Closure of the Strait of Hormuz may trigger a bioinvasion super-spreader event

In the modern history of ship lay-ups (vessels remaining at anchor or dockside for an extended period of time), the number of ships that have been trapped due to the closure of the Strait of Hormuz is unparalleled. More than 1500 large commercial ships remained idle in the Persian/Arabian Gulf and the Gulf of Oman, hereafter referred to collectively as the “Gulf region”, starting on February 28, 2026 (e.g., World Bank 2026). These vessels have arrived with biofouling organisms from a wide range of source regions and are now acquiring (and will likely depart with) extensive biofouling communities of microbes, algae, and invertebrates from the Gulf region. This massive intermingling of ships and biofouling will result in the inevitable release of non-indigenous species (NIS) at the lay-up sites and in the spread of marine species across the globe once the idle ships resume their trade routes.

The underlying inevitability of impending species translocations is based on the typical sigmoidal (s-shaped) curve that characterizes biofouling accumulation on submerged surfaces (Daly et al. 2025). Biofouling on static surfaces can reach the rapid growth phase after 10 days (e.g., Davidson et al. 2020), which is longer than typical residence times of less than 2 days for cargo vessels in most ports. When ships are idle beyond these time frames, biofouling develops aggressively as antifouling coatings become compromised from lack of movement (e.g., Casse and Swain 2006). The International Maritime Organization (IMO) recognizes that extended lay-ups substantially increase bioinvasion risks, recommending that vessels that idle for more than 18–30 days undertake “immediate actions before the next voyage” to manage biofouling (International Maritime Organaization IMO 2023).

Before this event, lay-ups have typically resulted in a single to a few hundred vessels becoming unexpectedly stationary. Lay-ups can occur due to corridor closures (such as the fully blocked Suez Canal for 6 days in 2021), or more typically because of vessel underemployment (loss of business or economic slowdowns) and port capacity bottlenecks (Ruiz et al. 2022). Ships may also be intentionally brought together in clusters of varying durations, such as for “Tall Ship” festivals, for jurisdiction claims, or when reserve military vessels are mothballed (e.g., Davidson et al. 2008). The connection between immobile biofouled ships and marine species invasions has long been recognized. For example, the invasion of the Australasian barnacle Austrominius modestus to western Europe during World War II was linked to New Zealand ships being held in English ports while awaiting convoy escorts (Southward 1998).

The Gulf shipping nexus, with the Strait of Hormuz its critical maritime chokepoint, supports transport of energy commodities (e.g., oil and gas), raw materials (e.g., fertilizers and minerals), and manufactured goods to global markets, especially in Europe and Southeast Asia (Fig. 1). It is not uncommon for well over 1000 vessels (including roughly 700–800 larger cargo ships and tankers) to be operating in the Gulf region at any one time, with an average of 130–140 of them (including between 60 and 70 very large oil and gas tankers) passing through the Strait of Hormuz each day (World Bank 2026). The majority of vessels in this region spend 1–3 days in port before departing. However, at the time of writing, ships in the Gulf region had remained idle there for at least 40-times longer than an average stay. Although a variety of vessel types and sizes operate in this region, the routine presence of a considerable number of very large ships (with correspondingly extensive submerged hull surfaces and niche areas vulnerable to biofouling) from around the world amplifies their exposure to diverse biofouling communities.

Here, we highlight the extreme worst-case scenario currently prevailing in the Gulf region, which exposes local and coastal ecosystems worldwide to elevated biosecurity risks. We further provide recommendations for immediate management interventions to mitigate these risks and identify longer-term monitoring and management measures that can be incorporated into contingency planning for both this potential super-spreader event and future vessel lay-up incidents.

