Fish Community Structure and the Role of Non-Indigenous Species Associated to Seagrass Ecosystems of Eastern Mediterranean

Abstract: Temporal and spatial variation in density, biomass and body size of littoral fish species associated with near shore Posidonia oceanica meadows were studied throughout an annual cycle in an area of eastern Mediterranean. Approximately 109350 littoral fish were collected, belonging to 34 families and 88 species. Density of fish peaked during the summer due to high densities of juveniles. Season was found to be an important factor in discriminating density of species, while number of species and biomass did not show any obvious seasonal pattern. Temporal variations in density and body size of fish were used to assess the seasonal and ontogenetic habitat use of each species and the affinity to seagrass was assessed in a comparison with each species distribution on sand. Four functional guilds were created to describe the habitat use of P. oceanica meadows for each species encountered including juvenile migrants, seagrass residents, seasonal migrants and occasional visitors. Several species associated with P. oceanica meadows mainly used this habitat for spending their juvenile stages during summer although many species were found to use this habitat both as adults and juveniles at the same time. Among the species encountered, eleven were found to be non indigenous of Indo-Pacific origin, three of them using segrasses mainly as juveniles, and four as residents. In an earlier study the composition of the diet of the Indo-Pacific cornetfish Fistularia commersonii from the SE Aegean Sea was described. Dietary comparisons were made reflecting the relationship between diet composition, time of year, and fish size. Correlation between predator length and prey length was significant. The diet of the blue cornetfish consists of 96% by number and 99.95% by weight of fish. Size classification and habitat of prey groups (benthic, supra-benthic, and pelagic) showed that with increased body length the blue cornetfish extended its diet to larger prey and more generalist feeding.