Biodeposit dispersion around a deep cage finfish farm in the Northern Persian Gulf

Due to increasing demand for food, intensive mariculture of finfish is a fast developing industry in the Iranian part of the Persian Gulf (PG). The environmental impacts of cage fish farming largely involve release of excessive organic matter (biodeposits). Careful monitoring of fish farms to avoid anoxia and bottom “dead zones” is an essential part of a sustainable mariculture industry. Monitoring data were collected for a fish farm north of Kish Island that had been in full operation for a period of one year. The data were then incorporated into a hydrodynamic model in order to estimate the quantity and dispersion pattern of biodeposits around the farm. Our results show that the farm had significant impacts on local environmental conditions, including water transparency, sulfide levels, and Eh values. Bottom water clarity near the cages was > 15 NTU. Underneath the cages, sediment sulfide values are > 800 μ mole, Eh is usually -100 or less, and the sedimentation rate is 6.8 g/m2/day. The number of live benthic foraminifera near the cages was 4.5/10 g aliquot, as opposed to 20 at a control site. Numerical modeling, combined with data from sediment traps, shows that the farm “footprint” is much larger than the actual area of the farm, and is elongated by tidal currents. Considering the relatively short operation of the farm, it is already close to generating anoxic bottom conditions over a 2 km wide area. This can potentially impact the fragile habitats of the Persian Gulf. Continued expansion of net pen fish farms in sensitive areas such as the PG should be carefully monitored.