Does Silence Erode Maritime Claims in the South China Sea? Acquiescence and UNCLOS

The South China Sea dispute centres on overlapping maritime and territorial claims by China and several Southeast Asian states, including the Philippines, Viet Nam, Malaysia, Brunei, and Taiwan, over a strategically vital and resource-rich body of water. Since the 1970s, competing claims to islands, reefs, and surrounding maritime zones, most notably in the Spratly and Paracel Islands, have intensified, driven by access to fisheries, hydrocarbons, and control over key sea lines of communication. China’s claims are unusually expansive, most prominently expressed through the so-called nine-dash line, which encloses the majority of the sea and extends far beyond what other claimants consider permissible under the United Nations Convention on the Law of the Sea (UNCLOS). These claims have been reinforced through patrols, administrative measures, and large-scale land reclamation and militarisation of disputed features. While an international arbitral tribunal ruled in 2016 that China’s claims lacked a legal basis under UNCLOS, Beijing rejected the decision and has continued to assert its position.At the same time, several Southeast Asian claimant states have oscillated between protest and restraint, often prioritising diplomatic stability and economic cooperation over sustained legal confrontation. This uneasy balance between assertion and silence forms the backdrop to ongoing tensions in the region and raises deeper questions about how prolonged inaction, accommodation, or “shelving” of disputes may affect the strength and credibility of maritime claims under international law.

International law does not treat silence as legally irrelevant. Under the doctrine of acquiescence, a state’s prolonged failure to object to another state’s known and adverse conduct may, under certain conditions, be interpreted as consent. International jurisprudence has repeatedly recognised that where a state is aware of a claim affecting its interests, has the opportunity to react, and nonetheless remains silent over a significant period of time, that silence can carry legal consequences.

The International Court of Justice has applied this reasoning most clearly in territorial disputes. In the Fisheries Case (1951), the Court held that the United Kingdom’s long-standing failure to object to Norway’s method of maritime delimitation precluded it from challenging that practice decades later. Similarly, in Temple of Preah Vihear, Thailand’s prolonged acquiescence to an incorrect boundary map was decisive in confirming Cambodia’s sovereignty.These cases illustrate that silence, when combined with time and consistent state practice, may solidify legal claims.

The application of acquiescence in the maritime context, however, is more contested. Unlike territorial sovereignty over land, maritime entitlements under UNCLOS are based primarily on legal entitlement, not effective control. UNCLOS deliberately limits the relevance of unilateral conduct at sea in order to prevent the consolidation of claims through force or coercion. As a result, effective presence or administration, and the absence of protest, does not automatically generate maritime rights in the same way it may on land.

This distinction is particularly relevant in the South China Sea. China’s expansive claims have been accompanied by a steady increase in administrative acts, patrols, fishing regulations, and physical presence. At the same time, several Southeast Asian claimant states have pursued a strategy of “shelving disputes,” prioritising economic cooperation and diplomatic stability over sustained legal or political protest. While this silence does not negate their maritime entitlements under UNCLOS, it may nevertheless have legal and strategic consequences.

Prolonged inaction risks normalising contested practices and weakening a state’s position in future dispute settlement. Even if silence does not extinguish maritime claims as a matter of law, it may erode a state’s ability to credibly challenge another state’s conduct, particularly where that conduct becomes entrenched over time. In this sense, silence may not eliminate legal rights, but it can reshape the factual and diplomatic context in which those rights are asserted.

This dynamic engages the process of customary international law formation, which Mendelson describes as a continuous cycle of “claim and response”. Within this framework, China’s physical patrols and administrative measures constitute active claims, while the prolonged silence of neighboring states functions as a form of State practice through "omission". Mendelson explains that when a state is "directly affected by a practice" and fails to object, its inaction may be legally interpreted as "knowing acquiescence" or "tacit adoption" of the new rule. If such silence becomes "constant and uniform" over a period of time, it builds the "density of practice" necessary for these contested claims to potentially crystallize into a rule of particular or general customary law that could eventually bind the region.

The South China Sea dispute therefore exposes a core tension within the law of the sea. While UNCLOS is designed to anchor maritime entitlements in legal criteria rather than effective control, international law more broadly continues to attach significance to patterns of state behaviour, including prolonged silence. China’s sustained assertion of authority through patrols, regulation, and physical presence, combined with periods of restraint or accommodation by other claimant states, risks reshaping the legal and factual environment in which maritime rights are asserted. Even if silence cannot, as a matter of black-letter law, extinguish UNCLOS-based entitlements, it may nonetheless contribute to the normalisation of contested practices, weaken future legal challenges,and feed into processes of customary international law formation through omission and acquiescence. In this sense, the question is not whether silence formally transfers maritime rights, but whether it erodes the credibility and enforceability of those rights over time. The South China Sea illustrates that in a contested maritime order, inaction is rarely neutral: it may preserve short-term stability and strengthen economic relations, but it carries long-term legal and strategic costs for both claimant states and the maritime system UNCLOS seeks to uphold.

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Reference List

Table of Cases

Fisheries (United Kingdom v. Norway), Judgment, [1951] ICJ Rep 116

Temple of Preah Vihear (Cambodia v Thailand), Merits, [1962] ICJ Rep 6

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