms on the water quality of Zarin Gol river using a physicochemical index (WQI).

The excessive growth of the aquaculture industry, in addition to using more natural resources, causes a disturbance to the biological and non-biological balance through effluent releases, and water quality indicators provide the possibility of better management of pollution and direction for targeted biological monitoring. The purpose of this research was to evaluate the nitrogen load of the basin and rainbow trout breeding farm on the water quality of Zarin Gol River in Golestan province, Iran, based on a standard index (WQI) by measuring water quality parameters including: pH, total phosphate, temperature, nitrate, dissolved oxygen, turbidity, and total coliform. Sampling was done seasonally at five stations (before and after the first and second fish farm and 1000 meters downstream from the second fish farm) in 2018. The results showed that the concentration of nitrogen and phosphorus in the River during the six months of fish breeding on the second farm was estimated to be 16.5 tons of nitrogen and 2.9 tons of phosphorus. According to the WQI, the results showed that the water quality values in all the studied stations are 79.7 to 87.8, which indicates excellent water quality according to the descriptive table. It seems that under the current conditions, the Zarin Gol River has the ability to self-purification the effluent of two fish farms with the expected capacity.