Policy makers in Vietnam face difficult choices when deciding reservoir water management strategies between irrigation and fisheries. In this paper, an economic optimization model for water management is developed to facilitate policy makers' decision making. The model includes the response of rice and fish yields to key factors including reservoir water levels, the timing and quantity of water release, and climatic conditions. The model accounts for variation in rainfall patterns, irrigation requirements, and the demand for low water levels during the fish harvest season. The model is applied to the Daton reservoir in the south of Vietnam to maximize profits in each of three production scenarios where the reservoir's water is used for: (1) only producing rice, (2) only producing fish, and (3) producing rice and fish. Key findings are: (1) for rice production, adequate water should be released to meet rice water requirement and residual water should be stored as a source of water in the case of low rainfall; (2) for fish production, maximum water should be released prior to the fish harvest; (3) for rice and fish production, although water should be released prior to fish harvest, sufficient residual water must remain to satisfy the water requirements of rice in its remaining stages of growth. The model could be applied to other multiple-use resources such as forests, river basins, and land.