At a time when ecological monitoring is crucial to implement efficient biodiversity conservation policies, we studied the perception of Marine Protected Areas (MPAs) agents on the place of ecological data in the functioning of MPAs. We interviewed agents working in eight of the main MPAs of the French Mediterranean Sea through qualitative surveys. We found that ecological monitoring encompasses multiple dimensions and fulfills several scientific, economic, psychological and political functions in the MPA while ultimately having limited effect on the implementation of biodiversity policies. The governance mode of some MPA and the chronic under-funding of marine conservation policies threatens both the quality of the ecological knowledge and its ability to affect decision-making. However, having ecological knowledge allows MPAs and their agents to establish themselves locally as essential institutions for marine biodiversity management. In this context, ecological monitoring not only justifies the existence of MPAs but can as a smokescreen, masking the absence of more stringent environmental regulations. Investigating how ecological monitoring is perceived within MPAs helps to unravel and describe the social and political processes underpinning ecological data and highlights its significance in contemporary marine conservation efforts.