The Strait of Hormuz connects the shallow, soft-sediment-dominated, hypersaline Persian/Arabian Gulf with the adjacent, deeper and more rocky oceanic Gulf of Oman. The Persian/Arabian Gulf is a semi-enclosed sea with an average depth of 35 m, characterized by high salinity and temperature, limited water exchange, and strong evaporation (Joydas et al. 2024). Summer surface-water temperatures can reach approximately 38 °C, and winter temperatures range between 15 and 20 °C (Alosairi et al. 2011), with salinities often exceeding 40 psu (Kashkooli et al. 2022). Recent data also revealed a significant increase in the spatial and temporal extent and average number of marine heat waves in this region (Paradan et al. 2025). Extreme environmental pressures, combined with coastal industrialization and urbanization, have also directly reduced biodiversity in Gulf ecosystems (Naser 2014). These conditions are relevant to biosecurity risk because the remaining stress-tolerant taxa are more likely to survive transport, persist during vessel lay-up, and establish in recipient regions with similar or stressful environmental conditions.

Given the Gulf region’s central role in global shipping networks (Fig. 1), there is a high probability of both the receipt and export of marine NIS. For example, a recurrent and escalating issue in the Gulf region is the occurrence of harmful algal blooms, including the non-indigenous dinoflagellate Margalefidinium polykrikoides near the Bandar Abbas and Qeshm Oil Terminals. Ship ballast water is considered a major pathway for the introduction of M. polykrikoides and other non-native algal species to the region (Mirza Esmaeili et al. 2021). A subsequent study listed 35 NIS from brackish and marine habitats in the region, with the great majority introduced through commercial shipping activities (Clarke et al. 2020). Notably, 57 cryptogenic and NIS biofouling species were identified growing on pontoons, docks, and research settlement panels surveyed at 12 sites along the western Persian Gulf coast, with species richness and relative abundance being greatest in ports (Chebaane et al. 2026).

Because most biofouling organisms are sessile, reproduction typically plays a large role in invasion success. Many marine invertebrates and algae reproduce in response to elevated temperatures and seasonal changes in environmental conditions (e.g., Donelan et al. 2022). Therefore, the Gulf region mass ship lay-up in the spring and summer of 2026 provides ideal conditions for biofouling growth and reproduction (Muthukrishnan et al. 2022). For example, the cosmopolitan barnacle Amphibalanus improvisus can release up to 36 larvae per adult per day at 30 °C (Donelan et al. 2025). Given the high densities of fouling organisms that can occur on ship hulls and niche areas (e.g., sea chests, rudders, and piping systems), individual vessels may have released millions of larvae during this lay-up period. The proximity of idle vessels provides abundant settlement substrate for these propagules, while the eventual resumption of shipping will facilitate their widespread dispersal.

Ship biofouling also poses hidden risks. Many biofouling species are globally distributed and adapted to diverse environmental conditions (e.g., Viard and Tepolt 2025). Introducing new genetic diversity into established non-indigenous populations can promote genetic introgression, accelerating adaptation, and increasing their ability to spread and persist across a wider range of conditions (Touchard et al. 2023). In particular, the warm waters of the Gulf region are likely to select for heat-tolerant genotypes, which, if introduced to thermally-limited populations, may give them a greater adaptive buffer against future warming (Griffiths et al. 2021).

Biofouling communities can also act as reservoirs and vectors for non-indigenous parasites and pathogens, including some that can cause epidemics in commercially important species (Georgiades et al. 2021). Introduced parasites can affect native hosts through novel host-parasite interactions, often causing higher infection levels due to the absence of co-evolved defenses (Goedknegt et al. 2016), with warmer temperatures and denser assemblages exacerbating these impacts (e.g., Cohen et al. 2018). Introduced disease outbreaks are unpredictable but can cause economic and ecological impacts that far exceed those of the introduced host species itself (Georgiades et al. 2021).

