Marine Pathways Project - Control and eradication of NNS

The central goal of the marine pathways project is to protect marine biodiversity in the UK and Ireland by managing key pathways by which NNS are introduced and spread. However, it is not possible to prevent all introductions of NNS. Therefore, work under the marine pathways project also included experimental studies concerned with control and eradication of NNS so that we are better equipped to deal with NNS in the event of their introduction. Three experimental projects were undertaken under the marine pathways project which aim to gather information which will aid the control of NNS following their introduction or spread. These are:

  • Project 1: ‘The control and eradication of Didemnum vexillum off the West coast of Ireland’.
  • Project 2: ‘The Dee Chinese Mitten Crab Project’.
  • Project 3: ‘The extent of Grateloupia turuturu in Wales‘.
  • Project 4: 'Survey of feral Pacific oyster in Scotland'.

Details of each project can be found below.

Project 1. The control of Didemnum vexillum off the West coast of Ireland.

The carpet sea squirt, Didemnum vexillum, is a colonial sea-squirt (tunicate) native to Japan. Over the past decade this species has spread rapidly outside of its native range and has now been recorded at over 100 locations across the U.K, U.S.A, Italy, Canada, New Zealand, The Netherlands, France and Ireland. Once established this species can grow rapidly, extensively coating living and non-living underwater surfaces.

This species was first documented in Ireland in June 2005 where it was discovered fouling immersed man-made structures within Malahide marina. The hulls of boats, pontoons, chains, ropes and buoys were extensively fouled and assemblages of native biota were overgrown by colonies of this invader. Subsequent to this initial discovery, it has been documented at a number of other sites throughout the country including; Clew Bay, Carlingford Lough, Galway Bay and most recently in Strangford Lough, Northern Ireland.

Impact of Didemnum Vexillum

This species has been of major concern globally because of its potential to alter marine ecosystems and the threat that it poses to aquaculture operations. Potential impacts include the following:

  • Overgrows native plants and animals
  • Forms dense mats on oyster bags reducing water flow and food supply
  • Significantly increases fouling on aquaculture equipment
  • Envelops economically important shellfish (oysters, mussels & scallops)
  • Dominates substrate in fouling communities
  • Reduces structural complexity, diversity and availability of marine habitats
  • Jeopardises the functioning of productive fishing grounds
  • Disrupts aquaculture processes and requires labour intensive management to remove & mitigate impacts
  • Increase labour costs & decrease profitability
  • Fouls boat hulls, pontoons, buoys, chains and ropes within marinas

Experimental work – assessment of two different control treatments.

Invasive species are a known threat to biodiversity and to the operation of aquaculture businesses. Any initiative to manage invasive species will have positive benefits for the achievement and maintenance of site integrity ultimately facilitating the continued operation and growth of the aquaculture sector. The project actions included research, awareness raising within the aquaculture sector, and testing control initiatives. The work carried out has identified an increased presence of Didemnum vexillum in Clew Bay compared with previous years. It establishes on hard substrates but once “anchored” can grow over various surfaces including seaweed.

Experimental work was carried out in Clew Bay using two simple control treatments; Acetic acid (vinegar), (a number of studies have identified this as an eco-friendly chemical found to reduce Didemnum vexillum cover by 80 – 100%) and bag turning (farmers found that fouling is reduced significantly when oyster bags were turned regularly, causing desiccation stress to Didemnum vexillum).

Experimental work on carpet sea squirt control in Clew Bay
These treatments wereapplied either individually or in combination to oyster bags over a six month period. In addition, the timing, frequency and variance of the application of the treatments were investigated to determine if any of these factors influence their success.

Over the course of the experiment, levels of Didemnum vexillum for each treatment were monitored and oyster health and condition recorded. Overall, this experimental work aimed to develop environmentally sustainable, cost effective and time efficient methods for managing Didemnum vexillum in aquaculture that protects stock and reduces the potential future spread of Didemnum vexillum. The outcome of this work will contribute to codes of practice that may be adopted by farmers for the management of invasive species in aquaculture.

