Understanding the stability of populations over a long time scale remains a major aim for research in a period when biodiversity is dwindling. Within this context, coral reefs provide a very interesting model, since their high degree of diversity can be compared to that of tropical forests. The Pacific Ocean islands are "life size" laboratories to study biodiversity and its evolution, insofar as migratory fluxes can be checked. Taiaro Atoll features a closed lagoon of 9 km² which only communicates with the ocean during exceptional storms or by particularly strong waves. The coral reef fish population in the lagoon was inventoried in 1977, 1992 and 1994 on sporadic missions. These surveys showed how extremely variable the population is, although we cannot really explain what this is due to. The proposed mission aimed to take new inventory of the fish population but also to take collections to determine the reason for variations noted. Furthermore, the Taiaro Atoll provides a unique structure to study the malleability and adaptation of the biological cycle of coral reef fish. The central issue here is to determine whether lagoon species can complete their biological cycle -especially the larval phase- in the lagoon itself without going through an oceanic phase. Therefore, it would be interesting to determine the origin of the larvae, taking several species chosen for their abundance.