Spatial and temporal changes in the assemblage structure of fishes associated to fish aggregation devices in the Western Mediterranean

We investigated the patterns of occurrence of FAD-associated fish off the coast of Sardinia, Sicily and Majorca (Western Mediterranean). Two working hypotheses are presented to test whether or not "differences exist in fish assemblages at i) the spatial and ii) the temporal level". Quantitative and qualitative experimental data were obtained from a total of 521 censuses at 17 fish aggregation devices (FAD) using underwater visual observations carried out in Sardinia and Sicily in May-October 2001 and 2002 and a purse seine data set on 3 FADs obtained from a previous investigation carried out in Majorca. The results from the multivariate analysis rejected both hypotheses. In all locations, patterns of fish assemblages were dominated by Coryphaena hippurus, Naucrates ductor, Seriola dumerili, Balistes carolinensis and Trachurus spp., a pattern evident from examining data at bi-monthly intervals. When the most dominant species highlighted with similarity percentage (SIMPER) analysis were analysed with an analysis of variance (ANOVA), a time effect was clearly apparent and subsequently confirmed for four of these five species (B. carolinensis being the exception); the location factor was also found to be relevant (except for the genus Trachurus). We concluded that differences noted were principally due to the "time shift" in the occurrence of these species in the three locations, leading to the assumption that a relationship did exist between the gradual but significant shift in seasonal temperature and colonization at the study locations. Defining selected community structure indices was also important because no reference points are actually available for the pelagic environment in the Mediterranean Sea.