This work was led by the Irish Sea Fisheries Board or Bord Iascaigh Mhara.

For further information on this work please contact Martina O’Brien, UCD, Dublin Ireland at

Project 2. The Dee Chinese Mitten Crab Project.

The Chinese mitten crab originates from eastern Asia, in temperate and tropical regions between Vladivostock (Russia) and southern China, including Japan and Taiwan. This species of crab can have serious impact on marine and freshwater ecosystems where it predates and outcompetes native invertebrate and fish populations. During its breeding season it may also burrow into river banks, increasing erosion, river turbidity and causing the collapse of river beds.
Chinese mitten crab

Image courtesy of the Food and Environment Research Agency (FERA)

The aim of this two year project (2013 – 2015) was to gain more of an understanding of mitten crabs in North Wales, focusing on the River Dee.

The main objectives of the project were to:

  1. Gain an estimation of population size.
  2. Explore the geographical extent within the catchment.
  3. Examine the environmental cues driving migration.
  4. Examine the effectiveness of methodology.
  5. Look at Pathways management and raise awareness of the mitten crab issue with stakeholders in the Dee catchment.
  6. Obtain mitten crab DNA samples for a related study based at the Natural History Museum in London.

This work was supported by Natural Resources Wales (NRW) and carried out by the North Wales Wildlife Trust.

Project report: Monitoring of Chinese mitten crabs (Eriocheir sinensis) on the River Dee (PDF) (Natural Resources Wales 2016)

Devil’s tongue weed, an invasive non-native species

Project 3. The extent of Grateloupia turuturu in Wales.

Grateloupia turuturu, or Devil’s tongue weed, is a NNS in the UK. It impacts the marine ecosystem by out-competing native species. G. turuturu was first recorded in Neyland Marina, South Wales, in 2008.

A survey was undertaken by Natural Resources Wales in 2013 to:

  1. Establish and record where this species is located within Neyland Marina and the surrounding Milford Haven area.
  2. Determine the feasibility of eradication of this species from this area.

Pontoons and other floating structures were surveyed, involving people snorkelling around the structures and recording presence/absence of G. turuturu. 38% of the pontoons and other floating structures surveyed had G. turuturu growing on them. Neyland Marina was probably the site of introduction as this was the epicentre of all of the G. turuturu records within the area.

The extent to which this species spread indicates that eradication will be very difficult. Ultimately this study highlighted that control and eradication of G. turuturu and other NNS will be improved by predicting where introductions are likely to occur and acting rapidly following detection of introduction.

This work was led by Natural Resources Wales

Pacific oyster

Project 4. Survey of feral Pacific oyster in Scotland

The Pacific oyster (Crassostrea gigas) has become the main species of oyster farmed in Europe. Originally native to Japan and North East Asia, it was first introduced for cultivation in the UK from Canada during the 1960s. Feral populations have become established in the wild, ranging from France north to Scandinavia, however in the UK established feral Pacific oyster beds have only been identified in the south east of England. Warming coastal seawaters around the whole of the UK may contribute to further developments in this trend.

A survey funded by the Scottish Aquaculture Research Forum (SARF) started in spring 2014 aimed to examine the existence, if any, of feral populations (or ‘self settlers’ rather than geographically displaced individuals, such as displaced from aquaculture or discarded) in Scotland. Intertidal surveys were carried out at a variety of potentially suitable locations around Scotland to determine if Pacific oysters were present, and record the biological and physical characteristics of the area surrounding any settlement of oysters found. Surveys took place throughout 2014, and reported in 2015. This project informed the development of policy and management in Scotland to support the sustainable development of Pacific oyster farming and the maintenance of protected sites.

This work was led by Scottish Association of Marine Science, Scottish Aquaculture Research Forum & Scottish Natural Heritage.

Project report: Survey of Wild Pacific Oyster Crassostrea gigas in Scotland (PDF) (Cook, E.J., Beveridge, C.M., Lamont, P., O’Higgins, T., Wilding, T., 2014)

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