Abstract
After substantial progress in emission reductions and water quality improvement, semi-enclosed seas often enter a post-remediation governance phase characterized by trade-offs among oligotrophication risks, ecosystem productivity, and fisheries livelihoods. However, how this shift is explicitly articulated and institutionally organized in policy language remains insufficiently supported by verifiable textual evidence. Using Japan’s Seto Inland Sea as a case, this study constructs a corpus of 41 statutory policy documents and traces long-term changes in policy discourse across three governance periods. Methodologically, this study applies a complementary text-as-data evidence strategy: word frequency analysis to identify period-specific core vocabulary and its shifts; paragraph-based co-occurrence networks weighted by the Jaccard coefficient to reveal concept couplings and issue modules; Kruskal-type multidimensional scaling to map the continuous semantic space and detect semantic neighborhoods and transition zones. Results indicate that the corpus consistently focuses on environmental conservation and is organized around an administrative action logic oriented to planning and the implementation of measures. Period 1 is dominated by legal-article structure and procedural vocabulary such as permission and notification, translating pollution control into verifiable statutory requirements and vertically coordinated enforcement arrangements. Period 2 retains the primacy of water quality regulation while strengthening performance-oriented narratives around total load control, load reduction, and target attainment, and integrating eutrophication risks, habitat restoration, watershed coordination, and infrastructure governance into a unified policy framework. Period 3 marks a structural turn: nutrient-salt governance shifts from one-way reduction to an appropriateness-oriented approach, and the discourse becomes tightly coupled with monitoring and effectiveness verification. This study clarifies the institutional mechanisms underpinning the shift from a clean sea to a clean and abundant sea and offers transferable institutional and methodological implications for other enclosed or semi-enclosed seas seeking to balance rule certainty with adaptive flexibility in the post-eutrophication stage.