Changes in Mediterranean rocky-reef fish assemblages exposed to sewage pollution

The fish fauna associated with shallow subtidal rocky-reefs in southern Apulia (SE Italy, Mediterranean Sea) was investigated to assess whether fish assemblage structure, and temporal trends in species richness and abundance of fishes were affected by sewage pollution. Fish assemblages were evaluated by visual census at 3 locations (3 to 4 km apart), 1 putatively impacted by a sewage outfall (discharge rate from 200 to 600 m(3) h(-1)) and 2 controls; within each location, 3 sites (100 to 300 m apart) were surveyed 4 times over the study year. In general, dissimilarities in assemblage structures between the 2 controls were always lower than between each control and the outfall location. During winter, however, the dissimilarities between controls were very low and similar to those between each control and the outfall location, suggesting that in this period, fish assemblages from the outfall did not greatly differ from those in the controls. Planktivorous and detritivorous fishes were more abundant at the impacted location and contributed most to dissimilarities in assemblage structures between the outfall and controls. Furthermore, the outfall significantly affected total fish abundance (which was consistently about 5-fold greater at the outfall than at the controls), and temporal trends in the average density of sparids and small serranids at the scale of locations, while small benthic fishes were influenced at the scale of sites (within locations). Particulate feeders and planktivorous fishes showed different temporal trends in average densities between outfall and controls at both spatial scales examined. Species richness and density of herbivorous fishes did not show significant differences attributable to the outfall. The data thus show that nearshore sewage discharges have the potential to alter spatio-temporal patterns in littoral fish assemblages