Stressed ecosystems, such as industrial ports, are highly vulnerable to bioinvasion because environmental disturbances reduce community resistance (e.g., Stachowicz et al. 1999). By suppressing native species, opening up ecological niches, and increasing resource availability, disturbances allow invasive, fast-growing opportunist (e.g., tolerant and resilient) species to establish themselves; the added influence of high resource availability may further promote invasibility (Crooks et al. 2011). In the Gulf region, invasibility is paired with massive shipping volume, providing ample opportunity for the development of NIS assemblages. Thus, the highest biosecurity risk will likely be at first ports of call (Fig. 1), in particular those with similar environmental conditions as the Gulf region, including the ports of Jeddah (Saudi Arabia), Mumbai (India), Colombo (Sri Lanka), Singapore, to the east of the Strait of Hormuz, and the ports of Alexandria (Egypt), Piraeus (Greece), Algeciras (Spain) and Rotterdam (Netherlands) to the west, as environmental matching between donor and recipient regions facilitates establishment success (Seebens et al. 2016).

Ideally, biofouling on idle ships in the Gulf region should be removed before departure. In-water cleaning (IWC) of ship hulls and niche areas is widely practiced to manage biofouling (e.g., Tamburri et al. 2021). However, IWC debris should be collected and properly disposed of to minimize the release of live organisms, and coating-associated biocides and microplastics, into surrounding waters (Tamburri et al. 2025).

The application of IWC will surely be incentivized because of the direct benefits on ship operations, particularly reduced drag and fuel consumption (e.g., Schultz et al. 2011). However, this scenario is unlikely for most vessels given the urgency of evacuating stranded seafarers and restoring shipping movements, and due to limited regional IWC capacity and operational constraints associated with safety and logistics. As a result, demand will likely also shift to subsequent ports of call. The capacity of IWC in many recipient ports may also be insufficient to accommodate the anticipated surge in needed services, and cleaning operations may be restricted or prohibited in some jurisdictions, creating additional bottlenecks to mitigate biosecurity risks. Finally, the current trade disruptions and shifting shipping routes are expected to reshape maritime networks and port connectivity, expanding and making less predictable biosecurity risks linked to Gulf vessel traffic. We therefore strongly encourage a coordinated global network of safe and effective IWC services.

Given competing priorities and operational constraints, most vessels are unlikely to complete IWC before arriving at their next ports of call. We therefore propose additional measures to mitigate expected biosecurity risks:

• Estimate vessel biofouling before departure. Direct underwater observations are preferred, but efforts to estimate the level and extent of biofouling at the ship waterline can still be informative. Although vessel crews are unlikely to determine specific species present, general taxonomic information can be helpful to inform biosecurity mitigation at subsequent ports of call (e.g., macroalgae, barnacles, etc.). Additionally, documentation of underway ship power loss (due to increased biofouling drag), especially when compared to these metrics from before the lay-up, can be a useful proxy for the relative amount of biofouling on the submerged surfaces and corresponding biosecurity risk.

• Forecast vessel routes and first ports of call. Ship route forecasting efforts can help predict which ports idle vessels are likely to call after departing the Gulf region. This can facilitate forewarning of ports and support preparedness and possible interventions. Additionally, information on vessel routes can help revise biosecurity threat assessments in that some routes could reduce invasion risk (e.g., freshwater exposure when passing through the Panama Canal) but many are likely to enhance it (e.g., short voyages with little changes in ambient water temperature and salinity). Important considerations for ports expecting vessels from the Gulf region are also their similarity in environmental conditions (e.g., Seebens et al. 2016), the number and size (submerged surface area) of arriving vessels, arriving ship residence times, and capacity of local IWC services.

• Implement early detection and rapid response at high-risk ports. Ports within the likely sphere of impact should, if not already present, conduct NIS monitoring for early detection. Low-cost, rapid assessments (e.g., O’Shaugnessy et al. 2023) and settlement plate monitoring networks could be rapidly deployed in locations with minimal infrastructure and resources (e.g., Sempere-Valverde et al. 2025), while more advanced molecular tools could be considered in others (Chebaane et al. 2026). This can provide early detection of new species and initiate appropriate potential containment and mitigation efforts, even in data-poor situations (Carvalho et al. 2023).

