Response of demersal fish communities to the presence of fish farms

The hypothesis that the presence of fish farms affects the communities of wild fish at large spatial scales was investigated through experimental trawl surveys. Three regions of the Aegean Sea with aquaculture zones were sampled in May and September (reproductive and recruitment periods, respectively for most species in the Aegean Sea). In each region, 2 sub-areas were sampled: 1 located nearby the fish farming zones (within 2 to 3 n mile) and a similar site ('reference site') far away from these zones (> 20 n mile). Three replicate hauls were made in each sub-area for each of the 3 investigated regions. Significant differences were observed in community structure between farming and reference sites, which were mainly due to quantitative rather than qualitative aspects, The overall abundance and/or biomass of wild demersal fish was significantly higher near the fish farms in every region studied during May without any significant change in diversity or biodiversity. During the recruitment period in September, total abundance and biomass were fairly similar. These findings support the hypothesis that the presence of fish farms affects the wild fish populations in the oligotrophic eastern Mediterranean at a spatial scale larger than the immediate vicinity of fish cages. It seems that during the recruitment period all sites were stocked with fish close to the carrying capacity of the respective areas, and by late spring the areas close to fish farming zones had suffered little losses during winter, whereas populations at the reference sites had decreased substantially.