Fish assemblages of Posidonia oceanica beds at Port Cros (France, NW Mediterranean): Assessment of composition and long-term fluctuations by visual census

Observations of the fish fauna of Posidonia oceanica (L.) DELILE seagrass beds were undertaken in two shallow stations and one deeper station in Port-Cros National Park, between November 1984 and September 1987. Demographic structure, density and biomass of the 19 most abundant species or taxa were assessed by several non-destructive sampling methods. The fish assemblages studied were dominated in number and biomass by a small number of species (Labridae: Symphodus ocellatus and S. tinca; Sparidae: Diplodus annularis is and Sarpa salpa). Large individuals were infrequent and small size class fish were dominant especially in sheltered shallow areas. Recruitment and migrations (short- or long-term) have been identified as the most important causes of abundance variations over the year. The four trophic categories were sampled only in the sheltered shallow zone, where the herbivore Sa,pa salpa represents a strong proportion of the fish fauna (40-70%) during summer. The fish assemblages differ between the three seagrass beds, and the depth appears as the most important factor affecting the fish assemblages in P. oceanica. The functional difference between shallow and deeper seagrass beds is therefore discussed.