The purpose of this study was to investigate the physical and chemical factors of seawater to determine the water quality index around two marine farms of rainbow trout in the south of the Caspian Sea. Each farm had four floating polyethylene cages with a final harvest of 60 tons of fish. The water sampling was done in January and March 2014 and May and August 2015 from around the cages (close: cage shade, 50 meters, and 100 meters, distance: 1000 meters) in three geographical directions: east, west, and south. The water quality parameters including pH, temperature, transparency, salinity, electrical conductivity, total dissolved solids, dissolved oxygen, and nutrients (nitrogen and phosphorus compounds) were determined. The results of the analysis of variance of data at both farms showed that changes in physical and chemical parameters of water had only significant differences at the time of sampling (P <0.05). The highest value of variance in the principal component analysis (PCA) (30.23% from 84.75%) was related to the parameters of electrical conductivity, temperature, salinity, total nitrogen, pH, and organic phosphorus. Iran Surface Water Quality Index (IRWQISC) at near and far distances from farms was determined to be moderate (40-55). The main reasons for this result can be attributed to the small-scale and short fish farming period and the hydrological conditions of the region.
