For decades, fishermen in most parts of the world have been experiencing a reduction in fish abundances. Overexploitation and increasing demands are the glaring visible causes, but weakness or absence of fishery management is the core problem. Faced with repeated failure of fishery management and the resulting overexploitation in fish stocks, new management plans are needed in order to preserve both fishermen's jobs and food security. A growing number of published sources have proposed marine protected areas (MPA) as a fishery management tool. The decision of MPA design requires close collaboration with local fishermen communities for it to be accepted and respected. This paper focus on the case of a Uruguayan lagoon, the Rocha lagoon, which is exploited by two fishermen communities. The lagoon is located inside a national park. Park authorities are in the process of designing a management plan that defines a MPA inside the lagoon. The plan also sets out the rules to be upheld for the artificial opening of the sandbar that separates the lagoon from the ocean. It is shown that it is relevant to study the local ecological knowledge (LEK) in order (1) to understand the fishery related ecological issues within the lagoon and (2) to highlight an existing conflict between two fishermen communities. Studying the LEK allowed a clear representation of the factors that must be taken into account when defining the management plan. Furthermore, the LEK study in itself creates an appropriate place for inter-community debate and it enhances the acceptance of the future management plan.