US seizures of MARINERA and M SOPHIA: Legal implications for shadow fleet enforcement

Central to the legal justification for these seizures is the interpretation of flag State jurisdiction under UNCLOS. Article 92 requires ships to sail under a single flag and presupposes continuity of nationality throughout a voyage. The operational history of “MARINERA” - involving frequent changes of name and flag, manipulation of tracking systems, and inconsistent registration - exemplifies the evolving modus operandi. US and UK authorities rejected the vessel’s purported Russian registration, treating it as legally ineffective due to the absence of clear de-registration from the prior flag State and full re-registration under the Russian registry. This approach reflects an interpretation of Article 92 under which rushed or opaque flag transfers, particularly during pursuit, may render a vessel without nationality under international law.

UNCLOS Article 92 is also concerned with the scope of “exclusive jurisdiction” exercised by flag States on the high seas. The 2020 Enrica Lexie Incident Award (Italy v. India)  adopted a broad conceptual interpretation, suggesting that flag State exclusivity extends to both prescriptive and enforcement jurisdiction. In practice, the Tribunal confined exclusivity to enforcement jurisdiction, reaffirming that only the flag State may enforce its laws against its vessels on the high seas, except where UNCLOS expressly provides otherwise, such as in cases of piracy.

While other States may exercise concurrent prescriptive jurisdiction under general international law, enforcement actions such as boarding or seizure remain, as a rule, the exclusive prerogative of the flag State. If the broader conceptual interpretation was adopted, that exception to flag State exclusivity would logically permit non-flag States not only to enforce but also to prescribe law in those circumstances, even absent a jurisdictional link. 

A key exception to exclusive flag State jurisdiction is statelessness. Under Article 110 UNCLOS, warships may board vessels on the high seas that are “without nationality.” The extent of permissible action following such boarding remains contested. The “Any State Theory” holds that any State may exercise jurisdiction over stateless vessels, while the “Nexus State Theory” requires a demonstrable connection - such as crew nationality or beneficial ownership - between the intervening State and the vessel. No settled international consensus exists on the scope of enforcement powers beyond boarding and inspection.

In the case of “MARINERA”, US authorities argued that the prior false flagging, refusal to comply with US Coast Guard instructions, and involvement in historic sanctions evasion justified its treatment as stateless at the time of interdiction. This position conflicts with the Russian assertion that it granted the vessel temporary permission to sail under the Russian flag in late December 2025. The US maintains that this registration did not comply with Article 92 and was therefore legally ineffective. If Russia’s claim of flag State jurisdiction were deemed valid, reliance on Article 110 would fail, and non-flag State enforcement would constitute a violation of freedom of navigation and flag State sovereignty.

The “M SOPHIA” was similarly boarded by the US Coast Guard in the Caribbean after falsely claiming Panamanian flag status and engaging in sanctioned oil transfers. Both seizures were executed under US judicial authority and reflect a growing willingness to challenge traditional assumptions of flag State protection where vessels are suspected of sanctions evasion. Together, these cases illustrate the grey zone in which shadow fleet enforcement now operates, as registry claims, operational conduct, and sanctions imperatives increasingly collide with supranational norms and historic interpretations of maritime law.

The seizures signal a broader shift in Western sanctions enforcement strategy. As the shadow fleet has expanded to over 1,400 vessels - representing up to a quarter of the global tanker fleet - its role in sustaining sanctioned oil exports has become more apparent. In response, the US and its allies have moved beyond registry reform and designation toward direct interdiction, seizure, and prosecution, through a reliance on expansive interpretations of UNCLOS and statelessness.

This approach carries substantial legal and diplomatic risk, including international disputes or kinetic retaliation. Russia’s muted response to the seizure of “MARINERA” may reflect a tacit reluctance to openly embrace the shadow fleet and the responsibility associated with formal registry. Historically, the modus operandi of the dark fleet lay in plausible deniability; bringing such vessels under genuine national registries exposes flag States to reputational and legal consequences. Intensified enforcement may push the shadow fleet to consolidate under flags of sanctioned States, increasing visibility and legal attribution, and enabling more robust application of secondary sanctions, price-cap regimes, and port State control.

The International Maritime Organization (IMO) has attempted to address the shadow fleet through guidance aimed at strengthening registries, improving transparency, and reinforcing flag State responsibility. These measures remain incremental and non-binding, constrained by political fragmentation and the absence of a universal registration framework.

By contrast, the US and its allies are increasingly relying on interdiction and seizure to impose immediate consequences. This shift from standards-based regulation to coercive enforcement raises questions about the long-term coherence of the international legal order. Growing divergence in the interpretation and application of UNCLOS may ultimately require adjudication before the International Tribunal for the Law of the Sea to reconcile unilateral enforcement with the need for predictable maritime governance.

The seizures of “MARINERA” and “M SOPHIA” marked a critical inflection point in the legal and strategic response to the shadow fleet. By challenging established interpretations of flag State jurisdiction and statelessness, these actions expose gaps in the international legal framework and signal a readiness to pursue more assertive enforcement. Whether this results in a more effective sanctions regime or accelerates erosion of consensus around UNCLOS will depend on States’ ability to balance decisive enforcement with durable, multilateral reform.