When Strait of Hormuz reopens, why these fouled ships could unleash silent invasion across oceans

US-Iran Tensions: The prolonged shutdown of the Strait of Hormuz has created an unusual problem at sea. Hundreds of ships have been stuck for months across the Persian Gulf and the Gulf of Oman. This has given sea creatures much more time than usual to settle on their hulls.

A new report published in the journal Biological Invasions warns that when these vessels eventually sail again, they could carry large numbers of marine organisms into ports across the world. Scientists associated with the study say the situation could turn into a major marine biosecurity problem, with ships acting as “super-spreaders” of invasive species.

According to the report, around 1,500 vessels have been stranded in the Persian or Arabian Gulf since February, while hundreds more are waiting in the Gulf of Oman. The ships have been caught in the disruption caused by the closure of the Strait, which is one of the world’s most important maritime routes.

Marine organisms such as algae, microscopic organisms and invertebrates can attach themselves to underwater structures and grow there. This process is known as biofouling.

Professor Mario Tamburri, a marine ecologist associated with the University of Maryland in the United States who is the lead author of the study, explained why ships provide such a useful surface for these organisms.

“There is a whole group of organisms that are typically invertebrates and algae. We call them sessile or sedentary. They spend their lives attached to a surface,” he said.

Whenever a new structure enters the sea, bacteria and other microorganisms are among the first to settle on it. They form a microbial film, which then provides a surface for other organisms.

“Then on that biofilm, you get spores of macroalgae like kelp or larvae of invertebrates like barnacles and worms. These communities then grow and spread because they need a hard surface to live on,” Tamburri said.

The Gulf provides a useful setting for this process. Much of the seabed is made up of mud, silt and sand. It leaves relatively little hard surface underwater. A ship therefore offers marine organisms a ready-made place to attach themselves.

The Gulf’s role in international shipping also adds to the problem. Ships arriving from different parts of the world can bring organisms with them, while vessels sitting in the Gulf for long periods can collect new organisms from local waters.

Tamburri described the existing situation as “unique” in modern history. The six-day blockage of Egypt’s Suez Canal in 2021 and the disruption caused by the Baltimore bridge collapse in the United States in 2024, which affected some vessels for around 11 weeks, were much shorter events.

Ships in the Gulf normally spend only one to three days in port before leaving. Researchers say biofouling can increase rapidly when a vessel is in one location for more than 10 days.

The International Maritime Organisation advised in 2023 that vessels kept stationary for long periods should take measures to deal with biofouling before sailing again.

Many ships in the Gulf have now been idle for months. “If you were to design a worst-case scenario for biofouling and the spread of invasive species, you could not come up with anything better than this,” Tamburri said.

The timing of the disruption has also given marine organisms favourable conditions for growth and reproduction.

Many marine invertebrates and algae reproduce according to seasonal changes. When the Strait was closed at the end of February, water temperatures were beginning to rise ahead of summer.

“Temperature increases often initiate reproduction, spawning, growth and the search for new places or surfaces to settle and thrive. If this route had been closed in the middle of winter, the risk would have been lower,” he said.

The report cites Amphibalanus improvisus, a highly adaptable brackish-water barnacle that has spread around the world through ships and is found in environments similar to the Gulf. At 30 degrees Celsius, each adult can release up to 36 larvae a day.

Researchers say individual vessels may have released millions of larvae while sitting in one location.

“It is really just the sheer size, scale and scope of it. And because of the reach of the ships leaving that area, they go everywhere,” Tamburri said.

The movement of these organisms into new waters can cause environmental and economic damage. Invasive marine species can displace native organisms and, in some cases, contribute to the loss of local species.

They can also affect fishing and aquaculture operations. Some organisms can settle inside cooling systems used by power plants and restrict water flow.

“Once invasive species become established somewhere, particularly in marine environments, they are difficult to remove,” he said.

The researchers also warn that marine organisms transported on ships can carry harmful parasites and pathogens. The warm waters of the Gulf can favour species that tolerate high temperatures. If these organisms reach other areas and temperatures rise in those locations over time, they could gain an advantage over native species.

Biofouling also creates a problem for the vessels. As organisms accumulate on a ship’s hull, they add weight and increase resistance as the vessel moves through the water. Ships then need more fuel to maintain their speed.

“You have to burn more fuel to maintain your speed,” Tamburri said.

He added that even a microbial biofilm on the bottom of a ship can increase fuel use by two to five per cent. A heavy layer of barnacles, mussels and other organisms can push the additional fuel requirement to as much as 25 per cent.

Ships use anti-fouling paint containing biocides to limit the growth of marine organisms. The system works better when vessels are regularly moving. Long periods of inactivity can reduce its effectiveness.

“These paints work well when ships are moving around a lot. But when ships sit in one place for a long time, they don’t work as well. That is because they are subject to more biofouling,” he said.

The research team assessed shipping routes, journey times and environmental conditions to identify ports that could face a higher risk when vessels from the Gulf begin moving again.

The ports identified 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.

Ports with environmental conditions similar to the Gulf, particularly warm and salty waters, could face a “very high risk”, according to Tamburri.

The ideal approach would be to clean ships before they leave the Gulf. That presents a practical problem, given the number of vessels involved and the amount of marine growth that may have accumulated on them.

Experts say organisms should be collected during cleaning and disposed of properly to prevent them from entering the surrounding water. The same care is needed to stop pieces of anti-fouling paint from breaking away during cleaning.

Robotic systems could be used to remove marine growth from ships while collecting the material, he said. However, underwater cleaning services in the Gulf often do not collect the material they remove because the technology is still relatively new.

The number of heavily fouled vessels also creates a capacity problem. Cleaning thousands of ships would require equipment, workers, time and suitable facilities at a scale that may not be available.

Researchers say the scale of the situation and the need to restart shipping and move stranded crews create several logistical difficulties. Cleaning every vessel thoroughly before departure may therefore prove difficult to organise.

The researchers say international cooperation will be needed as vessels start leaving the Gulf. Ports expecting ships from the region could prepare facilities and procedures to inspect and clean vessels after they arrive. This could help reduce the spread of invasive organisms.

“We are pretty confident that when the Strait reopens, there will be arrivals of invasive species. Some invasive species may be cryptic. They are there, but they are unnoticed or don’t have much of an impact. Others do,” he added.

The researchers say the scale of the eventual biological impact will depend on what has grown on the vessels and how countries, shipping companies and ports respond once maritime traffic resumes. The unusual period of inactivity has given marine organisms more time to colonise ships. The problem could spread far beyond the waters around the Strait.