• Establish rapid alert and knowledge-dissemination networks. Biosecurity warnings should be communicated quickly to all stakeholders, including port authorities, shipping operators, environmental agencies, IWC service providers, and regional biosecurity networks. Identifying trusted knowledge brokers across global recipient regions (e.g., Mediterranean, SW Atlantic, North Atlantic, Pacific, and Indian Oceans) will be essential to ensure that NIS warnings are translated rapidly into operational guidance, coordinated preparedness, targeted monitoring, and risk-reduction actions (Bortolus et al. 2025).

The prolonged mass lay-up of vessels in the Gulf region, together with conditions favorable for biofouling development, represents a significant risk factor for the large-scale dispersal of NIS upon the resumption of global shipping operations. Whether it becomes a global bioinvasion super-spreader event will now depend less on the biofouling communities already present on vessels than on the speed, coordination, and effectiveness of the responses that follow.

Biological invasions share common epidemiological features with infectious diseases, including transmission pathways, propagule pressure, network connectivity, amplification mechanisms, and the challenge of intervening before widespread dissemination occurs (e.g., Bortolus and Schwindt 2022). In this framework, biofouled vessels function as transmission vectors, prolonged lay-ups create conditions for the accumulation and amplification of potential invaders before their redistribution across multiple recipient regions, and ports act as nodes within a global connectivity network. As vessel operations resume, opportunities for prevention rapidly diminish, shifting management from risk avoidance to impact reduction or mitigation.

Lessons from epidemiology strongly suggest that the most effective responses prioritize prevention before large-scale dissemination. This principle is equally well established in invasion science and biosecurity, where prevention is consistently recognized as the most effective and cost-efficient management strategy compared with post-introduction control, containment, or eradication (Carvalho et al. 2023; Egawa et al. 2026). Early warning systems, risk forecasting, surveillance networks, targeted interventions, and coordinated communication are all fundamental components of disease preparedness and are equally relevant to marine biosecurity. The challenge is not simply to detect new species, but to identify emerging risks early enough to prevent their spread through highly connected global transport networks.

Current ship biofouling management and regulatory frameworks are insufficient to mitigate the impacts of large-scale, unplanned vessel lay-up events, which appear to be increasing in frequency and may pose substantially greater environmental risks than routine ship operations. While the human and economic consequences of events like the war in the Middle East will always remain the highest priority, ecological impacts cannot be overlooked. This underscores the need for coordinated action by governments, regulators, the maritime industry, and other stakeholders to strengthen biofouling management for both typical ship operations and varying lay-up scenarios.

Authors

1. Mario N. Tamburri
2. James T. Carlton
3. April M. H. Blakeslee
4. Alejandro Bortolus
5. João Canning-Clode
6. Susana Carvalho
7. Andrew L. Chang
8. Ian C. Davidson
9. Sarah C. Donelan
10. Lisa A. Drake
11. Amy E. Fowler
12. Amy L. Freestone
13. Linsey E. Haram
14. Chad L. Hewitt
15. A. Whitman Miller
16. Jim R. Muirhead
17. Okko Outinen
18. Tamara B. Robinson
19. Gregory M. Ruiz
20. Christopher Scianni
21. Evangelina Schwindt
22. Carolyn K. Tepolt
23. Thomas W. Therriault
24. Bella S. Galil

Cite this article

Tamburri, M.N., Carlton, J.T., Blakeslee, A.M.H. et al. Closure of the Strait of Hormuz may trigger a bioinvasion super-spreader event. Biol Invasions 28, 177 (2026). https://doi.org/10.1007/s10530-026-03893-5

Download citation

• Received: 05 July 2026

• Accepted: 08 July 2026

• Published: 22 July 2026

• Version of record: 22 July 2026

• DOI: https://doi.org/10.1007/s10530-026-03893-5

Keywords