The Loughs Agency has urged anglers to help catch thousands of farmed fish which have escaped into rivers in County Tyrone and County Londonderry.
The non-native rainbow trout entered the River Strule near Newtownstewart after it burst its banks and flooded the Rocks Lodge hatchery last week.
It is feared they could out-compete native species for food.
There was also a smaller, unrelated escape of farmed rainbow trout on the River Faughan.
- Northern Ireland floods: Clean up and questions
- How to cook rainbow trout
- Pacific salmon found in Irish rivers
The Loughs Agency, whose remit includes managing Lough Foyle’s inland fisheries, is to meet the company that owns the hatchery, Dawn Fresh, next week in relation to the incident.
But the agency said it was not planning to take any legal proceedings as there was no question of liability due to the nature of last week’s flash floods in the north west.
They came after 63% of August’s average rainfall fell within nine hours.
‘Kill them and take them home’
The agency said it could not remove the fish by normal methods such as electrofishing or netting due to the potential damage that would cause to wild stocks.
Seamus Cullinan, a fisheries inspector with the agency, said: “Our advice to anglers is, get out there, catch them and take them out of the system.
“Kill them and take them home.
“We have talked to the company and have been assured these fish are fine to eat, and apparently they are relatively easy to catch compared to wild species.
“I have already received reports of anglers catching hundreds of these fish,” he added.
The escaped fish have been modified and are not capable of reproducing, but the agency believes that they could damage other species of fish.
“They are bred in fish farms as a commercial species, designed to put on weight rapidly,” Mr Cullinan explained.
“They are naturally very aggressive feeders.
“They have the potential to outcompete our native species such as Atlantic salmon, sea trout and native wild brown trout for food, and also could potentially prey on the juvenile populations of these species,” he added.
The Loughs Agency is also encouraging anglers to note the time, date and place of capture and has asked them to leave samples at its headquarters in Prehen, County Londonderry.
What are Rainbow Trout?
Rainbow Trout (Oncorhynchus mykiss) and Brown Trout (Salmo trutta) are different species of the same family (Salmonidae)
They have the same ancestors but divided into two groups around 15-20 million years ago
They normally have a pink stripe running down each side
Dawn Fresh said it would review its flood protection plans and take any action required to reduce the risk of a similar incident in the future.
“We will be working with our staff, local public bodies and our staff to do all we can to make sure this kind of event doesn’t happen again,” said a company spokesperson.
“As a contributor to the local economy, both as an employer and through our local supply chain, it is in our own economic interests to minimise any loss of stock.”