This study describes the changes in the structure of the fish assemblage associated with Posidonia oceanica meadows after the establishment of a marine protected area. The survey of the ichthyofauna in the Marine Reserve of Tabarca Island was carried out by visual censuses with SCUBA. Inside and outside the reserve was sampled six times at random from summer 1997 to summer 1999 and in three different depths (1, 5 and 15 m) by applying transects of 150 × 5 m over the P. oceanica meadows. The data were analyzed with multivariant methodologies and with analysis of variance. The most represented families in number of species were the sparids and labrids. Mean abundance, biomass and number of species were higher inside the reserve in all studied periods. Inside the marine protected area, these variables decreased as depth increased; outside the protected area the pattern was the opposite, reaching the maximum at 15 m.

Fish farm facilities become colonized by biofouling, and in situ cleaning activities may increase the accumulation of biofouling, mostly shell-hash, on the sediment. However, there is a lack of knowledge about the effect of fish farming on this process. We evaluated the effect of fish farming on shell-hash accumulation on sediments in three fish farms in the Western Mediterranean in Spain. On the one hand, coverage of non-degraded shell on the seabed was estimated using an underwater camera attached to a frame of 1 × 1 m. On the other hand, superficial sediment samples were taken by a Van-Veen grab, and from a subsample, shell-hash was sorted at the laboratory, dried, and weighted. A significant increase of shells on sediment was detected under fish farms compared with the other treatments, with average values of 53 g kg-1, and 1.12% of cover. Shell-hash at zones close to the fish farm cages (Zone of Influence located between 40 to 60 m from the closest cage) did not show statistical differences compared to the reference zones, 300–500 m away from the concession limits, but the shell cover showed statistical differences. Fish farming activities produce a local increase in the sedimentation rate of shells under the cages. The derived ecological consequences of this accumulation need to be further studied.