Marine extractive reserves (MER) are being established in coastal areas of Brazil to protect ‘traditional’ coastal populations and the marine resources upon which their livelihoods depend. This approach to conservation is supported by common property theory that questions the inevitable destruction of collectively managed resources. This paper examines the challenges Brazil’s first open-water MER is facing in trying to achieve these goals. A brief review of the theoretical underpinnings of common property management through collaborative management is presented followed by a summary of the historic evolution and the creation process of MERs. The paper then investigates the quality of the institutions which have traditionally governed the beach seining community in Arraial do Cabo, Rio de Janeiro, Brazil. Finally, factors that constrain or provide potential for long-term participatory conservation are presented. Results from a pilot project in Arraial do Cabo, Rio de Janeiro suggest that significant social barriers to collective action exist and that local resource governing institutions are not robust. Consequently, fishers are not becoming decisive players in the decision-making process. The implications of these conclusions for future maritime conservation policy in Brazil are explored. The study concludes that extractive reserves in general represent the first conservation units in which specifically involve local communities in their design and management. These initiatives have enormous potential for conserving coastal areas and securing the livelihoods of coastal populations. This study suggests, however, that in order for these goals to be realized both parties must be willing and able to carry out their role in the process.