Non-indigenous Species in Mediterranean Fish Assemblages

Quantitative sampling in combination with classification of fish species into six major feeding guilds revealed the present position and contribution of non-indigenous species (NIS) in the food web of Posidonia oceanica and sandy habitats. In P. oceanica beds and on sandy bottoms 10 and five species, respectively, were non-indigenous fish of Indo-Pacific and Red Sea origin. The proportional contribution of MS individuals on P. oceanica beds was lower than that of sandy bottoms (12.7 vs. 20.4 %) a pattern that also followed for biomass (13.6 vs. 23.4 %), indicating that species-rich communities might be less impacted than low diverse systems. The two habitats had similar fish feeding guilds, but the biomass contribution from MS varied within each guild, indicating different degrees of impact on the available resources. This study showed that only few non-indigenous fish species contributed to the differences in biomass between habitats. Two of the aspects considered in this study, the chance of establishing and the chance of being very dominant will depend upon competitive abilities strongly coupled to size and grounds for habitat shift. However, success of establishment will also depend on appropriate food resources in the recipient community as well as competitive abilities and level of competition in the food web within habitats.