Impact of cage farming of fish on the seabed in three Mediterranean coastal areas

The impact of cage culture of marine fish on the benthic environment was investigated seasonally at three commercial fish farms (Cephalonia, Ithaki, and Sounion) established in coastal waters 20-30 m deep, with different types of sediment (from 80% silt to coarse sand) and varying current intensity. A transect of stations in the vicinity of the cages and a control station in each area were sampled for macrofauna and geochemical variables during July, October, and April. Redox potential in the sediment decreased under and near the cages but reached negative values only at the silty sediment site. Organic carbon and nitrogen content of the sediment near the cages increased by a factor of 1.5-5 and ATP content by 4-28 compared with the control. Azoic zones were not encountered, but the macrofaunal community was affected up to 25 m from the edge of the cages. At the coarse sediment sites, abundance and biomass under the cages were 10 times higher than at the control. Diversity indicated that the ecotone was in the vicinity of 25 m from the cages in all cases. Capitella cf. capitata dominated macrofauna up to 10 m from the cages in two fish farms, whereas the third was dominated by Protodorvillea kefersteini. Similar patterns of succession from the impacted to the normal zones were found, although macrofaunal composition differed among sites. Seasonal variability in geochemistry and macrofauna was higher in proximity of the cages. The results indicate that impacts of fish farming on benthos in the Mediterranean vary considerably depending on site characteristics. © 2000 International Council for the Exploration of the Sea.