Assessment of the ichthyofauna of an artificial reef through visual census and trammel net: comparison between the two sampling techniques

From July 1991 to June 1992 the fish assemblage associated to a concrete artificial reef located in the Central Adriatic Sea was assessed through visual census carried out by divers in the daytime and through a bottom trammel net fished in two different periods of the day: from dusk to dawn and from dawn to dusk. The aims of the study were: a) to assess the fish biomass at the reef through visual census; b) to compare the different sampling techniques on the basis of the species recorded; c) to compare the species regularly censused with both methods. The results obtained were: a) The mean fish biomass estimated at the reef through visual census was comparable to that reported for extensive multi species cultures. The maximum values were recorded in summer and near the structures. b) A higher number of species was recorded by night catches than by day catches and visual observations. Moreover, a higher number of individuals were caught in the night time than in the daytime. The benthic fish of the original sandy muddy bottom were generally better monitored through trammel net. Visual census revealed to be effective for obligatory or partially reef dwelling species. c) Only six species were both caught and observed; trammel net and visual census gave different information on the population of some of them.