Effect of fish farming on the water column nutrient concentration in a semi-enclosed gulf of the Eastern Mediterranean

The spatial and seasonal impact of fish farming on the water column of two fish farms in the Eastern Mediterranean was investigated, between July 2004 and April 2005. In the area of each fish farm, three stations were sampled. One was located at the centre of the fish farm, the other at a distance of 50 m from the centre and the last one at a distance of 300 m from the fish farm centre. At each sampling station, water samples were collected at the surface, at 5 m and at every further 10-m depth from the surface to the bottom, for nutrient analysis (ammonium, nitrite, nitrate, phosphate, silicate). Statistical analysis showed significant differences between seasons for all nutrients, with the exception of nitrite in one of the farms. Furthermore, ammonium, nitrite, phosphate and silicate values showed significant differences among sampling stations. The maximum concentrations of the above nutrients (1.64, 0.10, 2.57, 0.28 and 4.89 mu M respectively) were recorded at the sampling stations located at the centre of each fish farm and they were gradually reduced with increasing distance from the cages. The concentrations of nutrients did not appear to exceed permitted levels for the appearance of eutrophication. The maximum concentrations were considerably lower than those observed previously in the study area.