Differences Among Fish Assemblages Associated with Nearshore Posidonia oceanica Seagrass Beds, Rocky–algal Reefs and Unvegetated Sand Habitats in the Adriatic Sea

Abstract

Fish assemblages associated with nearshore Posidonia oceanica seagrass beds, rocky–algal reefs and unvegetated sandy substrates were studied at two sampling localities, Otranto (Apulian coast) and S. Domino (Tremiti Islands), located in the Southern and Central Adriatic Seas (Mediterranean Sea), respectively. Data were collected in situ by using non-destructive diver visual census methodology. A higher species richness and fish density were observed over thePosidonia seagrass and, in turn, the rocky–algal reef and unvegetated sand habitats. Planktivorous fish species (Spicara maena, Spicara smaris and Chromis chromis) showing patchy distributions were dominant in terms of abundance at both localities. The two former species did not show any particular habitat preference, while C. chromis was common over both the Posidonia and rocky–algal habitats and was only occasionally recorded over sand.Symphodus ocellatus , Diplodus annularis and Spondyliosoma cantharus were the most common species in theP. oceanica seagrass beds. Symphodus roissali, Symphodus tinca and Diplodus sargus were mainly associated with the rocky–algal reef habitats, while other species, such as Gymnammodytes cicerellus,Lithognathus mormyrus , Gobius geniporus, Mullus barbatus and Uranoscopus scaber, were censused over the unvegetated sand habitats. A cluster analysis performed on the entire fish density data set showed distinct groupings for seagrass, rocky–algal and bare sand fish assemblages, regardless of season and sampling site. Fish assemblages in the more structured (seagrass and rocky–algal reef) habitats were relatively similar to each other but quite different from those of the unstructured bare sands. With regard to small-sized specimens (juvenile stages including recruits), those of S. ocellatus, Symphodus mediterraneus, Serranus cabrilla, D. annularis, S. cantharus andSarpa salpa were mainly censused over P. oceanica beds, while juveniles of C. chromis inhabited predominantly rocky–algal bottoms rich in crevices. Small individuals of Coris julis were censused over both P. oceanica and rocky–algal habitats. No juveniles of any species were observed over bare sand. These results suggest that differences in fish species richness and abundance are primarily related to habitat structure. Different habitat preferences were evidenced for the juveniles and adults of several fish species. The ecological importance and need for protection of such shallow inshore habitats are discussed in relation to their crucial role as nurseries for many fish species, and their overall importance in maintaining littoral fish populations and species diversity is emphasized.