Environment chiefs have defended their plans to poison a Cumbrian lake to get rid of a predator species of fish.
Anglers are worried that the plan to poison topmouth gudgeon in Ratherheath tarn near Kendal will kill other harmless species such as roach.
But John Shatwell of the Environment Agency said there was "no other option" but to kill topmouth gudgeon, which eat native fish eggs and spread disease.
He said everything would be done to save native fish, but some would die.
In recent weeks, Agency officials have been netting the lake's native carp and bream, before moving them to a fish farm before the poison is introduced.
Mr Shatwell said: "It is a priority to eradicate this invasive species to protect the unique and sensitive ecology we have in the Lake District.
"We will have teams on stand by when the process begins to try and net native fish and save them, but we will not be able to remove every single one."
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.
BBC Radio Cumbria's angling correspondent, Patrick Arnold, said it would be possible to save fish even when the poisoning begins by getting them into clean water as quickly as possible.