Relationships between fish, sea urchins and macroalgae: The structure of shallow rocky sublittoral communities in the Cyclades, Eastern Mediterranean

Historical overfishing is the most likely explanation for the depletion of the shallow sublittoral communities in many areas not least in the Cyclades Archipelago, Greece. The present study is the first quantitative study of the shallow rocky sublittoral of the Cyclades based on in situ underwater surveys of algal cover, and fish and sea urchin abundance at 181 sampling sites in 25 islands to provide a baseline and investigate the relationship between these communities. Algal turf was the most abundant algal functional group, and canopy algae of the genus Cystoseira were more abundant at the northern islands. A range in fish biomass of almost two orders of magnitude was found across islands, but overall the Cyclades displayed much lower values than fished areas of the Western Mediterranean. We observed apex predators only in 25% of our sampling sites, and their biomass was uncorrelated to total fish biomass, indicating a depleted ecosystem. Sea urchin biomass was also low but similar to values found in other Mediterranean islands and was positively correlated with barrens. We observed a gradient of benthic community complexity from sea urchin barrens to communities dominated by Cystoseira spp. There was no correlation between sea urchins and their predators Diplodus spp., which presented extremely low densities.