The giant clam Tridacna maxima communities of three French Polynesia islands: comparison of their population sizes and structures at early stages of their exploitation

Several lagoons of the atolls of Tuamotu Archipelago and volcanic islands of the Australes Archipelago (French Polynesia) are characterized by enormous populations of the clam Tridacna maxima, a species considered as endangered in many locations worldwide. Even if this resource can still be considered as virtually intact, the growing harvesting pressure to feed Tahiti's market (up to 50 t of wet matter y−1), combined with the relatively small size of these lagoons, will soon call for management action to sustain a fishery that currently targets a large, virtually pristine stock. Hence, we present T. maxima population sizes and structures for two atolls (Fangatau and Tatakoto) and one island (Tubuai), where high clam densities and population sizes have promoted a small-scale, but growing, commercial fishery since the late 1990s. We followed an earlier pilot study, in which a combination of remote sensing and in situ data provided an estimate of the Fangatau clam population size (23.6 ± 5.3 million clams, mean ± 95% confidence interval, for 4.05 km2 of mapped lagoon). We obtain 88.3 ± 10.5 and 47.5 ± 5.2 million clams for Tatakoto (mapped area of 11.46 km2) and Tubuai (mapped area of 16.3 km2), respectively. Accounting for contrasted length frequency distribution curves and one common size–weight relationship, the total biomasses are 1485 ± 177 t, 1162 ± 272 t, and 2173 ± 232 t of commercial flesh for Tatakoto, Fangatau, and Tubuai, respectively. In addition, given the legal restriction on collecting clams smaller than 12 cm, the legally harvestable biomasses are 958 ± 114 t, 1038 ± 247 t, and 1971 ± 210 t of flesh for Tatakoto, Fangatau, and Tubuai, respectively. The ratio between legal and total stock is much smaller for Tatakoto because this atoll is dominated by small clams, unlike the other two sites. The differences in population size and structure are discussed in terms of natural environment (habitats, degree of aperture to the ocean, temperature variations), providing insights on the natural variability between two similar systems (Tatakoto and Fangatau), and between different systems (the two atolls and the volcanic island of Tubuai), suggesting that future management schemes will have to be optimized locally.