Simply asserting customary international law will not accrue all benefits of the convention
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International customary laws have developed out of “concordant practice by a number of states . . . over a considerable period of time,” when such practice is thought to be required by, or consistent with, the prevailing international law, and when such practice is generally accepted by other states.117 As mentioned in section III, the Convention itself is based in part on international customary laws. In addition, when an issue is not regulated by the Convention, the customary laws serve a gap-filling role, and because the Convention binds only its signatories, customary international law remains an important means of transacting with non-signatories of the Convention.118 However, the Convention expands the “existing norms to suit new developments where the existing norms are no longer sufficient,” creates new norms, and in some cases replaces old norms that are no longer appropriate.119 Thus, asserting customary international law will not secure all the benefits of the Convention for the United States because the signatories of the Convention do not have to extend specific rights established in the Convention, or those which are modifications of the existing rules, to non-signatories.120 For example, Canada may choose not to grant the United States the right of scientific research in the EEZ or in the continental shelf.121
Opponents of UNCLOS claim that the United States should not become a party because the United States already enjoys the benefits of UNCLOS through customary law and, therefore, should not unnecessarily incur the treaty's burdens. However, this ignores the fact that customary law can change and can also be influenced by how parties to UNCLOS decide to interpret its provisions.