What Is Flag State Jurisdiction on the High Seas and What Are Its Exceptions?

The high seas, defined under the United Nations Convention on the Law of the Sea (UNCLOS) as all parts of the sea that are not included in the exclusive economic zone, the territorial sea, or the internal waters of a State, or in the archipelagic waters of an archipelagic State, have long been cherished as a domain of freedom. This freedom, however, is not absolute. To maintain order and ensure lawful conduct in these vast areas beyond national sovereignty, international law relies primarily on the principle of flag State jurisdiction. This article delves into the concept of flag State jurisdiction and explores the well-defined exceptions that permit non-flag States to exercise authority over vessels on the high seas.

Flag State Jurisdiction: The General Rule

The foundational principle governing activities on the high seas is that ships are subject to the exclusive jurisdiction of the State whose flag they are entitled to fly. UNCLOS Article 92(1) explicitly states: "Ships shall sail under the flag of one State only and, save in exceptional cases expressly provided for in international treaties or in these Articles, shall be subject to its exclusive jurisdiction on the high seas." This means that the laws of the flag State apply to the vessel, its crew, and any activities conducted onboard, and it is primarily the flag State's responsibility to enforce those laws.

Nationality of Ships and the "Genuine Link":
A ship acquires the nationality of the State whose flag it is entitled to fly (UNCLOS Article 91). While international law requires that there be a "genuine link" between the State and the ship, the practical implications of this requirement have been debated. The International Tribunal for the Law of the Sea (ITLOS) in the M/V "Saiga" (No. 2) Case (Saint Vincent and the Grenadines v. Guinea) (Judgment of July 1, 1999) indicated that the determination of the criteria for a genuine link is largely left to the flag State, and other States may have limited grounds to challenge it. The flag State has the duty to effectively exercise its jurisdiction and control in administrative, technical, and social matters over ships flying its flag (UNCLOS Article 94). However, the prevalence of "flags of convenience," where ships are registered in states with lax regulations and minimal connection to the vessel's ownership or operation, continues to pose challenges to effective flag State control.

Exceptions to Exclusive Flag State Jurisdiction

Despite the paramountcy of flag State jurisdiction, international law recognizes several specific and narrowly defined exceptions. These exceptions are generally aimed at addressing activities that threaten the common interests of the international community or the particular interests of coastal States.

A. Right of Visit (UNCLOS Article 110)

Warships or other duly authorized ships or aircraft on government service have the right to approach any vessel on the high seas to verify its right to fly its flag. If, after checking the documents, suspicion remains, the warship may proceed to a further examination on board the ship (boarding and inspection). This "right of visit" is permissible only under specific circumstances:

1. Piracy: If there is reasonable ground for suspecting that the ship is engaged in piracy.
   • Definition of Piracy (UNCLOS Article 101): Piracy is defined as any illegal acts of violence or detention, or any act of depredation, committed for private ends by the crew or the passengers of a private ship or a private aircraft, and directed:
     • on the high seas, against another ship or aircraft, or against persons or property on board such ship or aircraft;
     • against a ship, aircraft, persons or property in a place outside the jurisdiction of any State.
   • This definition excludes acts committed within a State's territorial waters (which may be classified as "armed robbery against ships" under the International Maritime Organization's definition) and acts committed by the crew or passengers against their own ship (such as mutiny or hijacking, which are addressed by other legal frameworks like the Convention for the Suppression of Unlawful Acts against the Safety of Maritime Navigation (SUA Convention)).
   • The "private ends" requirement has historically served to exclude acts committed by insurgents for political purposes. The Belgian Court of Cassation in the Sirius case (1986) controversially found that acts by an environmental group against a vessel discharging waste constituted piracy, interpreting "private ends" broadly.
   • Universal Jurisdiction: All States have the right to seize a pirate ship or aircraft, or a ship or aircraft taken by piracy and under the control of pirates, and arrest the persons and seize the property on board. The courts of the State which carried out the seizure may decide upon the penalties to be imposed (UNCLOS Article 105).
   • Modern Piracy (e.g., off the Coast of Somalia): The resurgence of piracy, particularly off the coast of Somalia, in the late 2000s and early 2010s prompted significant international action. UN Security Council resolutions, acting under Chapter VII of the UN Charter, authorized states cooperating with Somalia's Transitional Federal Government to enter Somali territorial waters and even conduct land-based operations to repress piracy. International naval task forces were deployed, and many states enacted or updated national anti-piracy legislation (e.g., Japan's Anti-Piracy Measures Act of 2009). However, prosecuting suspected pirates has posed significant challenges, including securing evidence, finding states willing to prosecute (burden-sharing), and ensuring fair trials and humane detention. While incidents off Somalia have decreased significantly due to these efforts, the underlying causes in Somalia remain, and piracy continues to be a threat in other regions.
2. Slave Trade: If the ship is suspected of being engaged in the slave trade (UNCLOS Articles 99 and 110(1)(b)).
3. Unauthorized Broadcasting: If the ship is engaged in unauthorized broadcasting from the high seas intended for reception by the general public contrary to international regulations (UNCLOS Articles 109 and 110(1)(c)).
4. Stateless Vessels: If the ship is without nationality (UNCLOS Article 110(1)(d)). A ship that sails under the flags of two or more States, using them according to convenience, may be assimilated to a ship without nationality (UNCLOS Article 92(2)).
5. Ship of Same Nationality: If, though flying a foreign flag or refusing to show its flag, the ship is, in reality, of the same nationality as the warship (UNCLOS Article 110(1)(e)).

