Background and Theoretical Foundations: The effluents of cage culture, mainly uneaten food and faecal, are released directly into the environment and result in many environmental problems such as eutrophication, and changes of benthos communities .Therefore, studies related to the effects of this type of activity on marine ecosystems seem to be very necessary. In this study, the possible effects of cages culture Asian sea bass fish (Lates calcarifer) on water quality in coastal waters of Rigo region of Qeshm Island in Hormozgan province during a period of six months during 2008 were investigated.
Methodology: :Water sampling was performed to measure some of water quality parameters in three stations, including the first station in the center, the second station at the end of the cages and the third at a distance of 1000 meters from the cages (reference site). A total of 540 water samples were collected using a Nansen sampler bottle to measure nutrients (nitrate, nitrite, total ammonia and phosphate). All nutrient samples were measured based on colorimetric method by spectrophotometer. Measurements of temperature, pH, salinity, dissolved oxygen, oxygen saturation, turbidity and chlorophyll a were performed by multiparameter in the water column. To classify water quality, trophic status and risk of eutrophication in coastal waters of the study area, scale and upscale trix indices were used.
Findings: The water temperature range in this study is equal to 22.56-31.48 C^o; Salinity, 36.98-37.55 ppt; Dissolved oxygen, 3.03-5.92 mg/l ; pH, 8-8.46; Turbidity, 4.3-27.4 NTU; Nitrate, 0.005-0.04 mg/l; Nitrite, 0.001-0.003 mg/l; Total ammonia, 0.003-0.04 mg/l; Phosphate was 0.002-0.043 mg/l and chlorophyll a was equal to 0.1-3.8 µg /l. The results of one-way analysis of variance test showed that there was a significant difference (P<0.05) between the studied at the location of the cages culture and the refrences site in terms of measured water quality parameters (except water temperature, pH and salinity) also the concentrations of nitrate, total ammonia, phosphate, nitrite, turbidity and chlorophyll a measured at the location of the cages were much higher than farther from the location of the cages (the references site). The mean value of Trix index, eutrophication risk index, trophic efficiency coefficient and N: P ratio in the coastal waters of the study area were 4.97, 3.98, 0.2 and 3.6, respectively. Based on the amount of Trix index, the level of trophy at the location of the cages culture was high, the water quality was moderate, and the trophy efficiency coefficient and the Eutrophication risk were high.
Conclusion: In general, the results of the present study showed that fish farming activities in cages could decrease water quality, increase trophic levels and the risk of eutrophication in the waters adjacent to the cages, while the effects of such activities on the coastal waters of the study area were ineffective at the time of the study.