Native fish were removed from Ratherheath Tarn near Kendal, after topmouth gudgeon were discovered.

Topmouth gudgeon are native to Asia, but were accidentally introduced to Europe in the 1960s. They are invasive and feed on the eggs of native fish.

The Environment Agency says the drastic measures, which involved removing native fish, have been successful.

Agency officials said the alien fish posed a "serious threat" to the ecology of local rivers and lakes. They are now preparing to re-stock the tarn with native fish.

The tarn is a popular fishery with both local and visiting anglers Jeremy Westgarth, Environment Agency

Ratherheath Tarn is run as a coarse fishery by the Windermere, Ambleside and District Angling Association (WADAA).

Environment Agency fisheries team leader Jeremy Westgarth, said: "It was essential for us to take action to protect the unique ecology of the Lake District.

"We're privileged to have internationally-important fish stocks on our doorstep, including salmon in the River Kent and rare species like char and schelly in the nearby lakes.

"Earlier this year, we netted native fish from the tarn and took them to one of our fish holding units so we could keep them safe while we carried out work to eradicate the topmouth gudgeon.

"We will now be working with WADAA to release the native fish back into Ratherheath Tarn.

Native fish

"The tarn is a popular fishery with both local and visiting anglers and we want to have it back to normal and fully-stocked with healthy native fish as soon as possible."

The tarn was treated with a chemical known as a "piscicide", meaning that it is harmful to fish but not to other wildlife, to kill the topmouth gudgeon quickly and humanely.

The piscicide, made from a naturally occurring organic substance, has broken down harmlessly, and the water is now ready for native fish to return.