The conservation of marine species not targeted but adversely affected by fishing is often limited to the top-down designation of marine protected areas. Contrary to this general rule, fishers along the German Baltic Sea modified their fishing practices voluntarily to avoid the bycatch of harbour porpoises (Phocoena phocoena) and diving sea ducks. This research applies the Institutional Analysis and Development framework, combining document analysis and stakeholder interviews, to trace the emergence of the underlying voluntary agreement that was signed between fishers, the federal Ministry of the Environment, and a local nongovernmental information centre. We find that two key factors enabled collective action: First, the government’s threat to impose stricter formal regulations prompted fishers to collaborate to retain autonomy. Second, the involvement of the information centre as a policy broker, who mediated the negotiations between fishers and government decision-makers, fostered trust and social capital between actors, despite the policy threat. The final agreement highlights the interplay between top-down and bottom-up conservation action: We conclude that some top-down incentive or threat may be necessary for initiating collective action to protect non-target species. Beyond that initial trigger, trust, shared mental models, and relationships between actors played a major role in the rulemaking and enforcement process. The study emphasises the added value of integrating relationships and dynamics between actors into institutional analysis.
