Shadow vessels and the law of the High Seas

Sunday 28 December 2025

Modern maritime law rests on an understanding that the high seas are open to all, but each ship is meant to belong, legally, to one state whose flag she flies. That “flag state” bears the primary responsibility for regulating the vessel and enforcing rules on her, even when she is far from land. This bargain is written into the United Nations Convention on the Law of the Sea (UNCLOS), which frames both the rights of trading vessels and the narrow circumstances in which other states may interfere. Shadow fleets, of vessels containing sanctioned cargoes, take advantage of this by sailing the high seas under flags that provide no effective control over the vessels purportedly under their jurisdiction because their flags are essentially "for sale".

The basic right: freedom of navigation, under flag-state control

On the high seas, merchant ships benefit from the freedom of navigation and related freedoms (trade, communications, fishing, scientific research) exercised “under the conditions” of UNCLOS and general international law. 

The corollary is exclusive flag-state jurisdiction: as a rule, only the flag state may board, inspect, arrest, or prosecute a ship on the high seas. This is the core protection for trading vessels: she is not supposed to be stopped merely because another state disapproves of her cargo, trading partner or route. That is why sanctions enforcement at sea is legally difficult unless it can be tied to an accepted exception or a treaty-based boarding regime. 

The narrow exceptions: when interference is allowed

UNCLOS does not leave the seas lawless, but it is intentionally sparing about coercive powers.

Right of visit (boarding to verify status)

A warship may stop and board a foreign vessel on the high seas only on limited grounds, such as reasonable suspicion of piracy, slave trading, unauthorised broadcasting, statelessness, or where a treaty confers a specific power. This is not a general power to enforce sanctions. 

Statelessness and false flagging

If a ship is genuinely stateless, or if she is in effect pretending to have a nationality (for example by using a false flag), other states have wider latitude to board and treat her as lacking the protections that accompany flag-state jurisdiction. Shadow fleets try to sit in the grey zone: enough paperwork to look plausibly flagged, enough opacity and “flag hopping” to make responsibility evaporate when problems arise. 

Hot pursuit

A coastal state may pursue a foreign vessel onto the high seas if the pursuit begins while the ship is in internal waters, the territorial sea, or certain other maritime zones, and if strict conditions are satisfied. Hot pursuit is about enforcing coastal-state law after a violation close to shore, not about policing distant sanctions evasion as such. 

Treaty-based boarding

Treaties can widen interdiction powers by pre-agreement. The most relevant model for modern maritime security is the SUA framework (Suppression of Unlawful Acts against the Safety of Maritime Navigation) and its 2005 Protocol, which provides for boarding requests and co-operation procedures between the boarding state and the flag state in specified circumstances such as where the violence on board, damaging navigation facilities, or where the vessel is communicating false navigation information.

Why sanctions interdiction is hard on the high seas

Sanctions are typically domestic or regional measures, even when co-ordinated amongst allies. Unless a sanctions regime is embodied in a United Nations Security Council resolution (which can authorise maritime enforcement), sanctions do not automatically become a universal legal basis to stop ships on the high seas. As a result, many enforcement strategies focus on:

• port-state controls (denying entry, services, bunkering, insurance recognition)

• financial and insurance chokepoints

• de-flagging and safety enforcement via flag states and classification societies

• criminal jurisdiction when a ship enters territorial waters or a port

Shadow fleets exploit the asymmetry: they can move value across oceans, while enforcement is concentrated only at ports, insurers, and compliant registries.

The Venezuela context: what recent interdictions illustrate about the legal pressure points

Recent reporting about United States actions against tankers linked to Venezuelan oil exports illustrates the practical methods states use when sanctions alone are not enough: interception near coastal waters, heavy reliance on the flag state’s regulatory authority, and arguments about non-compliance with maritime rules (for example switching off transponders). 

A notable theme is flag-state leverage. Panama, for example, publicly emphasised that a Panamanian-flagged tanker intercepted by the US Coast Guard had violated Panama’s maritime rules, and that the flag state can take measures including revocation of registration. That is a reminder that the international system is designed to make the flag state the first responder. Where the flag state is serious, shadow shipping becomes riskier. Where she is indifferent or captured, it becomes lucrative. 

