The extent to which flag States are bound by the conservation rules of regional fishery management organisations is an important question in the quest to reduce unregulated fishing. The European Union implemented a trade suspension against Cambodia under Council Regulation 1005/2008, in response to unregulated fishing by Cambodian vessels in high seas areas managed by regional fishery management organisations. Limitations in the arguments underpinning the decision evidence the flaws of unregulated fishing as a legal concept, underlining the need for it to be appropriately interpreted and contextualised in the international legal framework. Clarity on the differences between conventional and customary sources of international legal obligation, and their implications for State consent, should guide the implementation of the Regulation to maximise the normative potential of resulting practices. •Commission must underpin implementing decisions with legal rationale.•Unregulated fishing and UNFSA Article 8.4 customary status not suitable.•State practice in RFMOs and arrangements should provide such rationale.•Due regard underpins such rationale.•Due regard insufficient to ensure conservation by itself.