The southwestern coast of Taiwan used to be a vast lagoon with rich fisheries. This water attracted not only aboriginals to settle along the shore, but also merchants and commoners from southern China to trade, fish and hunt in the area. However, in merely five decades towards the end of the eighteenth century, this region witnessed rapid and large-scale deposit of sediment because of human exploitation, changes in river courses and natural disasters. This article is a case study of Gangkou (港口) area located at northeastern Old Taijiang Inland Sea (the present-day Anding District, Tainan City, Taiwan). With reference to past literature, historical materials, including a contract dated at 1814/15 among Gangkou villagers, and fieldwork findings, this article traces the historical and social development of Gangkou for a better understanding of how the villagers responded to a dramatically changing environment. Besides facing natural disasters that not only threatened their lives and properties but also transformed the natural environment and their subsistence style of livelihood, Gangkou villagers had to compete with inhabitants of neighboring areas for the dwindling marine resources. The situation was further complicated owing to state involvement in response to local armed conflicts. Both capable and insightful, the elites of Gangkou decisively united villagers with different opinions to cope with the strong competition from neighboring villages and harassment of corrupt governmental officials.