<p>Gaining public support and involvement is essential for implementing effective social-ecological system (SES) management. Earlier studies focused on an aggregate level, i.e., measured by average; however, we aimed to address public heterogeneity, determine their support for policy, and identify their involvement in SES management. Accordingly, we applied audience segmentation and a messaging approach in coastal SES management to address oligotrophication in part of the Seto Inland Sea (SIS), Japan. In our experimental study, audience segmentation was applied based on a single-item self-categorization measure regarding public opinion about coastal SES issues and management, and the impact of two messaging conditions on the segments was assessed. Respondents living in coastal SES areas were recruited (N = 1800) and randomly divided into three groups: a control group without messaging and two groups with either “Negative consequences of oligotrophication” or “Collective public involvement” messaging. Four primary segments were identified: alarmed, concerned, cautious, and disengaged-dismissive. The groups exhibited distinctive personal characteristics and decreasing levels of policy support and involvement. For example, those in segment “Disengaged- Dismissive” were generally younger and physically and psychologically distant from the sea, showed less recognition of the problems, and exhibited infrequent related behaviors (e.g., consuming local seafood), with lower values of the sea. Such knowledge informs policymakers in targeting decisions. The two messaging conditions comprised varied impacts on different segments, suggesting that policymakers should select messaging conditions based on the target segment. Our study highlighted the importance of audience segmentation and targeted messaging in enabling policymakers to effectively reach the public sector.</p>