The Cargo You Can’t See Leaving the Gulf: Biofouling, the Shadow Fleet, and a Submerged Supply-Chain Risk at the Strait of Hormuz

The Ocean Foundation watches the Strait of Hormuz the way the energy desks do — but our attention is below the waterline. As the world’s community foundation for the ocean, we see the current crisis in the Strait through two lenses that the freight and commodity markets tend to miss: the transfer of invasive species and the climate and pollution costs of a global fleet now operating under abnormal conditions. Both are converging into a supply chain risk that will not remain submerged.

The Biofouling Problem in Extended Waiting

Since the conflict that began on 28 February 2026 — what we regard as a war of choice against Iran — the Strait has been functionally closed. Roughly 138 vessels transited Hormuz daily before hostilities; open transits have since fallen to near zero, with growing clusters of loitering ships on both sides of the Strait. At the peak, on the order of 1,000 vessels carrying up to 20,000 seafarers were unable to transit. More than three months on, the chokepoint remains a near-empty “ghost route,” with owners still reluctant to return. The supply-chain conversation has understandably centered on oil, gas, insurance, and schedule reliability. We want to flag what is accumulating on the hulls of those waiting ships.

Why extended waiting in the Gulf is a biofouling problem. Biofouling — the build-up of algae, barnacles, tubeworms, and other organisms on a hull — is governed by two variables that the Gulf maximizes.

Fouling occurs substantially faster in warm waters, and the risk rises sharply whenever a vessel idles or operates at low speed. The Persian/Arabian Gulf is among the warmest and most saline semi-enclosed seas on Earth, and the vessels now anchored or loitering there for weeks or months are sitting in close to ideal fouling conditions. The consequences are not cosmetic. Biofouling accounts for roughly 9% of all fuel consumed by ships, and heavy growth can cut fuel efficiency by up to 40% with a corresponding rise in emissions. When this backlog of vessels finally clears Hormuz, operators will face a simultaneous, fleet-wide drag and fuel penalty — and, as importantly, hundreds of hulls carrying Gulf-origin organisms outward into the Gulf of Oman, the Arabian Sea, and onward to discharge ports worldwide. That is a coordinated biosecurity pulse, not a series of isolated events. Port states are already empowered to act on it: vessels have been refused entry and forced into costly unplanned cleaning when high-risk fouling was detected, a scenario that turns a hidden problem into a hard schedule disruption.

Fouling occurs substantially faster in warm waters, and the risk rises sharply whenever a vessel idles or operates at low speed

The Shadow Fleet as an Uncontrolled Vector

The shadow fleet is the uncontrolled vector. This is where the risk we have been tracking sharpens. The Iran conflict is swelling the sanctioned and “dark” tanker trade that already moves a large share of constrained-supply oil. The IMO’s 2023 resolution describing the dark fleet pointed to several hundred older tankers with substandard maintenance, unclear ownership, and a severe lack of insurance; by late 2025, analysts counted well over 1,900 such vessels, with roughly a tenth of the global tanker fleet now tied to sanctioned or illicit trades. These ships disable or spoof AIS, conduct ship-to-ship transfers, defer maintenance, and operate largely outside classification societies, P&I coverage, and routine port-state inspections. The conservation community has rightly begun to name this fleet a marine-environmental crisis in its own right — an aging, poorly maintained, under-inspected fleet of tankers that experts call a “ticking time bomb” for spills. The biofouling dimension follows directly: vessels that skip drydock, evade inspection, and loiter for extended ship-to-ship operations are precisely the ones operating outside any biofouling management regime — undocumented, unmanaged hull-borne vectors for invasive species, with no record book and no accountable owner to clean them.

Recommendations for the Legitimate Fleet

What we recommend: For the legitimate fleet, the framework already exists, and we urge operators and charterers to use it now rather than after a detention. The anchor is the IMO’s 2023 Biofouling Guidelines, which replaced the 2011 voluntary version, requiring a ship-specific biofouling management plan, a record book, and — critically for current conditions — contingency action plans triggered when monitoring identifies elevated fouling risk. Prevention through modern coatings and proactive grooming is far cheaper than reactive cleaning. Where in-water cleaning is necessary, we recommend capture-based systems only, because cleaning without capture releases both contaminant paint particles and viable organisms — many of which regrow from fragments — into the receiving waters. Regionally, this should align with the established marine-environment bodies: ROPME for the Gulf, and the GloFouling-supported regional strategy that PERSGA member states have drafted for any vessels continuing toward the Red Sea and Suez.

Best Practices Before Exiting the Gulf

Best practice before a vessel exits the area. The guiding principle is simple: do not move Gulf organisms into a new bioregion. We advise inspecting and documenting the hull condition and proactively grooming before transiting Hormuz — rather than relying on reactive cleaning at the next port, where weather, cost, and queueing for scarce drydock capacity will all work against the operator. Any pre-departure cleaning near sensitive or enclosed waters should use capture. And we are candid about the limits: in-water cleaning infrastructure in the region is scarce, the security situation disrupts planned drydocking, and none of this applies to the shadow fleet, which, by design, answers to no one. That last gap is the one that should most concern the supply-chain community, because the costs of an invasive-species introduction or a poorly maintained hull’s failure are borne not by the evading operator but by the coastal states, ports, and downstream shippers left to absorb them.

The Ocean Foundation’s view is that the prudent response is the same one good supply-chain managers already practice: treat the predictable as predictable. The vessels waiting out this war will leave the Gulf carrying more than their declared cargo. Planning for that now — through the existing IMO framework, regional coordination, and real pressure on the enabling infrastructure of the dark fleet — is cheaper than cleaning it up later.

The opinions and views expressed in this article are solely those of the author, Mark J. Spalding, President & CEO of The Ocean Foundation, and do not necessarily represent the positions or views of The Supply Chainer.