The find was made in recent weeks during a fish stock assessment survey on the River Inny, downstream from Ballymahon in Longford, by the Shannon Regional Fisheries Board and the Central Fisheries Board.

Over 30 chub fish, both male and female, were located at several locations over a 2km section of the river. The chub is a European freshwater game fish with a thick spindle-shaped body.

These fish are not native to Ireland and, according to Matt Nolan, inspector with Shannon Regional Fisheries Board, "they were probably brought here by English anglers either living here or on holidays here."

The introduction of foreign fish into Ireland is illegal without an import licence from the Department of Communications, Marine, and Natural Resources. People importing fish without a licence face prosecution.

Nolan says the introduction of unauthorised fish to Irish rivers poses two serious threats to the quality of Irish fish stock.

Firstly, foreign fish species, such as the chub, carry dangerous parasites and fish diseases which could affect our native fish. To combat the introduction of other unwanted fish to Irish rivers, the Shannon Regional Fisheries Board is monitoring and inspecting anglers moving into the Shannon area.

"The trouble with chub and other foreign fish species is that they carry fish diseases and parasites that we don't have here in Ireland. We don't want these diseases attacking Irish native fish and we don't need chub here, so to ensure this doesn't happen again we're carrying out checks and surveillance on anglers coming into our region. Only lately, English anglers on the way to Ireland were caught in Holyhead with a container full of live carp they intended to bring here illegally, " said Nolan.

Although checks are being carried out to prevent any further influx, Nolan says the damage is already done. The second disadvantage of chub is that they compete with trout for habitat and eventually drive the trout out. The chub species, if not caught and killed, could wipe out trout stock in the Inny which in turn would affect the River Shannon system.

"Chub fish need to be caught and exterminated. These fish are not edible and are of no advantage. Any chub we've caught so far we've killed, and over the next two weeks we will be trying to eliminate all of them. It's a difficult task and although we might get rid of 95% of them, its not achievable to erase them entirely from our rivers, " said Nolan.

A muscular fish with silvery sides and white belly, the chub varies in size and can resemble a roach in shape in their first and second years. Irish coarse fish species, which include roach and rudd, do not compete with trout the same way chub does.

"Ireland's coarse fish are regarded as probably the best in Europe and are central to the angling business here. By adding chub to our cocktail of Irish fish, we are seriously interfering and reducing the quality of our fish stock, " said Nolan.