The objective of this work is to understand why oysters Crassostrea gigas shows low settlement success in the Thau lagoon despite the fact that biomass of adults is elevated and that larvae show normal development in the water column. Our hypotheses are that oysters compete for food or space with other filter-feeders animals such as sponges, ascidians and other bivalves. To test the trophic hypothesis, we analysed the entire settler community harvested on four-month-old collectors deployed in the Thau lagoon during the summer 2009 for stable isotopes and fatty acids. Based on the isotopic, crustacean amphipods and barnacles can be rejected as a potential food competitor of juvenile oysters. However, other bivalves such as Mytilus sp., Anomia sp., and sponges seem to feed on the same diet. Our results also suggest that the ascidian Ascidiella sp. is the major food competitor of oysters followed by two other ascidian species and the bivalve Lima sp. To test the space hypothesis, sets of collectors were (1) maintained always underwater, which is normally the case in the Thau lagoon where tides are marginal, or (2) manually emerged every weeks to reduce the development of subtidal species. After few weeks, the collectors maintained underwater were dominated by sponges and ascidian whereas the regularly emerged collectors were successfully colonised by oysters and barnacles. This experiment clearly suggests that settlement of pacific oysters in the Thau lagoon is constrained by competition for space with other subtidal species. From a practical standpoint, it seems that it is possible to enhance spat collection of oysters until attaining commercial profitability by regularly emerging the collectors.