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 in vestigated seasonally at three commercial fish farms (Cephalonia, Ithaki, a nd Sounion) established in coastal waters 20-30 m deep, with different type s 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 J uly, October, and April. Redox potential in the sediment decreased under an d near the cages but reached negative values only at the silty sediment sit e. Organic carbon and nitrogen content of the sediment near the cages incre ased by a factor of 1.5-5 and ATP content by 4-28 compared with the control . Azoic zones were nor encountered, but the macrofaunal community was affec ted up to 25 m from the edge of the cages. At the coarse sediment sites, ab undance and biomass under the cages were 10 rimes higher than at the contro l. Diversity indicated that the ecotone was in the vicinity of 25 m from th e 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 Prot odorvillea kefersteini. Similar patterns of succession from the impacted to the normal zones were found; although macrofaunal composition differed amo ng 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 charact eristics. (C) 2000 International Council for the Exploration of the Sea.