The authorities are scouring Lough Neagh to see if the UK's largest freshwater lake has been colonised by an invasive alien species.
Zebra mussels have been found in Lough Neagh
Five zebra mussels have been found on the hull of a boat in Kinnego Marina.
The mussels affect lake ecology and the Department of the Environment wants to see if the lake has been colonised.
Bob Davison of the Environment and Heritage Service said the boat had not been in any other lake such as Lough Erne where the mussel is established.
"No other zebra mussels have yet been seen in the lake," he said.
"We have started an investigation into how widely they have colonised the lake. We do not know if the mussels are confined to Kinnego Bay or if they are already present throughout the lough.
"We are very concerned that zebra mussels may be established in Lough Neagh.
"Invasions of alien species are the biggest threat to native biodiversity after habitat destruction and the zebra mussel is ranked among the world's 100 worst invasive species."
Zebra mussels first invaded Ireland in 1994 and have spread rapidly throughout the Shannon-Erne waterway and connected navigable waterbodies.
In July, the DoE expressed major concern that zebra mussel sightings have been confirmed in Carran Lough, Derrygonnelly, which is not connected to the navigable Shannon/Erne system.
This was the first confirmed sighting of zebra mussels outside the Erne Navigation in Northern Ireland since they were discovered in Lough Erne in 1997.
Should a zebra mussel population become established in Lough Neagh there will be unpredictable implications for fish, birds, water abstraction, navigation, sand dredging and other interests, the DoE said.
The zebra mussel is a stripy, freshwater mussel native to the Caspian sea area of eastern Europe.
They form large colonies that attach to almost any hard surface and have already affected private and public water abstraction stations on Lough Erne, resulting in modifications at the Killyhevlin Water Treatment Plant costing over £100,000.