Modeling the effects of water level decreases on the optimum distance of fish pens in Gorgan Bay

By reducing the water level in the Gorgan Bay, as a result of the decreasing in the Caspian Sea water surface, the optimum distance will change between fish farms. The purpose of this study was to investigate the optimal distance variation due to water level reduction, Hydrodynamic and Transport modules of the Mike 21 model was implemented under 15 scenarios by considering western prevailing wind and in 5 levels. Wave radiation tension was implemented using the Near Shore Wave module in 15 scenarios and used as input in hydrodynamic models. Dispersion coefficient was extracted by salinity modeling in the year 2012 and at a level of 26.5 m and used it in other models. The modeling results in most scenarios indicate that the pollutant displacement is in the direction of the dominant wind from west to east in the southwest farm and was from east to west in the northeastern farm opposite to the dominant wind. Mostly, wind speed increasing and water levels decreasing will increase the optimum distance and will reduce time to dilute pollutants. The maximum mean optimum distance in the northeast and southwest farms were 487 and 2615 meters in level of 28 meters, respectively. The minimum mean optimum distance in the northeast and southwest farms were 194 and 999 m in level of 26.5 and 26 meters, respectively. In order to reduce the ecological and economic disasters caused by the effects of the Caspian Sea level reduction on the optimal distance between farms in future, it is recommended to restructure fish farms in the Gorgan Bay.