The influence of marine cages on the sediment chemistry in the Western Mediterranean Sea

Few studies have been carried out regarding the environmental impact of marine cages in the Western Mediterranean Sea. In fact, many more have been published concerning the northern Atlantic areas. A study of the marine fish farm impact of a was carried out to improve knowledge regarding these arguments in the Western Mediterranean. A marine fish farm, with light fish biomass, was selected to evaluate the sensitivity of environmental variables and measure their efficiency over increasing distances. The following environmental parameters were used: hydrodynamic (water current speed and dominant direction in the whole water column) and geochemical (sediment water content, porosity, total volatile substance, sulfide, redox potential, total nitrogen, total and organic carbon, total phosphorous, and total sulphur). The sea current was measured, in the whole water column, with an ADCP (Acoustic Doppler Current Profiler). Four sampling campaigns were performed in January and July 2001, and in May 2002 and October 2002 according to the different reared biomass. The results showed that the most efficient, practical and sensitive parameters were organic carbon, total carbon, total sulphur, sulfide, potential redox, total phosphorus and total nitrogen. The more detectable effects, highlighted by high levels of organic enrichment, were observed in the stations just beneath the cages; moderate impact was present in the area 50 m around the cages and no disturbance was observed further away from this area. The strong but spatially limited organic enrichment found would seem to suggest that the implementation of best management practices should include fallowing and rotation of sea cages to allow sediment recovery.