Diversity, structure and function of fish assemblages associated with Posidonia oceanica beds in an area of the eastern Mediterranean Sea and the role of non-indigenous species.

Temporal and spatial variation in density, biomass and body size of littoral fish species associated with nearshore Posidonia oceanica meadows was studied over an annual cycle in an area of the eastern Mediterranean Sea. A total of 109,350 littoral fishes were collected, belonging to 34 families and 88 species. Density of fishes peaked during the summer due to high numbers of juveniles. Season was a significant factor determining density, although number of species and biomass did not show any obvious seasonal pattern. Throughout the study, schooling planktivorous fish species such as the picarel Spicara smaris, the bogue Boops boops and the damselfish Chromis chromis were dominant, both in terms of density (80%) and biomass (70%). Temporal variation in density and body size of fishes was used to assess the seasonal and ontogenetic habitat use of each species, with their affinity to seagrass assessed by comparing their respective distribution on sand. Four functional guilds were created (juvenile migrants, seagrass residents, seasonal migrants and occasional visitors) to describe the habitat use of P. oceanica meadows by each species. Several species associated with P. oceanica meadows used this habitat mainly as juveniles during summer, although many others were present concurrently as adults and as juveniles. Among the species encountered, 11 were non-indigenous of Indo-Pacific origin, of which three used seagrasses mainly as juveniles and four as residents. The non-indigenous silverstripe blaasop Lagocephalus sceleratus ranked among the 10 most dominant species in terms of biomass (2%) and was classified as a seagrass resident.