With the number of marine invasive and non-native species (INNS) in the UK ever-increasing, the chances of finding one or more on-site have never been higher.

It should be noted that the majority of INNS are not ‘invasive’ (i.e. they do not have the ability to spread and cause damage to the environment, the economy and our health). Invasive species, however, are one of the greatest threats to biodiversity as they can rapidly colonise a wide range of habitats and exclude native flora and fauna. It is therefore increasingly important to develop robust Biosecurity Plans with Risk Assessments for marine developments to prevent introduction and spread of INNS.

With a high likelihood of finding INNS at a given site, it is important to understand what options are available once they have been identified. In the first instance, a detailed survey of the area will confirm the presence and distribution of any INNS, and once this is complete the severity of the problem can be determined. If some species are identified which are not invasive and are widespread in the local area, then no specific measures may be required. If INNS are identified, then appropriate biosecurity measure should be considered and proposed. Regardless of scale of coverage or location, responses fall into three main categories and, depending on the findings, one or all of the options may be necessary as follows:

  1. Monitoring
  2. Confinement/mitigation
  3. Control/Eradication


Ongoing monitoring to assess the current state of INNS is typically the first step at any site. The assessment can be conducted in many ways, but the easiest and quickest ways are:

Rapid Assessment Survey (RAS) – There are a number of rapid assessment methods that can be used, but they typically take one hour or less, focussing on all epibiota growing on submerged features such as pontoons, boat hulls, pilings, walls or submerged artificial substrates. This would include pulling up and examining ropes, cages, and fenders where possible. Samples are photographed in situ and preserved for laboratory identification if required.

Settlement panels – Normally conducted in association with the rapid assessment, settlement panels are deployed to measure the recruitment of fouling species on the site. They can be made of plastic squares suspended below the water and left for around six months, at which point they are then removed, and the biota analysed in the laboratory.

Vessel inspections – Vessel inspection should be a routine part of biosecurity mitigation measures and should take place at all stages of the project lifecycle. To assess the extent of biofouling a rapid vessel inspection can be conducted from the surface if the water is clear enough. Alternatively, a pole camera, Remotely Operated Vehicle (ROV) or experienced marine ecologist diving or snorkelling can be used to examine the amount of growth. Guidance is available to score different levels of biofouling.

Confinement /Mitigation

Once the extent and number of INNS has been established, the next step would be to develop and implement a biosecurity plan incorporating a risk assessment which would highlight any activities that could spread INNS. This process looks at the extent of the area being covered and assesses the risks of introduction and spread of INNS into and out of the area. These will mainly consist of activities associated with movement of INNS, such as vessel movements (e.g. via hull fouling and ballast water), recreational activities and aquaculture (e.g. INNS hitchhikers). Control methods to reduce or limit the risk each identified activity present would be considered. Ideally these control methods can take a range of forms, from reducing the level of the activity in the area, to reducing the risk of activities via biosecurity measures. Additionally, the plan should include provisions for contingency planning if a new species is detected. The main objective is to limit as far as possible the risk of introduction or spread of INNS to the site and reduce the likelihood of INNS being translocated off-site. A robust Biosecurity Plan would ideally be implemented before any INNS have been recorded at the site but will serve to limit spread if INNS are present.


The final option in most cases is the application of population level control or eradication, this can be as simple as removing the affected item from the water or it can be as complex as a site-wide eradication attempt. Most sites will require a range of methods to be effective; however, selection will largely be driven by cost and logistical feasibility. As such, control/eradication can be an effective tool to manage the spread of INNS. Control/eradication methods fall into four main categories:

  1. Removal/air drying
  2. Chemical treatment
  3. Smothering
  4. Mechanical scouring

These methods are used individually or in combination to tackle different elements of a single site.

Removal/Air drying – The cheapest and easiest way to treat small to medium-sized objects is to simply remove the item from the sea and air dry for between 48-72 hours, as this will typical desiccate any attached aquatic organisms. This works well on anything from a rope up to small/ medium-sized boats but is not practical for larger objects or structures.

Mechanical scouring – Often used in conjunction with Removal/Air Drying, Mechanical Scoring can be conducted using jet washers or scrapers to remove material from the desired object. Every care should be taken to contain the material removed using screens or booms and capture for sending to landfill or composted where appropriate. Although not typically conducted underwater, mechanical removal can be undertaken in the same way as on land, but extra care must be taken not to fragment and disperse INNS whilst doing this. As such mechanical removal in the water column has limited practical applications.

Smothering – Smothering is used when the object requiring treatment is immovable (e.g. the seabed) or it is not practical to move (such as a marina). Various methods of smothering have been implemented to remove INNS, including: covering the seabed with non-breathable material (plastic or soil), wrapping objects with polyurethane to make them watertight and making custom made bags to fit around pontoons or other structures. The aim is to stop the water flow and create anoxic conditions, thus containing and removing the INNS.

Chemical accelerants – Chemical treatment can be used in a number of ways to accelerate smothering or air drying and has been successfully applied in a range of scenarios. Bleach can be used in high concentration to speed air drying and is frequently used in aquaculture sites to treat mussels, where the accelerants kill epiphytes but not the mussels. Bleach has also been used to speed up smothering, where high doses were added to bags to increase effectiveness. Bleach rapidly dissociates in the marine environment into its constituent ions, making it a fairly “environmentally safe” option.

In purely marine environments, freshwater pulses can be another effective way of treating large areas such as docks, where the salinity of the environment is reduced to a level below the natural tolerance levels of the INNS being eradicated.

Does eradication work and is it still an effective management tool?

When tackling established INNS or NNS in the marine environment no eradication method to date has been 100% effective and consequently, eradication is often deemed ineffective or not cost-efficient; however, eradication has been successful as a rapid response to newly established or isolated patches.

A combination of the methods outlined above for a specific site can be an effective way to control the further spread of INNS or slow the establishment on site. On land, millions of pounds are spent each year to reduce the spread of rhododendron, Japanese knotweed and other highly invasive plants, often resulting in highly localised control and small-scale habitat restoration and biodiversity net gains. Applying the same principles to the marine environment would greatly aid in improving the quality of our marine habitats.

How APEM can help

APEM can provide a range of services to assist with any stages of the NNS survey and assessment requirements outlined above such as:

  • Marine monitoring – including rapid assessment surveys using a range of techniques to suit the location – conducted by non-native species specialists
  • Stakeholder engagement
  • Provision of site or project specific INNS biosecurity plans
  • Advice on bespoke site or project-specific control and eradication methods

For more contact us on enquiries@apemltd.co.uk



Other websites of interest

GB NNSS Website 

    • Biosecurity in the field (including biosecurity for boat users, submerged structures and event biosecurity support pack)

RYA website 

The Green Blue

    • Antifoul and Invasive Species
    • The Green Guide to Boat Washdown

IMO Guidelines for The Control And Management Of Ships’ Biofouling To Minimize The Transfer Of Invasive Aquatic Species

IMO Guidance for Minimizing The Transfer Of Invasive Aquatic Species As Biofouling (Hull Fouling) For Recreational Craft

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