The effects of marine aquaculture on the environment were evaluated by studying the water quality of Gulluk Bay (Turkey). Marine aquaculture, both extensive and intensive, is one of the most important activities carried out in this area. The intensive culture of fish (Dicentrarchus labrax) is the most important polluting element in Gulluk Bay. Records of long term (seasonally) monitoring of the ambient water dissolved oxygen concentration, the inorganic nutrients (ammonia, nitrate, nitrite, and phosphate) and chlorophyll a concentrations were generated to assess sea water quality of Gulluk Bay. Surface water (0.5 m) samples were collected from seven fish farm areas in the cage of Gulluk Bay (here after reported as the cage stations). Reference surface water was also concurrently sampled at three stations. Modifications in the cage stations water quality were assessed as the difference between the magnitude of a specific parameter recorded at cage station and the concurrently recorded value of the parameter at the reference station, relative to the mean value at the reference station.