If suspicions prove to be unfounded, and provided the ship visited has not committed any act justifying them, it shall be compensated for any loss or damage that may have been sustained (UNCLOS Article 110(3)).

B. Hot Pursuit (UNCLOS Article 111)

The right of hot pursuit allows a coastal State to pursue a foreign vessel that has violated its laws and regulations when that vessel is within the coastal State's jurisdictional waters, and to continue that pursuit onto the high seas if necessary.

Conditions for Lawful Hot Pursuit:

• The pursuit must be commenced when the foreign ship is within the internal waters, archipelagic waters, territorial sea, or contiguous zone of the pursuing State. It may also be commenced from the exclusive economic zone (EEZ) or the continental shelf, but only for violations of the laws and regulations applicable to those specific zones.
• There must be good reason to believe that the ship has violated the laws and regulations of the coastal State.
• A visual or auditory signal to stop must have been given at a distance which enables it to be seen or heard by the foreign ship before the pursuit begins.
• The pursuit must be continuous and uninterrupted.
• The pursuit must be undertaken by warships or military aircraft, or other ships or aircraft clearly marked and identifiable as being on government service and authorized to that effect.
• The right of hot pursuit ceases as soon as the ship pursued enters the territorial sea of its own State or of a third State.

The use of force during hot pursuit must be limited to what is necessary and reasonable in the circumstances. The I'm Alone Case (Canada v. United States) (Joint Interim Report, 1933; Final Report, 1935) and the M/V "Saiga" (No. 2) Case confirmed that force should be avoided if possible, and if used, must not go beyond what is strictly necessary to effect the arrest.

C. Pollution Intervention on the High Seas (UNCLOS Article 221)

Coastal States have the right to take and enforce measures beyond their territorial sea, proportionate to the actual or threatened damage, to protect their coastline or related interests (including fishing) from pollution or threat of pollution following a maritime casualty which may reasonably be expected to result in "major harmful consequences". This right, which originated from the Torrey Canyon incident in 1967, is also codified in the International Convention Relating to Intervention on the High Seas in Cases of Oil Pollution Casualties (1969) and its 1973 Protocol covering other hazardous substances.

D. Illicit Traffic in Narcotic Drugs or Psychotropic Substances (UNCLOS Article 108)

UNCLOS Article 108 requires all States to cooperate in the suppression of illicit traffic in narcotic drugs and psychotropic substances engaged in by ships on the high seas contrary to international conventions. It also provides that any State which has reasonable grounds for believing that a ship flying its flag is engaged in such illicit traffic may request the cooperation of other States to suppress it. This article itself does not grant non-flag States a unilateral right to board or arrest, but it establishes a strong basis for cooperative enforcement actions, often operationalized through bilateral or regional shipboarding agreements. Many states, including the United States, have concluded such agreements, particularly with prominent flag of convenience states, to facilitate drug interdiction on the high seas.

E. Fisheries Enforcement on the High Seas

Traditionally, freedom of fishing was a core component of the freedom of the high seas. However, to address overfishing and conserve marine living resources, international law has evolved. UNCLOS itself obliges States to cooperate in the conservation and management of living resources in the areas of the high seas (Articles 116-119).
More specific enforcement powers for non-flag States exist under certain treaties:

• The 1995 UN Agreement on Straddling Fish Stocks and Highly Migratory Fish Stocks (UNFSA) provides for member States of regional fisheries management organizations (RFMOs) to board and inspect fishing vessels of other State Parties on the high seas within the RFMO's area of competence to ensure compliance with conservation and management measures. If clear grounds for a serious violation are found and the flag State fails to respond or take action, the inspecting State may, under certain conditions, bring the vessel to a port.

F. Counter-Proliferation of WMD and Suppression of Maritime Terrorism

Recent international efforts have focused on preventing the maritime transport of weapons of mass destruction (WMD) and related materials, and suppressing acts of maritime terrorism.

• The 2005 Protocol to the Convention for the Suppression of Unlawful Acts against the Safety of Maritime Navigation (SUA Convention), though not yet universally ratified at the time of earlier reviews, significantly expands the list of offenses and provides a framework for States Parties to request and authorize the boarding of vessels on the high seas suspected of involvement in offenses covered by the Protocol, including WMD transport and terrorism-related acts. Flag State consent remains a key principle, but the Protocol includes provisions for implied consent or streamlined authorization procedures in certain situations.
• The Proliferation Security Initiative (PSI), launched in 2003, is a multinational initiative aimed at stopping trafficking of WMD, their delivery systems, and related materials. Participating states commit to take action to interdict such shipments, relying on existing national legal authorities and relevant international law, including flag state consent for actions on the high seas, or action in their own territorial waters or based on intelligence shared among participants.

These mechanisms reflect a growing international consensus that certain activities on the high seas pose such a significant threat that cooperative, and sometimes more assertive, enforcement actions by non-flag states are warranted, usually within a framework that still respects the ultimate authority of the flag state.

Conclusion

The principle of exclusive flag State jurisdiction remains the bedrock of public order on the high seas, ensuring that every vessel is subject to a system of law and accountable to a sovereign authority. However, this exclusivity is not absolute. International law, primarily through UNCLOS and other specialized treaties, has carved out carefully defined exceptions. These exceptions—ranging from the universal right to combat piracy to cooperative efforts against drug trafficking, illegal fishing, and WMD proliferation—reflect a balancing act: upholding the freedom of navigation and the primary authority of the flag State, while empowering the international community to address common threats and ensure that the high seas are not a lawless domain. For maritime businesses, understanding both the scope of flag State authority and the circumstances under which other states may intervene is essential for safe and lawful operations on the world's oceans.