This paper explores the externally-led vertical differentiation of third-party certification standards using the case of the Marine Stewardship Council (MSC). We analyze this process in two dimensions. First, fisheries employ strategies to capture further market value from fishing practices that go beyond their initial conditions for certification and seek additional recognition for these activities through co-labelling with, amongst others, international NGOs. Second, fisheries not yet able to meet the requirements of MSC standards are being enrolled in NGO and private sector sponsored Fisheries Improvement Projects (FIPs), providing an alternative route to global markets. In both cases the credibility and authority of the MSC is challenged by new coalitions of market actors opening up new strategies for capturing market value and/or improving the conditions of international market access. Through the lens of global value chains, the results offer new insights on how such standards not only influence trade and markets, but are also starting to change their internal governance in response to threats to their credibility by actors and modes of coordination in global value chains.