Anglers have been warned to be vigilant after a breed of mussel which can cause significant ecological damage was found near Wrexham.
Zebra mussels breed quickly and use up food sources
The zebra mussel has been found at Monks Pool in Johnstown - the first confirmed sighting in north Wales.
The striped mussels breed quickly, blocking water intakes and robbing other species of food.
Fishermen are being urged to wash equipment in disinfectant to help prevent spreading the mussels' larvae.
Environment Agency Wales are also asking anglers not to use the molluscs as bait.
Spokesman Richard Pierce said: "The zebra mussels' larvae can be carried on boats or fishing equipment and can seriously damage the quality of the fishing."
Zebra mussels filter water so effectively they remove most of the algae and zooplankton that forms an important part of the diet of young fish.
They breed rapidly, displacing native species, and can reach up to 5cm in length.
They are known to have caused extensive damage in Ireland and North America, with densities of up to 700,000 mussels per square metre.
The Environment Agency said it was working with Johnstown Angling Club to reduce the risk of spread.
Anglers are encouraged to wash all equipment - including nets - with disinfectant.
Graham Humphreys, secretary of Johnstown and District Angling Club, said: "We're working very hard with the Environment Agency to ease the problem, but I don't think we'll ever get rid of these things completely."
Anyone who thinks they spot a zebra mussel is asked to call the Environment Agency on 0800 807060.