Contenu du sommaire : Récifs et lagons de Polynésie française

In considering the 84 atolls of French Polynesia, the atoll on Scilly, which was studied during an expedition in 1979, was chosen as representative of a category of those low islands characterised by the absence of passes (Scilly is a closed atoll) and by an atoll rim a large part of which is submerged thereby increasing the exchange between oceanic and lagon waters. Does the existence of the submerged crown bring with it living lagoon populations such as those found on open atolls (great richness of species, weak dominance) or those of closed atolls (poor in species, strong dominance by some species)? The existence of leading lines of beach rock in the process of erosion on the submerged borders of the crown (west and south of the atoll) demonstrates the former and recent existence of islets which 'closed ' the atoll to oceanic influences; these islets were composed of material swept into the lagon by cyclones or tsunami. Here is one explanation for this particular geomorphological structure found infrequently in the atolls of French Polynesia. The benthic fauna of the lagoon is characteristic of the populations of lagoons of closed atolls: poor in species and strong dominance of certain of these species. The noticable absence or presence of certain species or groups can be noted. The existence of a submerged border, as exists on Scilly, does not seem to allow oceanic influences as strong or as determinative for the lagoon environment and its populations as does a pass.

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.

The terrestrial flora of Scilly Atoll has never been studied in detail by a botanist. The only plant collection (18 species) was made in a few days in 1921-1922 by the ornithologist E. H. Quayle. More recent general observations on the flora and vegetation are available. Although coconut plantations have replaced much of the natural plant cover, a fair sampling of the mesophile forest of the interior remains, as well as several types of shore scrub and forest. The forest of Pemphis acidula on old island reef conglomerate is especially striking. The known flora, from Quayle's specimens and later sight records, includes about 45 indigenous and introduced plants, but quite a few species have certainly been missed.

The Scleractinian coral communities of the atoll of Takapoto (Tuamotu archipelago, French Polynesia) were quantitatively studied in the lagoon and on outer reef areas. Coral communities settled on the ocean slope of the atoll displayed a high coverage rate of the substratum and a bathymetric distribution typical of Indo-Pacific reefs: upper zone (0 — 15 m) dominated by the genus Pocillopora, Acropora and Pavona; medium zone with Porites and a lower zone dominated by Agariciidae (Leptoseris and Pachyseris). Takapoto is an enclosed atoll and the communities of the lagoon emphasize the peculiar environmental conditions prevailing in such a biota (high salinity, high sedimentation rate). The coral communities are poorer than those of the outer reef of the atoll and quantitatively dominated by a few species. Difficult settlement conditions for larvae induce patchiness in the pattern of distribution of the species.

Quantitative data are given on the distribution of the main herbivorous reef fishes (Acanthundae and Scaridae) of the enclosed atoll of Takapoto (Tuamotu Archipelago, French Polynesia). Visual counts were carried out by diving along transects (200 m long and 5 m wide) at 11 stations located inside the lagoon and on the outer reef to 30 m deep. The highest density of herbivorous fishes was recorded in shallow waters (5 m) around the lagoon pinnacles and between 10 m and 20 m on outer reef areas. The population structure of herbivorous fishes varied much according to biota. Parrotfishes were numerically dominant in the lagoon and formed plurispecific groups composed of many juveniles whereas Surgeonfishes dominated on outer reef areas. Species of Acanthuridae occupied specific zones of the atoll and a change in their population structure was noted and related to a change in the health conditions of the reef.

In May and June was done a diving research up to 45 m depth in the outer slopes of the wall reef and in the reefs and the lagoon of Moorea, Society Archipelago. All in all there were established 74 Scleractinia and 2 hydrocorals. Psammocora profundacella, Montipora cf. tuberculosa, Pontes lutea and Synaraea convexa were growing in the surface water as well as in more than 40 m depth, in the open ocean as well as in the lagoon, in waters with dense suspensions and angered chemical quality by influence of a river mouth. There are significant euryoecious species. Other species were living on the outer slopes above 20 m only and may be called "hydrodynamophile" species. The genus Echinopora and in addition 12 other species were ascertained for the first time in the central Pacific.

A study of growth and productivity has been carried out on eight abundant species of French Polynesian mollusks : Nerita plicata, Tectarius grandinatus, Erosaria obvelata, Mitra mitra, Area imbricata, Chama iostoma, Tridacna maxima and Cardium fragum. If we endeavor to extrapolate our results on the scale of French Polynesia's coral reef complexes, we may establish the following facts : Production potentialities are about the same on the atolls' outer reefs compared with those of the lagoon (about 50 kg/ha/y). But in the coral reef complexes of high volcanic islands, productivity is most important for the Mollusk group (about 230 kg/ha/y).

Ciguatera, a neurointoxication caused by eating fish which have concentrated toxins produced by a bottom-dwelling dinoflagellate, is distributed unevenly in space and through time. Morbidity statistics are reviewed and a hypothesis is presented which helps to explain the higher incidence of ciguatera in the eastern Pacific. Islanders have adapted to the phenomenon over time and the strategies are reviewed. With population shifts to urban centers and the development of small scale commercial fisheries, the impact of ciguatera is exacerbated. The implications are explored in terms of both nutrition and marine resource development, using Pacific Island examples. It is stressed that ciguatera need not hamper small scale fisheries development.

The transport of pigs, as domestic or wild animals, into regions east of the Wallace line, has resulted in four different types occurring outside the natural range of the genus. One of these is of hybrid origin. Characters of each are given, and a survey of Sus systematics.

The paper discusses the discontinuous distribution of pig and dog in Polynesia. It suggests that in some cases the absence of one or both species may result from economic causes. Since the diet of both pig and dog in Polynesia was based on agricultural foods, their exploitation required a heavy investment of economic resources. As human populations increased, with consequent economic pressure, so would the relative cost of animal husbandry. Whether animal exploitation remained viable was largely related to the availibility of alternative sources of protein, in particular of marine resources.