Investigations are under way after 600 crayfish were killed by crayfish plague in a Co Cavan river.
Preliminary results indicate the cause of deaths in the River Bruskey, near Ballinagh, to be the incurable, waterborne, fungus type disease.
If this plague becomes established, there is a high probability that white-clawed crayfish, Ireland's only freshwater crayfish species, will be eliminated from much of the island.
The disease may have been accidentally introduced from contaminated equipment that was previously used in affected waters in another country.
However, if the disease was caused by the introduction of non-native crayfish, then it is likely to become established with a severe and probably irreversible ecological impact.
Many US crayfish species are resistant to the plague but can act as a carrier of the disease, and can also out-compete Ireland's smaller native crayfish.
The introduction of non-native species is illegal and the white-clawed crayfish has already been completely eliminated from much of its European range.
Until now, Ireland has been free of crayfish plague and is the only European country without any established non-native crayfish species.
The only means of protecting native crayfish stocks is to prevent the introduction and spread of the disease, and this is possible if the disease was accidentally introduced from contaminated equipment.
The disease has no direct threat to human, fish or other animal life.
Non-native crayfish can also destabilise canal and river banks by burrowing, and have a severe impact on other freshwater species such as salmon and trout.
While non-native crayfish may be a potential cause, there is no evidence to date that they have been introduced to Ireland.
Members of the public are asked to alert the authorities to any reports of mass mortalities of crayfish as well as sightings of unusual crayfish, and in particular to disinfect boots or angling equipment before moving from one body of water to another.