"It out-competes our native fish" says Dave Mee, from the Environment Agency.
"Water becomes totally dominated by these small fish."
"They're small, they're a nuisance and they're not viable for anglers to fish for, so they're not a targeted species".
More top news
Britain's Got Talent star Ryan Howell has died of bone cancer, aged just 18.
Community Furniture Aid in Bridgend says families have been left sleeping on hostel floors due to a lack of donations.
A 'bizarre' combination of poisonous metals was found in the body of a Cardiff woman allegedly murdered by her lover, a court has heard.