Temporal variability of wild fish assemblages associated with a sea-cage fish farm in the south-western Mediterranean Sea

The abundance and size structure of wild fish aggregated around a floating sea-cage fish farm were assessed and compared with control sites over a period of 1 yr. The farm was located 3.1 km offshore from El Campello (Alicante, SE Spain) and cultivated Sparus aurata and Dicentrarchus labrax in 14 sea-cages. Fish assemblages were evaluated by visual census using SCUBA on three separate days in each season. On each sampling day, six 5-min visual counts were made around the cages and six counts were made at control sites, 200 m distant from the farm. Assemblages of wild fish differed greatly between the control sites and the farm, with 12 species observed in farm counts and only 4 in the control counts. Abundance, biomass and diversity were greater in the farm-associated fish assemblage. In most controls, fish were not observed. The most abundant families were Sparidae and Carangidae (>98% of aggregated fish). Assemblages at farms differed among seasons, with Trachurus mediterraneus dominant during spring, Oblada melanura in summer and Boops boops in autumn and winter. The results indicate great temporal variability in the farm-associated fish assemblage, which may be related to recruitment periods for juveniles (e.g. O. melanura in summer) and specific preferences by other species for warm (e.g. T. mediterraneus) or cold (e.g. B. boops) water periods.