Maritime authorities routinely hail vessels to verify identity, navigational status, and intended movements. When a foreign vessel does not respond, the operational risk is collision or miscalculation in congested waters. Importantly, under UNCLOS, a non-responsive vessel’s silence is not, by itself, a breach of international law.
The stabilizing value lies in UNCLOS’s clear rules for each maritime zone, which ensure that a vessel’s non-response—operationally described as silence—is treated as a factual condition to be monitored rather than presumptive illegality. By interpreting silence as a signal requiring observation and documentation, rather than as an automatic ground for enforcement, States protect navigational safety, reduce unnecessary tension, and preserve the balance UNCLOS establishes between coastal-State authority and the freedoms of foreign vessels.1
1. Core Rule
Under UNCLOS, a non-responsive vessel’s radio silence alone does not constitute a breach of international law. Authority to act depends on three factors: (1) the vessel’s location within a defined maritime zone, (2) observable conduct beyond mere non-response, and (3) vessel status, including sovereign immunity for warships and State vessels on non-commercial service. Coastal States may track, shadow, and document, but lawful measures may be considered only when a vessel’s non-response is accompanied by conduct that infringes a specific UNCLOS-based jurisdictional interest. Enforcement must remain proportionate, with restraint as emphasized by ITLOS jurisprudence.2
2. What “Non-Responsive” Means
A “non-responsive vessel” is one that fails to reply to lawful hails via recognized maritime communication channels (e.g., VHF bridge-to-bridge, coast-station calls). This definition is operationally narrow: it refers to communication silence, not to broader misconduct. It is not, by itself, evidence of unlawful passage or breach of UNCLOS. Non-response must be distinguished from AIS carriage or operation issues, which are governed by SOLAS/IMO rules and domestic enforcement regimes rather than UNCLOS.3 4 A non-responsive vessel’s silence is therefore a factual condition, not a legal violation.
3. Doctrine: Elements and Tests
a. Zone Determination
UNCLOS authority is structured by maritime zone: internal waters, territorial sea, contiguous zone, the exclusive economic zone (EEZ), and the high seas. Each zone defines the scope of coastal-State jurisdiction and the limits of flag-State freedoms. The closer to land, the stronger the coastal-State powers; the farther out, the more navigation freedoms dominate. For maritime authorities, the first doctrinal test is to confirm the vessel’s location, because any lawful response must be matched to the jurisdictional powers of that specific zone.5
b. Vessel Status
UNCLOS distinguishes between merchant vessels and sovereign-immune vessels such as warships or government ships on non-commercial service. Merchant vessels are subject to coastal-State jurisdiction within applicable zones, but sovereign-immune vessels are protected from boarding, detention, or judicial process. The doctrinal implication is clear: measures against warships are limited to denial of entry, direction of departure, or diplomatic protest. Identifying vessel status is, therefore, a critical step before considering further action.6
See also: Sovereign Immunity at Sea Under UNCLOS: Meaning, Boundaries, and Operational Use
c. Observable Conduct
A non-responsive vessel’s silence alone is not a breach of UNCLOS. The legal basis is conduct—what the vessel is actually doing. If a ship refuses to comply with lawful safety directions, engages in activities that undermine innocent passage, or disregards collision-avoidance rules, those behaviors may justify coastal-State action. The doctrinal test requires documentation of observable conduct: speed, course, radar tracks, and hailing attempts. Without such evidence, measures may lack lawful authority.7
d. Legal Basis for Action
Every maritime zone establishes clear legal bases for jurisdiction. In the contiguous zone, powers are limited to customs, fiscal, immigration, or sanitary enforcement. In the EEZ, jurisdiction relates to resource management, pollution control, and marine scientific research. The doctrinal requirement is that enforcement must be tied to one of these recognized bases. General suspicion or discomfort with a vessel’s non-response is not sufficient. Without a clear legal hook, action risks violating UNCLOS’s measured framework.8
e. Proportionality and Restraint
Even when enforcement is lawful, UNCLOS and ITLOS jurisprudence require that measures should be reasonable and proportionate to the objective. Disproportionate action is unlawful and undermines legitimacy. The doctrinal anchor is proportionality: action taken must be limited to what is necessary to achieve the lawful purpose. This principle ensures that a communication failure does not develop into unnecessary tension, preserving both safety and legal integrity.9
