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Biosecurity Pathway: Commercial Vessels

Asian Date Mussel
Asian Date Mussel © University of Southampton
Asian Date Mussel

Marine invasive species can be introduced and spread by commercial or military vessels on hulls, anchors, and rudders, or in sea chests and ballast water. IMO research showed that in the past commercial vessels that used ballast water were a key mechanism for global invasive species transfer (now regulated). Vessels such as barges, dredgers and fishing vessels present a particular risk due to slower transit speeds, water retaining niche areas and long periods of time spent in coastal waters, often stationary, where they are subject to biofouling recruitment. The IMO have produced a table that provides a list of high-profile Invasive Aquatic Species that are capable of being translocated via biofouling.

The greatest risk of marine invasives introduction is when a vessel (particularly slow moving), arrives at your site from another country, region or water body, with similar environmental conditions (temperature, salinity). Conversely vessels leaving the Solent can export marine invasives to other sites as the Solent is a known UK marine invasives hotspot.The risk is increased if the vessel is covered in biofouling (anything more than a thin, green ‘slime’ coating for vessel hulls) or hosts algae and animals (‘hitch-hikers’) within other parts of its structure. 

Work vessels/barges such as dredgers that move from site to site, and have to hold position for long periods of time or spend extended periods of time in port, are more vulnerable to having invasives settling on their hull or water filled niches.  For faster moving vessels like ferries and cargo ships that are well maintained, antifouled and regularly move there is unlikely to be a problem with marine invasive spread due to minimal hull fouling. The table below illustrates a ranking system for biofouling of vessels, the boxes highlighted orange indicate when action needs to be taken.

Hull Biofouling Classification



Visual estimate of biofouling cover


No visible fouling

Hull entirely clean, no biofilma on visible submerged parts of the hull.


Slime fouling only

Submerged hull areas partially or entirely covered in biofilm, but absence of any plants or animals.


Light fouling

Hull covered in biofilm and 1–2 very small patches of one type of plant or animal. 1–5 % of visible submerged surfaces.


Considerable fouling

Presence of biofilm, and fouling still patchy, but clearly visible and comprised of either one or more types of plant and/or animal. 6–15 % of visible submerged surfaces.


Extensive fouling

Presence of biofilm and abundant fouling assemblages consisting of more than one type of plant or animal. 16–40 % of visible submerged surfaces.


Very heavy fouling

Many different types of plant and/ or animal covering most of visible hull surfaces. 41–100 % of visible submerged surfaces.

Source: New Zealand ranking of fouling

Ballast Water

Ballast water can contain organisms, such as plankton, bacteria, viruses, and even small fish or larvae, which can pose a threat to marine ecosystems if discharged in a different region from where they were taken. The IMO has developed several conventions and guidelines to address the issue of invasive species through ballast water discharge. The Ballast Water Management Convention (BWMC) requires all ships to have a ballast water management plan and comply with specific standards for treatment and discharge of ballast water. It entered into force in 2017 and has since been ratified by 86 countries.

Case study: Decontamination Berth

These berths are intended for more routine use in marinas and harbours that are dealing with an invasive species incursion or are aiming to prevent one arriving. Vessels suspected of harbouring an unwanted species on their hull moor inside the berth for a few hours while the chemical treatment is introduced. Once completed, the biocide is pumped back out into a reservoir for re-use, and the vessel leaves.

Vessel Maintenance

Vessel maintenance is an important aspect of preventing the spread of marine invasive species. Marine invasives should be considered by vessel owners when they are sent for maintenance and refit, the biosecurity policies of repair facilities should be a consideration when selecting an operator. This is particularly important for vessels that have been at anchor for some time or for slow moving vessels (< 5 knots) like dredgers and barges. As the Solent is a known hotsport for marine invasive species it is helpful if this information is conveyed to repair facilities outside the Solent for Solent based vessels undergoing refit/maintenance.

Case study: EcoSubsea

The Port of Southampton uses the services of ECOsubsea for vessel hull cleaning. Their remotely operated underwater vehicles remove all organic matter without damaging vessel coatings. All matter is automatically collected for biogas production/recycling. Its mobile cleaning stations can be operated from quays or barges.

Case study: Royal Navy Ships

UK scientists are seeking to ensure Royal Navy ships can operate more sustainably and limit their impact on marine biodiversity. Heavily-fouled hulls spread marine organisms beyond their natural ranges, with the potential for negative environmental impacts. Defence Equipment and Support (DE&S), the procurement arm for the UK Armed Forces, has been working with QinetiQ and PML Applications to design and manufacture a specialist raft which can test anti-fouling coatings on five different classes of Royal Navy ships at the same time – the RFA Tide Class, Type 23 Frigate, Type 45 Destroyer, Landing Platform Dock (LPD) and Sandown Mine Sweeper.


Dredging can create a pathway for invasive species to enter new environments, either by transporting them along with the dredging spoil or through ballast water/hull fouling from dredging vessels. There are studies underway to look at whether grab samples can be tested for marine invasives using Environmental DNA, (eDNA), a technique used to identify species in water bodies. It is good practice to ask dredging contractors about what biosecurity measures they have for their vessels and equipment. Questions that could be asked are whether they clean the hopper, dredge equipment and plough prior to sailing from a port, do they photograph and report any suspected marine invasives found within the dredge arisings and do they have a ballast water treatment system (if needed).

Case Study: Heron - Commercial Dredging Company (NZ)

Heron has an overarching Biofouling Management Plan to document its policy and processes, it has a Biofouling Management Plan for each vessel and a Vessel Record Book. To prevent biofouling, its vessels use a high end antifoul system and are routinely hauled out every 2.5 years, they are surveyed in between times by divers. Its underwater inspection reports check hulls, fittings, niches and voids and are photographed by divers, vessels are cleaned when required, with detailed records kept. Records are also kept when the vessels are laid up in the water for any period of time. It’s now a normal part of operations for the business to have an underwater hull inspection and a cleanliness certificate issued for all its vessels before they move from port to port.

Abandoned Vessels and Fishing Gear

Abandoned, lost, or discarded vessels and fishing gear, such as fishing nets, lines, and traps, can continue to trap and provide a home to marine organisms, including marine invasives even when no longer in use. Fishing gear that drifts or is transported across oceans can serve as a hitch hiking vehicle for invasive species spread. It is important to ensure that abandoned vessels and ghost gear are disposed of properly and care is taken to check for marine invasives before vessels are broken up, especially if this is done at an external location.

Biosecurity Actions

Please refer to our Biosecurity Action Plans for measures that you can take regarding commerical vessels.

Commercial Vessel Resources

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