Depth and substrate type driven patterns in the infralittoral fish assemblage of the NW Mediterranean Sea

Fish fauna was studied in the coastal area of Portofino Promontory (Ligurian Sea) by means of visual census techniques to explore the relationships between fish assemblage, depth and substrate type. Because of the complexity of substrate types (Posidonia oceanica, rocky bottom, pebble, sand) and depth ranges (0-3; 3.1-12; 12.1-24 m), two different visual census methods were used. Sampling was carried out between 1998 and 1999. The data showed that species richness was greater at shallower depths and positively related with the structural complexity of the substrate type. Data analysis indicated a strong affinity between fish assemblages associated with pebble, P. oceanica meadow and rocky bottom, and confirmed the difference between ichthyofauna from sand and other substrate types. Depth was the main factor affecting variability in fish assemblages associated to P. oceanica, pebble and rocky bottom. The medium size class of fish was generally predominant in all depth and substrate type, except for the shallower depth range over sand and the deeper depth range over rocky bottom, where succeed respectively small and large size classes. The strong evidence of the role of depth and substrate should be taken into consideration in planning future studies to characterise fish assemblages in other Italian and Mediterranean coastal areas.Original Abstract: Influences de la profondeur et du type de substrat sur la distribution des peuplements de poissons dans l'infra-littoral de la mer Mediterranee nord occidentale. L'ichtyofaune de la region cotiere du promontoire de Portofino (mer Ligure) a ete etudiee a l'aide de techniques visuelles afin d'etudier les liens entre les peuplements de poissons, la profondeur et le type de substrat. Une strategie d'echantillonnage specifique a ete elaboree suivant deux methodes de recensement en raison de la diversite des types de substrats (Posidonia oceanica, fonds rocheux, galets, sable) et de la profondeur (0-3 ; 3,1-12 ; 12,1-24 m). L'echantillonnage a ete effectue entre 1998 et 1999. Les donnees recueillies montrent que la richesse en especes est plus importante pour de faibles profondeurs et que l'abondance est correlee positivement avec la complexite structurale du substrat. L'analyse des donnees a montre une forte affinite entre l'ichtyofaune associee aux galets, aux herbiers de P. oceanica, et aux fonds rocheux. A l'inverse,il il existe une difference entre l'ichtyofaune associee au sable et celle des autres substrats. La