Unregulated fishing on the high seas and the formation of customary international law

Custom is an essential source of international law. Yet the process of customary law formation has received much less attention than its identification. This thesis studies the rise of an obligation on third states to cooperate with regional fisheries management organisations (RFMOs) to manage high seas fishery resources and examines the evolution of this duty to contribute to understanding the formation of customary law.

The thesis examines whether the duty to cooperate is leading to the formation of a customary rule. It concludes that global practice through RFMOs, and their actions against unregulated fishing, suggests this obligation is emerging as custom, performing normative effects beyond treaty commitments. The interactions based on claim and acceptance, acquiescence or protest explain the legal consequences of emerging custom. The dynamics of the formation process challenge common criticisms of customary international law and demonstrate that developing states play a crucial role in custom development.

The success of RFMOs in confronting uncooperative non-member states illustrates the interplay of treaties, international organisations and non-binding instruments in custom formation. Customary law can indeed form around multilateral treaties and in a legal environment defined primarily by treaty obligations. The rise of the duty to cooperate through RFMOs suggests that international organisations contribute to custom formation both as catalysts and legal actors creating relevant practice. It also reveals the decisive role of non-binding instruments in sparking customary law development.