Benthic community response to sediment organic enrichment by Mediterranean fish farms: Case studies

There is a great deal of scientific information about the biogeochemical processes in sediments near fish farms and how those processes may be driven under nutrient enrichment to environmental negative effects, as biodiversity decrease is observed in benthic assemblages. Within an adaptive management framework, a good monitoring program can be used to adjust farm management to avert serious and persistent impacts to the sediment. Macrobenthic community represents a good indicator able to detect environmental effects resulting from organic enrichment and, based on this, was selected to monitor Italian marine fish farms. In order to assess the environmental sediment disturbance resulting from marine aquaculture along the Italian coasts three marine fish farms were considered based on their different environmental features: two located in the Tyrrhenian Sea and one in the Adriatic Sea. The selected cage fish farms produce European sea bass Dicentrarchus labrax (L.), gilthead sea-bream Sparus aurata (L.) and meagre Argyrosomus regius (A.) The study ranges from 2001 to 2012, at different times in each fish farm, and was planned according to the fattening cycles. The samples for benthic community analyses were collected by means of a van Veen grab in selected stations located from 0 to 1000 m from the sea cages, the farthest as control stations. In total 134 stations were sampled for more of 20,000 individuals belonging to more than 450 species. Data analyses were performed with statistical methods and biotic indices were calculated. The results showed changes in benthic communities at the stations located just beneath the cages and connected with high cultured biomass. The impacted sites showed loss of biodiversity and increase of opportunistic species such as Capitella Sp. I and Nebalia strausi. Changes in the benthic communities were more or less evident in relation to substrata composition, hydrodynamic conditions and cultured biomass. The observed impact was limited to a few tens of meters around the sea cages of all the three studied fish farms.