The rainbow trout has become the grey squirrel of waterways in the north west – with anglers being encouraged to take part in a cull.
Normally the Loughs Agency operates a policy of 'catch and release', but when it comes to the rainbow trout, that's all about to change.
Like the American grey squirrel – which threatens to squeeze out its red-coated Irish cousin –the rainbow trout is an invasive species that has adapted so well to its new environment that it now threatens to take over the habitats of the native salmon and brown trout.
The Loughs Agency will use data supplied from anglers to try to solve the mystery of how the rainbow trout entered the River Faughan and River Derg.
Gerry Quinn of Faughan Anglers alerted the Loughs Agency to the invasion. He said there was already a significant number of rainbow trout in both rivers.
"Rainbow trout is not an indigenous species and are essentially a nuisance, but in sufficient numbers they can also affect the supply of salmon and brown trout," he said.
"I alerted the Loughs Agency a while back when our members noticed a significant number of rainbow trout in the Faughan and Derg.
"Both of these rivers are Areas of Special Scientific Interest so it is worrying.
"It is important to find out how these rainbow trout got into the two rivers because there are any number of fish farms around the north west.
"There is nothing wrong with farming rainbow trout in contained water but it is illegal to allow the fish into the natural waterways and it is a breach of the terms of a licence issued by the Loughs Agency."
The agency wants fishermen to note the time, date and location of their catch, take a picture of the fish and send it in.
"If possible, leave samples of rainbow trout caught into the Loughs Agency headquarters at Prehen or contact a fishery officer and ask him/her to transport these samples to Prehen for further evaluation and analysis," a spokeswoman said.