The aim of this study was to investigate the impact of rainbow trout (Oncorhynchus mykiss) cage culture on the facultative anaerobic bacteria in the environment around the fish farm. This study was carried out at two fish cage culture farms on the southern coast of the Caspian Sea. In the fish farms(Klarabad and Abbasabad), the sediment samples were collected from three sides (east, west, south) and four stations on each side (distances of 5, 50, 100 and 1000 m) from 2014 to 2015 years. The samples were cultured as Pour plate method in the TSA medium. Results showed that the distance from the cage and different geographical directions in each fish cage culture (in each sampling period) did not differ significantly (P<0.05). Also, at different sampling periods in each fish cage culture, there was significant difference only in August compared to other periods (P<0.05). But in comparison to the mean of facultative anaerobic bacteria in the sediment around the fish cage culture, there was a significant difference between the two sites Klarabad(2.2×10 6 cfu/ml) and Abbasabad(2.4×10 5 ) except March (P<0.05). It is concluded that a significant increase in facultative anaerobic bacterial populations in August could be one of the reasons for a significant decrease in total organic matter in August compared to other months. It seems that the activity of breeding rainbow trout in cages in Abbasabad and Klarabad regions had a relatively small effect on bacterial factors and this regions is suitable of aquaculture
