The 'Invisible Killer' of the Hormuz Crisis: 1,500 Ships Stuck Long-Term, Risking Global Marine Biological Invasion Disaster

An international team of marine biologists is warning that the closure of the Strait of Hormuz represents a 'looming ecological crisis' and a 'major international biosecurity threat.' Their report was published in the journal Biological Invasions.

Scientists say that with ships stranded for months, invasive species have had unprecedented time to attach themselves to vessels. Biologists warn that once these ships resume sailing and head to destinations around the world, they could trigger a 'massive, super-spreader event of marine biological invasion.'

The report estimates that since February, approximately 1,500 ships have been stranded in the Persian/Arabian Gulf (the Gulf), with hundreds more in the Gulf of Oman.

'The Worst-Case Scenario'

Many marine microorganisms, algae, and invertebrates rapidly attach and grow on ships when they remain stationary for long periods. This phenomenon is known as biofouling.

Professor Mario Tamburri, lead author of the report and a marine ecologist at the University of Maryland, USA, explains: 'There is a group of organisms, typically invertebrates and algae, that we call sessile or attached species—they spend their lives fixed to a surface.' He co-authored the warning with scientists from the United States, Argentina, Portugal, Saudi Arabia, New Zealand, Finland, South Africa, Canada, and Israel.

Tamburri explains that whenever a new structure is placed in the ocean, it is first colonized by bacteria and other microbes, forming a microbial film. 'Then, this biofilm is colonized by spores of larger algae like kelp, or larvae of invertebrates such as barnacles and worms... These communities then continue to grow and expand, as they need hard surfaces to settle on.'

The seabed in the Gulf has almost no hard surfaces. Tamburri explains that the seafloor is mostly mud, silt, and sand, so when these organisms encounter hard surfaces like ships, they rapidly attach.

Moreover, because the Gulf is central to global shipping routes, it serves both as a point where ships bring in fouling organisms from elsewhere and as a place where ships accumulate them while stationary.

With the Strait of Hormuz closed, a large number of massive vessels have been idle for extended periods. Researchers describe this situation as 'unprecedented in modern history.' Tamburri notes that the 2021 Suez Canal blockage lasted only six days, and the 2024 disruption from the Baltimore bridge collapse affected only a 'small number of ships' for about 11 weeks.

Scientists say ships in the Gulf typically stay in port for one to three days before departing. They warn that after more than 10 days, invasive species can rapidly accumulate on vessels.

In 2023, the International Maritime Organization (IMO) recommended that ships idle for long periods take measures to address biofouling before resuming operations.

Multiple ships have already been moored in the Gulf region for months. Tamburri says: 'If you were to design the worst possible scenario for ship biofouling and invasive species spread, you couldn’t come up with anything worse than this.'

Additionally, many marine invertebrates and algae reproduce seasonally. When the Strait of Hormuz closed in late February, sea temperatures were beginning to rise, entering the summer phase.

'This warming often triggers reproduction, spawning, growth, and the search for new habitats and surfaces to grow on... If this closure had occurred in midwinter, the risk would have been lower,' Tamburri explains.

The report highlights that the highly adaptable estuarine barnacle Amphibalanus improvisus, already spread globally via shipping, has been found in the Gulf. At 30°C, each adult can release up to 36 larvae per day.

The report estimates that a single vessel during this mooring period may have released millions of larvae.

Tamburri says: 'The issue is the sheer scale, scope, and impact. And because ships leaving this region travel globally, they go everywhere.'

'Enormous' Damage

Tamburri says that if these organisms are introduced to new habitats, they could cause 'enormous' economic and ecological damage.

He adds that they could outcompete native species, cause local extinctions, disrupt fisheries and aquaculture, and even clog power plant cooling systems. 'Once invasive species establish populations, especially in marine ecosystems, they are extremely difficult to eradicate.'

The report also notes that these organisms may carry harmful parasites and pathogens. The Gulf’s warm seawater may favor heat-tolerant algae. If introduced elsewhere, they could outcompete native species as global temperatures rise.

Biofouling is also harmful to the ships themselves. As organisms grow, they increase the vessel’s weight and drag in water, slowing it down.

'Ships end up burning more fuel to maintain speed,' Tamburri explains. 'Even a microbial film on the hull can increase fuel consumption by 2% to 5%. If heavily fouled with barnacles and mussels, fuel use could rise by 25%.'

Antifouling paints containing toxic biocides have been developed to deter these unwanted organisms. 'When ships sail frequently, these paints work well... but when ships are idle for long periods, they are less effective,' Tamburri explains, because the paints 'get overwhelmed by biofouling growth.'

Considering shipping traffic, voyage duration, and environmental conditions, researchers have identified several ports particularly vulnerable to receiving these species.

These include Jeddah in Saudi Arabia, Mumbai in India, Colombo in Sri Lanka, Singapore, Alexandria in Egypt, Piraeus in Greece, Algeciras in Spain, and Rotterdam in the Netherlands.

Tamburri notes that ports with similar environmental conditions—such as warm, saline waters—may be 'highly vulnerable.'

International Cooperation

Scientists say ideally, idle vessels should be cleaned before leaving the Gulf.

But cleaning such a large number of massive, heavily fouled ships is a challenge.

Experts emphasize that collecting and properly disposing of these organisms is crucial to prevent them from escaping into the water and to avoid damaging antifouling coatings containing toxins and microplastics.

Robotic technologies can collect these organisms during hull cleaning, but Tamburri notes that most underwater cleaning services in the Gulf region do not currently collect debris, as the technology is still relatively new. He adds that processing capacity is also a major issue given the sheer number of severely fouled vessels.

Given the scale of the problem, the urgency of resuming shipping and evacuating stranded crews, and logistical constraints, scientists conclude that thorough pre-departure cleaning is not a realistic solution.

They stress that international cooperation will be essential, and ports expecting ships from the region should prepare to clean vessels upon arrival to reduce the risk of invasive species spread.

Tamburri says: 'We are quite certain that once the Strait of Hormuz reopens, non-native species will be introduced.'

'Some invasive species may go unnoticed—they’re there but not conspicuous or impactful. Others will have major consequences.'

Researchers say whether this becomes a global 'biological invasion super-spreader event' now depends less on what organisms are on the ships and more on how the international community responds in the future.

This article was translated using AI assistance from the original English version. BBC journalists reviewed the translation before publication.