By Alex Kirby BBC News Online environment correspondent

A scheme to reintroduce highly endangered water voles to the wild in the UK is getting a hi-tech boost.

Ready for the wild (Image: Martin Chainey)

Of the 70 animals being released, 15 have been fitted with specially designed radio collars.

The collars will allow scientists to monitor the welfare of the voles, all of which have been ID chipped.

The scheme is part of a programme to reverse the species' disastrous recent decline.

The release, at a secret location near Bristol in the west of England, is being completed on 23 April.

The voles are captive-bred animals from groups at Bristol Zoo Gardens and Wildwood Discovery Park in Kent.

Wildwood breeds other endangered species for release, including red squirrels, sand lizards, dormice, wild cats, pine martens and beavers.

Lethal resemblance

The UK is thought to have had about seven million water voles in 1989, but fewer than 900,000 in 1996.

The pre-release health check (Image: Martin Chainey)

Causes include riverside development and subsequent habitat loss, pollution, and the depredations of mink.

Voles are also often mistaken for brown rats, and poisoned as pests. But while rats have protruding ears, pointed noses and almost hairless tails, voles have small ears, blunt noses and furry tails, and are shorter and rounder than rats.

Slightly confusingly, the vole was immortalised in the fictional person of Ratty, the water rat in Kenneth Grahame's book The Wind in the Willows, published a century ago.

The first release to the wild of radio-fitted voles took place in London in 2001. The animals being set free near Bristol have all been checked by the zoo's veterinary department to make sure they are disease-free and likely to survive.

They have been put in holding pens in an area which at the moment has no voles, and where there is plenty of suitable habitat and a network of waterways.

The pens, stocked with food, will let the voles become used to their surroundings and allow them to dig burrows and tunnel their way to freedom.

Trail blazed already

The zoo director, Dr Jo Gipps, said: "It is vital that action is taken now to halt the decline in water vole numbers across the UK.

At home in the zoo (Image: Bristol Zoo Gardens)

"For the past four years Bristol Zoo Gardens has been breeding water voles successfully for reintroduction initiatives, helping to re-establish communities on the Kennet and Avon canal near Bath.

"However, this is only the first step in a long and careful process of educating people about the threats to them and the importance of reintroducing new breeding gene pools to ensure a long and healthy future."

Early bolter

Dr Gipps told BBC News Online: "The area where the voles are being released is mink-free, so far as we know, which should help.

"Clearly some of them will be lost to natural causes, including birds of prey, but that's an ecological fact.

"We're hoping the voles will breed and become a stable, self-sustaining population. One of them has burrowed out of its holding pen already."

The zoo is working on the scheme with the Environment Agency, the lead partner in the UK Biodiversity Action Plan for the vole.