Zebra mussels were first found in Northern Ireland in 1997
A mussel considered to be an underwater pest has spread to a Fermanagh lough, the Northern Ireland Environment Agency has warned.
The Zebra mussel, an invasive alien species, was discovered in Lough Bresk.
Zebra mussels can filter between one and two litres of water a day, taking away algae and other organisms which birds and fish thrive on.
They can also cause blockages in pipes which increases costs to water treatment plants and boat users.
Zebra Mussels were first discovered in Northern Ireland in Lough Erne in 1994, and they have since spread to Lough Neagh.
They are native to the lakes of southeast Russia, and attach themselves to stony lake bottom and to hard surfaces like boats, buoys and intake pipes, where they form hard clusters.
The mussels have previously affected private and public water abstraction stations on Lough Erne , resulting in modifications at the Killyhevlin Water Treatment Plant costing over £100,000.
They have significantly altered fish communities in Lough Erne and are a major risk to the future of some freshwater fisheries.
One of the most noticeable impacts has been on the native Swan mussel, which has become extinct in Lough Erne.
Zebra mussels have also led to excessive weed growth in the lough, which has caused serious problems for boat users and tourism.
Bob Davidson, the chair of the NIEA's Zebra Mussel Control Group, said it was vital to stop the organisms spreading to other lakes.
"Prevention of spread is an issue of shared responsibility and we are asking all water users to follow the guidance issued.
"Unfortunately Zebra mussels have spread to a number of unconnected lakes since their first arrival in the Erne system in 1994.
"I would encourage water users to be aware of their presence and if they do find them outside the Lough Erne and Lough Neagh system to report them to NIEA, as the anglers did in this case," he said.