Derbyshire Wildlife Trust said the minks have thrived in this county's rivers over the past four decades.

The American minks prey on voles - the animals made famous by Kenneth Grahame's The Wind in the Willows, with the character "Ratty".

Now the trust has enlisted the help of walkers, anglers and birdwatchers to spot mink strongholds so it can protect the voles.

Water voles are often mistaken for rats

Philip Precey, of the trust, said: "Our research into water vole numbers in the county suggests that they are disappearing wherever mink are common.

"Mink are particularly successful at catching water voles as they can pursue them through water and on land, and the female is slender enough to enter water vole burrows.

"If we can find out how many mink there are and where their strongholds are, we can take action to protect our remaining water voles."

He added: "Anyone who spends a lot of time in the countryside, particularly around rivers and other wetland areas, can help us by recording sightings of mink."

The trust has produced a form with background information on mink, as well as a card comparing it with similar species such as polecat and pine marten, to help with identification.