4. Case Illustrations
a. M/V SAIGA (No. 2) (Saint Vincent v. Guinea, 1999)
The SAIGA, a Saint Vincent-flagged vessel, was detained by Guinea for alleged customs violations while refueling fishing vessels outside Guinea’s EEZ. ITLOS held that Guinea’s enforcement measures were unlawful and emphasized that even when a State has jurisdictional grounds, enforcement must be proportionate and restrained. The Tribunal underscored that excessive force in boarding or detention violates UNCLOS. The doctrinal takeaway is that a non-responsive vessel’s silence or non-compliance cannot justify disproportionate measures; proportionality is the baseline requirement for all enforcement at sea.10
b. M/T San Padre Pio (Switzerland v. Nigeria, 2019)
The San Padre Pio, a Swiss-flagged tanker, was detained by Nigerian authorities while operating in Nigeria’s EEZ. Nigeria alleged violations of domestic petroleum laws, while Switzerland argued that Nigeria had exceeded its jurisdiction under UNCLOS. ITLOS ordered provisional measures requiring Nigeria to release the vessel and crew, underscoring that enforcement in the EEZ must be strictly tied to the jurisdictional authorities recognized by UNCLOS, such as fisheries, pollution control, or marine scientific research. The Tribunal emphasized the principle of due regard for flag-State rights and reiterated that enforcement must be proportionate and fact-based. The doctrinal lesson is clear: a non-responsive vessel’s silence cannot justify detention unless linked to a specific UNCLOS basis.11
c. M/T Heroic Idun (Marshall Islands v. Equatorial Guinea, 2022)
The Heroic Idun, a Marshall Islands-flagged tanker, was intercepted and detained after being accused of oil theft in Nigerian waters and later prosecuted under Equatorial Guinean law. The Marshall Islands sought prompt release before ITLOS, arguing that the detention violated UNCLOS. ITLOS highlighted that enforcement actions must rest on clear and specific UNCLOS authority, and that refusal to comply with coastal-State orders cannot substitute for lawful jurisdictional grounds. The Tribunal stressed proportionality, requiring that measures be limited to what is necessary for the lawful objective. The doctrinal takeaway is that a non-responsive vessel’s silence or disputed communication does not authorize boarding or detention without a valid UNCLOS basis.12
Conclusion
A non-responsive vessel’s silence at sea should be seen as a factual condition of communication, not automatically as a violation of international law. Under UNCLOS, the key question is whether that silence is paired with actual behavior that infringes on the rights or jurisdiction of a coastal State in a particular maritime zone. The rules of authority flow from the sea zones themselves: in internal waters and the territorial sea, coastal States have broader control; in the EEZ and on the high seas, their powers are limited to specific areas such as resource management or pollution control.13
The guiding principle is proportionality, affirmed by ITLOS in M/V SAIGA (No. 2). This means that even when a State has the legal right to act, its response must be restrained and no stronger than necessary to achieve the lawful objective. In practice, this ensures that a simple communication failure does not develop into unnecessary tension and that any measures taken remain balanced and lawful.
By treating a non-responsive vessel’s silence as a signal to be assessed rather than a basis for enforcement, States preserve navigational safety, maintain the balance between coastal-State authority and flag-State freedoms, and prevent unnecessary tension. This approach reflects UNCLOS’s design: a framework that manages risk and promotes stability without turning communication gaps into disputes.
- United Nations Convention on the Law of the Sea arts. 2–3, Dec. 10, 1982, 1833 U.N.T.S. 397 (hereinafter UNCLOS). ↩︎
- M/V “SAIGA” (No. 2) (Saint Vincent v. Guinea), Judgment, Int’l Tribunal for the Law of the Sea [ITLOS], Case No. 2, July 1, 1999, ITLOS Rep. 10, ¶¶ 155–156 [hereinafter M/V “SAIGA”]. ↩︎
- Int’l Maritime Org., AIS Transponders, SOLAS Overview (IMO publication, n.d.) [hereinafter IMO AIS Overview]. ↩︎
- Int’l Maritime Org., AIS Transponders, Note on Maintaining AIS Operation (IMO publication, n.d.) [hereinafter IMO AIS Operation Note]. ↩︎
- UNCLOS arts. 2–3. ↩︎
- Id. art. 32. ↩︎
- Id. art. 19. ↩︎
- Id. arts. 33, 56. ↩︎
- M/V “SAIGA”, ¶¶ 155–156. ↩︎
- Id. ↩︎
- M/T “San Padre Pio” (Switzerland v. Nigeria), Provisional Measures, ITLOS, Case No. 27, Order of July 6, 2019, ¶¶ 72–74. ↩︎
- M/T “Heroic Idun” (Marshall Islands v. Equatorial Guinea), Prompt Release, ITLOS, Case No. 30, Order of Nov. 2022, ¶¶ 98–100. ↩︎
- UNCLOS, arts. 2–3, 33, 56. ↩︎