A small Cumbrian lake is to be intentionally poisoned in an effort to rid it of an unwanted species of fish.
The Environment Agency says the move is necessary to rid Ratherheath Tarn, near Kendal, of topmouth gudgeon.
Native species are being removed from the lake, before experts poured in an organic piscicide - fish poison - to kill off the Asian invaders.
Topmouth gudgeon prey on the eggs of native fish, as well as spreading disease and competing for food.
Topmouth gudgeon, which are about 10cm long, are thought to have been accidentally introduced to the tarn in the 1960s.
Efforts to control their numbers have so far failed.
Agency officials were netting the lake's native carp, bream and tench, before moving them to a fish farm before the poison was introduced.
It is likely to take several weeks for the poison to take effect.
BBC Radio Cumbria's angling correspondent Patrick Arnold said: "Topmouth gudgeon breed very rapidly and have a ferocious appetite.
"They eat the food in the lake and feed on fish eggs, so they affect the native fish in the tarn.
"Nothing like this has been done before here. It's been done in America. This was the only option left to the Environment Agency.
"We do not want the topmouth gudgeon getting into other lakes in the Lake District."
A spokesman for the Environment Agency said: "This has to be done to protect the unique ecology of the Lake District.
"Topmouth gudgeon are very resilient, extremely invasive and thrive at the expense of our native fish species."
A spokesman for the Windermere, Ambleside and District Angling Association, said closure of the tarn for a long spell would be damaging to local anglers.
He said: "The closure of the tarn for a summer period would be a great loss to local and visiting anglers and also cause serious financial difficulties for the association.
"However, the committee are already considering potential re-stocking policy, but cannot make definite plans until the success of the netting operation is known."