Invasive Alien Species in Northern Ireland

Dreissena polymorpha, Zebra Mussel

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Dreissena polymorpha
© Environment and Heritage Service
Dreissena polymorpha
© Roy Anderson
Dreissena polymorpha
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Dreissena polymorpha (Pallas, 1771)
The zebra mussel (Dreissena polymorpha) is a freshwater mussel which was accidentally introduced into Ireland in 1994. They attach to hard surfaces in rivers and lakes including rocks, anchors, boat hulls, intake pipes, unionid mussels and plants.
The zebra mussel is small, up to 3-4 cm in length with distinctive stripy shells. It is the only freshwater mussel which attaches to hard surfaces using byssal threads.
Country of origin
The zebra mussel is originally from the Ponto-Caspian region which includes the Black, Azov and Caspian seas.
Current distribution
Zebra mussels are now present in Sweden, Finland, France, the former USSR, Germany, The Netherlands, Italy, Switzerland, Britain, Ireland, Spain and North America.
Location in Ireland
Zebra mussels were first discovered in Ireland in Lough Derg in 1997, but probably arrived in 1994 attached to the hulls of imported second-hand boats. They are well established in the Shannon, Boyle and Erne navigations and to date they have spread to over 50 lakes including Carron Lough and Lough Neagh.
Life cycle
The zebra mussel produces millions of microscopic larvae called veligers when water temperatures are above 12°C (May to September in Ireland). Larvae drift in water currents and settle on hard surfaces (mainly from June to October), develop the adult shape and typically live for two to three years.
Wildlife and habitat impacts
Ecological impacts are varied and unpredictable. Zebra mussels change nutrient cycles, filter out phytoplankton which forms the basis of the food chain, increase water clarity and increase plant growth around lake margins. They reduce the amount of zooplankton, an important fish food. Colonies of zebra mussels change the habitat of other invertebrates and they attach to native mussel species causing their local extinction. Changes in fish populations can also occur as zebra mussels colonise spawning grounds, change habitat and food sources.
Human impacts
Fouling of hard structures in waterbodies can increase maintenance costs, for example, water intake pipes at water treatment works may need modifications. Boat hulls become fouled and engines blocked and boat owners need to clean hulls or apply antifouling treatments. Zebra mussels can also cause problems for anglers by shells cutting and snagging fishing lines. This may affect important recreational tourist fisheries.
Key vectors
Spread among connected waterbodies occurs as zebra mussel larvae drift in water currents. Spread between isolated waterbodies usually occurs via human mediated vectors. There are seven major vectors that may transfer zebra mussels to new lakes in Northern Ireland:
  • Recreational boating
  • Intentional introductions
  • Restoration of the Ulster canal
  • Angling activities
  • Fisheries and aquaculture activities
  • Illegal eel fishing
  • Scientific research and conservation work.

    What you can do as an individual
    Individuals should prevent further spread of zebra mussels by following the guidance issued by EHS and outlined below.
    Advice to specific groups relevant to this species
    There are specific steps that anglers and boaters can take to ensure zebra mussels are not spread by their activities:
    • Ensure that all angling equipment is fully dried out before use in a different lake or immersed in hot water
    • Do not place bait bucket water in an uninvaded lake
    • Do not reuse bait if it has been exposed to infested waters
    • Do not use zebra mussels as bait in uninfested waters
    • Check your boat hull for zebra mussels, if the surface is rough, young stages of mussels may be attached
    • If mussels are found, dry out the boat or steam clean. Zebra mussels will die once they are removed from the water but this may take three to four weeks or more under damp and dull conditions. Boats should be stored in dry and airy conditions where the mussels can dry out for the sufficient time period. If infestation is heavy or if there is insufficient time to kill the mussels by drying, an effective method for cleaning a boat is by high-pressure steam cleaning. The water temperature needs to exceed 40°C and large mussels need longer to be killed. The material removed should not be allowed to enter a water body of any sort
    • Never move your boat to an uninvaded waterbody without cleaning it first
    • Remove any visible vegetation from items that were in the water, including the boat, trailer, and all equipment
    • Boats should be kept in the water for the shortest possible time. If boats are left in the water over the summer they are more likely to be fouled with mussels
    • If you have to keep your boat in the water, antifoulants can reduce the numbers of settling mussels by killing or discouraging them from settling. Some products are designed for use in freshwater; nevertheless ensure you use an approved brand using the instructions given.

      Management measures
      The Zebra Mussel Management Strategy for Northern Ireland (2004-2010) was published by Environment and Heritage Service in 2005. Management measures include awareness raising and policy and legislation. Fact sheets and leaflets are available for water users outlining actions that can be taken to prevent the spread of zebra mussels (see literature section for references). Any sightings of zebra mussels outside the Erne system should be reported to EHS, Tel: 028 9054 6550.
      Management groups
      The Zebra Mussel Control Group (ZMCG) is a forum for sharing information, coordinating joint education and awareness campaigns and identifying research needs. Contact: Joanne Livingstone, EHS, Tel: 028 9054 6550.
      Further information
      Environment and Heritage Service ( for research, publications and further links.
      Western Region Zebra Mussel Control Initiative (
      The World Conservation Union (IUCN) has an Invasive Species Specialist Group and the web site contains information on invasive species and species profiles (
      Zebra Mussel Management Strategy for Northern Ireland 2004-2010
      Advice for anglers and water users (
      Zebra mussels in Northern Ireland (
      Management of the impacts of zebra mussels in Northern Ireland and determination of effects on fish populations in Lough Erne through alterations of the food web
      See the Invasive Species Ireland web site for further information -
      Text written by:
      Dr Cathy Maguire