This article solves the puzzle of why developing countries such as Ghana are responsive to the European Union's (EU) promotion of sustainable fishing despite the fragmentation and contestation of environmental norms. Analysing the EU-Ghana interaction with rationalist and constructivist perspectives on norm diffusion, this article reveals EU counterparts' motivations for domestic fisheries reforms. It argues that although the EU exercises both 'normative power' and 'market power' to encourage sustainable fishing, EU partners are more reactive to the manipulation of material benefits associated with the European market access. By highlighting such motivations, and thus the relational aspect of EU power, this article also contributes to the EU-as-a-power debate from the often-overlooked perspective of EU partners. The findings capture the characteristics of EU influence and further illuminate the fisheries policymaking and collective action mobilisation needed for broader environmental protection.