Inland Fisheries Ireland is investigating the origin of newly-arrived pike in Aughrusbeg Lough near Claddaghduff in north Conamara.
The presence of the pike in the lake was found during a fish stock survey by the State agency - the first time this has happened in Aughrusbeg Lough.
Several years ago, pike was introduced into the upper sections of the Owenriff catchment in Oughterard, which caused the virtual collapse of salmon fishing in the area.
Chairman of Clifden Trout Anglers Martin Conroy is extremely disappointed with the discovery and said it could be devastating to resident fish populations.
New introductions are also potentially a carrier of fish disease and parasites.
Aughursbeg Lough was a big attraction to fishermen from all over Europe, Mr Conroy said, and this discovery will be a big blow to the local economy.
Officials are investigating if the pike was introduced to the Conamara lake through human activity, by the illegal movement of fish between watercourses.
Francis O’Donnell, Chief Executive Officer of Inland Fisheries Ireland said: "Ireland’s inland waterbodies are ecologically important ecosystems, which support significant recreational fisheries for native and established fish species. Introductions of new species threaten these ecosystems that they support, potentially in unforeseen ways, and are a major cause for concern for Inland Fisheries Ireland."
Inland Fisheries researchers are currently analysing samples from the latest fish stock survey to establish the age and distribution of the pike population discovered in Aughrusbeg Lough and this will determine the steps that need to be taken by the organisation.
