Coastal lagoon fisheries largely depend on themaintenance of the sea mouth. Exchange with the sea affects alterations in chemical and physical conditions, leading to varying availability of fishery resources in the coastal lagoon environment. Adaptive efforts to reopen closed sea mouth have been taken place in many parts of the world. This article explores the impacts of sea mouth opening in the Hokkaido region of Japan using case studies, focusing on the decision-making process among relevant stakeholders. The case studies highlight the possibility of the sea mouth management becoming a social dilemma over when, where, and how opening of the sea mouth should be initiated. Sea mouth management is strongly characterized by uncertainty and heterogeneity inherent in the different interests and needs of various stakeholders. The lessons from the case studies put strong emphasis on the significance of scientific consensus and packaging of scientific knowledge in the decisionmaking process while creating institutional arrangements for sea mouth opening as an integral part of sea mouth management. Based on these discussions, this article presents challenges to adapting to changes in sea mouth condition toward sustainable coastal lagoon fisheries.