Most Unwanted: Zebra mussel
Dreissena polymorpha
Habitat: Freshwater
Threat: Fouling, negative impacts on native species and local extinction of unionids
Status: Established
The zebra mussel is perhaps one of the most well known invasive species in Ireland. This species was first documented here in 1997. Since that date has spread to many of Irelands freshwater ecosystems and significantly altered these habitats, threatening many native species including protected and/or species of economic value.
Download the Zebra Mussel Management Strategy .
What is it?
The zebra mussel has a distinctive stripy shell and individuals are small, growing up to 3-4 cm in length. Unlike other freshwater mussels the zebra mussel, attaches to hard surfaces using byssal threads. Zebra mussels typically live for two to three years. They will settle on a wide range of surfaces including rocks, anchors, boat hulls, intake pipes, unionid mussels and plants.
Impact
When zebra mussels invade a waterbody there can be a wide range of economic and ecological impacts.
Zebra mussels can have varied and unpredictable ecological impacts. They can change nutrient cycles, filter out the microscopic plants called phytoplankton that form the basis of the food chain, increase water clarity and increase plant growth around lake margins. They can reduce the amount of zooplankton, an important fish food. Zebra mussels will also attach to native mussel species and this can dramatically reduce unionid populations or cause their local extinction. Changes in fish populations can also occur as zebra mussels colonise spawning grounds, change habitat and food sources. In Lough Erne, County Fermanagh, there has been an increase in water clarity and a major reduction in the amount of plankton. Very few live specimens of the native mussel Anodonta are now found and the local extinction of Anodonta can be expected in the near future. The fish populations have also changed with much higher levels of perch recruitment.
In Northern Ireland, zebra mussels have blocked water intake pipes at Killyhevlin water works, Enniskillen, County Fermanagh and modifications were needed at a cost of over £100,000 to date.
Zebra mussels will also attach to the hulls of boats and block boat engines causing problems for boat owners. This increases maintenance costs with the need for hull cleaning or application of antifouling treatments. Zebra mussels can cause problems for anglers, with their sharp shells cutting and snagging fishing lines. Many of Northern Ireland's waterbodies support important recreational tourist fisheries that make a significant contribution to the local economy. If zebra mussels affect fish populations, this may have an economic impact.
Where is it from?
Originally from the Caspian and Black Sea region but has become invasive in Europe since the late 1700’s after the construction of an extensive canal network. Zebra mussels are now present in Sweden, Finland, France, the former USSR, Germany, The Netherlands, Italy, Switzerland, Britain, Spain and North America.
Where are they in Ireland?
Zebra mussels were first discovered in Ireland in Lough Derg in 1997, but. Zebra mussels have continued to spread in Ireland and are well established in the Shannon, Boyle and Erne navigations and, to date, they have spread to over 50 lakes.
How did it get here?
The zebra mussel probably arrived in Ireland in 1994 attached to the hulls of second-hand boats imported from Britain or the Netherlands.
How are they spreading in Ireland
The zebra mussel continues to spread by natural dispersal in connected waterbodies. The zebra mussel life cycle involves the production of 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 and develop the adult shape.
Zebra mussels continue to be spread by the activities of society. Hull fouling and movement of colonised material from an infested site to new waterbodies has lead to this species becoming established in new lakes and rivers.
There is also a history of deliberate introduction to non impacted sites. A history of misinformation has lead to some individuals believing that the zebra mussel can help ‘clean up’ a waterbody and remove pollution.
What you can do
Do not introduce zebra mussels to new sites.
Prevent fouling on your boat.
Dry out angling equipment.
Report all sightings.
