IMPACT OF CAULERPA-TAXIFOLIA COLONIZATION ON THE LITTORAL ICHTHYOFAUNA OF NORTH-WESTERN MEDITERRANEAN-SEA - PRELIMINARY-RESULTS

After its accidental introduction in the French waters of the Mediterr anean, the green seaweed Caulerpa taxifolia is now spreading along the coast of the Alpes-Maritimes, invading various biotopes (rock, sand a nd Posidonia oceanica seagrass beds). During the spring and fall of 19 92, we evaluated the impact of C. taxifolia on the infralittoral ichth yofauna of Cap Martin (Menton) and compared invaded and reference site s, in shallow (3-8 m) and deep (11-25 m) stations. Caulerpa taxifolia apparently does not have any effect on the composition and global spec ies richness of the ichthyofauna. Therefore, no exclusion phenomenon h as been observed and fish behavior has remained normal (feeding habits , reproduction, recruitment). However, fish density and biomass are sl ightly lower in the colonized sites. Negative regression was observed, in shallow sites during the spring season, between the percentage of substrate invaded by C. taxifolia and the average density of fish asse mblages (slope = -0.13, r = 0.26, p<0.01). Despite these observations, C. taxifolia meadows seem to be a favorable environment for the recru itment of some species of Labridae (Coris julis, Symphodus ocellatus), Sparidae (Diplodus annularis) and Serranidae (Serranus cabrilla) in t he fall. The relative importance of C. taxifolia as environmental para meter and of other habitat characteristics (structural complexity, sub strate slope, hydrodynamism, depth, fishing pressure) is discussed to explain these differences. The results of this first study indicate th at there exists no simple relation between the presence of C. taxifoli a and fish assemblages.