Comparison between an artificial reef and a control site in the Adriatic Sea: analysis of four years of monitoring

A fishing survey was carried out over four years (1988-1991) using a trammel net as a sampling device at reef and unprotected control sites about 2.5 miles apart located along the central Adriatic coast on a sandy-mud bottom (depth 10-11 m). At the same time, qualitative visual observations were carried out by scuba divers at the reef to obtain information independent of catch samples. Objectives were to evaluate the effectiveness of the sample gear as a descriptor of the reef community, to test the effectiveness of the artificial reef to enhance fish assemblage and yield and to evaluate these effects seasonally. Seasonal fluctuations in fish abundance were confirmed by visual observations at both sites. Lowest values were recorded in winter, when most of the species migrated to deeper and warmer waters. In spite of this, the average total yield was 10.9 kg'year~1 at the artificial reef, compared to 8.1 kg·yeac I at the control site and the difference was statistically significant. During the fishing survey the fish reported from the reef always exceeded in average weight those obtained from the control site. The influence of the reef was particularly apparent for nekto-benthic fish. In fact pelagic fish always dominated the fish catches at the control site. The same was true at the reef during the first two survey years. After that the nekto-benthic fish, mainly represented by partially and/or obligatory reef-dwelling species, such as sparids and sciaenids, gradually increased and reached the same importance as the pelagics. Conversely, no significant differences were recorded for molluscs and crustaceans between the two sites. Higher total species richness (S,), mean species richness (Sm) and species diversity (H') were recorded at the artificial reef in comparison with the control site. The difference was statistically significant for Sm and not significant for H'. Differences in the fish species composition were also recorded between fishing samplings and visual observations at the reef. Fewer species of fish, crustaceans and molluscs were recorded by visual observations than by trammel net. This last method better described the population of the original sandy-muddy bottom. Conversely the partially and obligatory reef-dwelling species were better recorded visually