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 paper examines the challenges Brazil’s first open-water MER is facing in trying to achieve these goals. 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.