Diversite de l'ichtyofaune corallienne a Rapa (Polynesie franchise)

Diversity of coral reef fish at Rapa Island (French Polynesia). The South Pacific island of Rapa (27°36'S, 144°21'W) is the southernmost inhabited island of French Polynesia. It is sufficiently isolated (the closest island lies 520 km northwest of Rapa) to be regarded as a separate geographic unit. In 2002, an international team of scientists conducted a marine biodiversity survey of Rapa (algae, corals, fishes and molluscs). The present study presents the diversity offish, with two objectives: i) establish the checklist of coral reef fish species present at Rapa, and ii) study the spatial distribution of coral reef fish communities. In 1990, Randall, Smith and Feinberg published a list including 274 coral reef fish species at Rapa. During our expedition at Rapa, fish surveys were conducted to sample specimens at 66 collecting stations from 0 to 78 m of depth by visual survey and/or capture with the ichthyocide rotenone and by spearing. Combining these fish records with the list of Randall et al. brings the number of coral reef fish species known from Rapa to over 383 species. The Rapan fish fauna is different from those already recorded in French Polynesia. Some shallow-water families or genera, of which there are wide-ranging species in French Polynesia, were not found at Rapa: Clupeidae, Carapidae, Pseudochromidae, Malacanthidae, Mugiloididae, Soleidae, Scorpaenopsis and Pseudanthias. Moreover, 10% of species are currently known only from Rapa. Visual surveys of coral reef fishes were conducted at various stations around Rapa. Spatial variation in fish communities was compared to changes in environmental variables (water depth: 0, 3, 6, 12, 18, and 24 m; reef zone: bay vs. outer slope; exposition to the swell: North, South, West, and East). The total species richness of coral reef fishes at Rapa was low (108 species recorded on 60 transects of 100 m). Mean abundance of reef fishes was also low (1.81 +0.29 SE fish per m), with more than 49% of all fishes belonging to the family Pomacentridae and 31% to Scaridae. The spatial structure of fish was characterized by four communities. In the bay, the fish community was dominated by the Pomacentridae (72.7%) with Chromis atripectoralis and Dascyllus aruanus as the most abundant species. In contrast, the deep reef slope community (> 12 m) from North to South-Western coast was characterised by high densities of the pomacentrid Chrysiptera galba, as well as high densities of the acanthurid Acanthurus leucopareius. The second deep reef slope community encountered on South-East to East coast was characterised by high densities of Scaridae (64.1 %), mainly Scarus psittacus and S. schlegeli. Lastly, the shallow reef slope community (< 12 m) was dominated at 51.3% by Pomacentridae {Chromis atripectoralis) and at 34.1% by Scaridae {Scarus psittacus and S. schlegeli).