A comparison of fish assemblages from two types of algal beds and coral reefs in the south-west lagoon of New Caledonia

Whereas fish assemblages of tropical seagrass beds have been widely studied , those of tropical macroalgal beds are poorly known. To provide informatio n about these assemblages, the fish of two different types of algal beds (d ominated by Sargassum sp. and Halimeda sp.) were sampled using visual censu s in the south-west lagoon of New Caledonia, and were compared to coral ree fs. The number of species present on Sargassum beds (202 spp.) and reefs (1 87 spp.) was more than twice that observed on Halimeda beds (86 spp.). Only 42 species were shared among the three habitat types. Mean species richnes s and density decreased sharply from reefs (52 species per transect, 6 fish /m(2)) to Sargassum (35, 1.5) and Halimeda beds (11.5, 0.2). Differences be tween the two algal beds probably resulted from the higher structural compl exity of the Sargassum beds (presence of a higher coral, rocky and algal co ver than in Halimeda beds). Pomacentridae greatly dominated fish assemblage s on reefs and Sargassum beds whereas Lethrinidae were prevailing on Halime da beds, important similarities were observed between the dominant species from reefs and Sargassum beds (e.g., Neopomacentrus spp., Pomacentrus moluc censis, Scolopsis bilineatus), whereas major differences existed with Halim eda beds on which Lethrinus genivittatus was dominant. Species diversity am ong trophic categories was relatively similar between habitat types but the density and the biomass varied markedly, especially between Halimeda beds and the two other habitats. Territorial or sedentary fishes were dominant o n reefs whereas a higher proportion of the large roaming fishes was found o n algal beds. This study gives a useful description of the poorly known fis h assemblages in tropical algal beds and demonstrates that different algal habitats support different fish assemblages.