How the law might be amended to make interdiction easier

Any reform has to navigate two competing aims:

• preserving predictable freedom of navigation for ordinary commerce

  
  

• reducing the ability of bad-faith operators to exploit openness and jurisdictional fragmentation

The most plausible route is not to abolish the flag-state system, but to widen narrowly-defined grounds for boarding and to harden the legal meaning of “genuine” nationality and compliance.

Create a sanctions-evasion boarding protocol, modelled on SUA 2005

A practical amendment would be a new multilateral protocol, or a set of linked treaties, that:

• defines sanctions-evasion conduct that triggers an interdiction request (for example deliberate identity concealment, systematic AIS (transponder) manipulation, ship-to-ship transfers designed to falsify origin, forged flag documentation, or operation on lists maintained by participating states)

  
  

• requires a standardised “boarding request” process to the flag state, with time limits for response

  
  

• provides default rules if the flag state is non-responsive (for example deemed consent after a short window, subject to safeguards)

  
  

• sets evidentiary, safety, and compensation standards to reduce abuse

This mirrors the logic of SUA: you make boarding easier by pre-negotiating consent pathways and procedures, rather than by asserting a unilateral right to stop anyone. 

Tighten the concept of “statelessness” for false-flag and paper-flag cases

UNCLOS already allows boarding where a ship is suspected of being without nationality. Reform could clarify that a vessel should be treated as functionally stateless if she:

• cannot authenticate her registry and call sign through an interoperable verification system

• uses conflicting registry documents

• claims a flag whose administration fails to confirm her within a defined timeframe

• repeatedly changes flags in patterns associated with sanctions evasion, without a verifiable beneficial owner chain

This would not make sanctions themselves a boarding ground, but it would make the common shadow-fleet tactic of identity laundering far less protective.

Make AIS and ownership transparency obligations enforceable, not merely “best practice”

Switching off transponders is often presented as a safety or compliance matter, yet it is also a sanctions-evasion technique. A reform package could:

• require continuous AIS use on the high seas subject to tightly limited exceptions

• treat unexplained “dark periods” as a rebuttable presumption supporting inspection when combined with other indicators

• mandate beneficial ownership disclosure to registries and port states, with auditability

The Panama comments reported in relation to a tanker that turned off her transponder show how states already frame this as non-compliance. A treaty could convert that framing into harmonised enforcement triggers. 

Expand port-state powers and create a shared denial-of-services regime

Even without high-seas interdiction, the most powerful leverage remains what ships need to keep trading: ports, pilots, repairs, insurance, finance, spare parts, classification, and lawful cargo documentation. States could adopt a centralised system involving:

• mutual recognition of “denial of port entry” lists for shadow fleet vessels

• mandatory reporting of ship-to-ship transfers and high-risk routing

• civil forfeiture or cargo seizure rules that activate upon port entry when a vessel has engaged in defined concealment conduct

This keeps the strongest coercive measures within territorial jurisdiction, where states have clearer legal competence.

Where politically feasible, embed maritime sanctions enforcement in Security Council practice or regional security mandates

The cleanest legal authority for interdiction on the high seas is a Security Council resolution. In practice, Russia’s veto often blocks this for Ukraine-related sanctions, but the broader point matters: when sanctions are backed by collective security authority, interdiction becomes far less controversial legally. Short of that, regional arrangements can approximate collective authority if they remain anchored in treaty obligations.

Build due process safeguards to prevent the cure becoming worse than the disease

Any expansion of interdiction powers should include:

• minimum standards for reasonable suspicion

• clear limits on use of force and treatment of crews

• rapid review mechanisms, and compensation for wrongful interference

• transparency reporting by interdiction states

Without these measures, reforms will be rejected by trading nations that fear politicised harassment at sea.

Make opacity costly, not navigation fragile

Shadow fleets succeed because the law heavily penalises interference and because verification is slower than evasion. The most realistic legal reforms therefore concentrate on two pressure points:

• making it easier to verify nationality and compliance quickly (and to treat unverifiable ships as boardable)

• making boarding possible through treaty protocols, rather than unilateral reinvention of high-seas policing

That approach preserves the core freedom of navigation for ordinary trade, while shrinking the space in which sanctioned cargoes can hide behind paper flags, dark transponders and jurisdictional inaction.