Conduct of the Parties as a RELEVANT CIRCUMSTANCES in delimitation process in law of the sea and customary international law

Normally the influence of the conduct of the parties is very limited in jurisprudence and State practice relating to maritime delimitation. So far the only exception is the Tunisia/Libya judgment, which clearly took such conduct into account. In that case, the ICJ attached great importance to a de facto line drawn from Ras Ajdir at an angle of some 26± east of north, which resulted from concessions for the offshore exploration and exploitation of oil and gas granted by both parties. It was the de facto line which effectively governed the delimitation in the first segment to be delimited. The Tunisia/Libya judgment seems to suggest that only when the conduct of the parties can prove the existence of a modus vivendi or a de facto line, or an agreement to apply a particular method, may such facts be taken into account by the courts. Nonetheless, the Court’s approach entails the risk of introducing the idea of effectiveness or occupation into the law of maritime delimitation.
The rights over the continental shelf are attributed to the coastal State ipso facto and ab initio. Accordingly, the idea of effectiveness would be incompatible with the fundamental character of legal rights over the continental shelf. Furthermore, by giving excessive weight to the conduct of the parties, unilateral acts of occupation of the continental shelf may be encouraged. In view of these questions, it appears that the Tunisia/Libya judgment cannot have general application in the law of maritime delimitation.