Premier inventaire et distribution de la faune ichtyologique de l'atoll de Scilly (Polynésie française).

Abstract

For its geographical position as the most Western Island of French Polynesia, for its morphology of closed atoll with important communications between the lagoon and the Ocean through some hoa and over a wide submerged outer reef flat, for its deep lagoon on the course to silt up, and with few coral patches never reaching the water surface, the atoll of Scilly should have a fish population with specific characters. To study them, various stations have been established in both lagoon and outer reef flat, but owing to the stormy weather it has not been possible to survey the outer reef brow and slope. The investigation has been carried out by using several technics as direct observation, gun, net sampling and punctual rotenone poisoning. The results of the survey point out the following features : 1. A number of 180 species has been inventorized. This number is far more superior to that observed in the Tuamotu Islands in the lagoons of the closed atoll of Taiaro and of the open atoll of Mataiva. It is at the present time approximatively the same as the one registered in the closed atoll of Takapoto, which is considered as not restrictive at all. 2. The percentage of ubiquitous species (7%) is consistent with the previous data obtained in French Polynesia or in other places of the Pacific area. 3. In the lagoon, the populations of vagile species (Chanidae, Mugilidae and Carangidae) are opulent, whereas the sedentary species are scanty. This data can be directly related to the scantiness of the live coral covering. 4. The presence, in the lagoon of this closed atoll, of tunas (Euthynnus affinis), barracuda (Sphyraena forsteri), and scads (Selar crumenophtalmus) gives evidence of exchanges between oceanic and lagoon waters. The density of population for these species, and their different size indicate that these exchanges are many and frequent. 5. The most opulent area of this atoll stands on the northern reef flats, where we have found 92 species. Among them, the herbivorous species (Acanthuridae and Scaridae) are dominant, but the number of carnivorous fish (Lutjanidae Holocentridae and Sparidae) has to be taken in account. 6. In spite of the presence of potentially toxic species, the ciguateric risk seems presently minor in Scilly owing to the fact that the ciguatoxigenic dinoflagellates Gambierdiscus toxicus are scanty. However the presence of the causative agent of ciguatera should make possibly periodically the occurrence of small outbreaks of fish poisoning concerning the big sized reef carnivorous fishes.