A hundred and fifty water voles were last week settling into new homes on the riverbanks of Hertfordshire. The animals had been released from pens the previous week as part of a campaign to halt the devastating drop in Arvicola amphibius number across the British Isles over the past 50 years.

Once widespread in the UK, water voles – whose most known incarnation is Ratty in Wind in the Willows – have suffered a 90% drop in population since the 1970s. Feral mink, which kill young voles in their nests, in combination with major changes in land use, have resulted in the widespread eradication of a species that was once a ubiquitous presence along the banks of Britain’s rivers.

“Water voles, although small, have quite an impact on riverbanks and fields, and their presence is a sign that an area is ecologically healthy,” said Josh Kalms, a conservation officer for the Wildlife Trusts, the charity spearheading the campaign to save the water vole.

“However, land use changes along with mink farm escapees have devastated populations over the past 50 years. They have become the fastest declining mammal in the country, and they face extinction unless we do something to save them.”

In several areas of Britain, reintroductions have been launched in a bid to bring water vole numbers back to healthier levels, but conservationists stress that restorations of any species are doomed to failure unless the factors responsible for the initial population declines are removed. If not, then numbers will start to decline again. And in the case of the water vole, this means they must be protected against mink.

Mink escaped from fur farms in the 1950s and 1960s, and although these farms were later made illegal, the animal now breeds across most of the country. It is an active predator, feeding on ground-nesting seabirds and water voles. Mink are good swimmers and the females are small enough to enter the waterline burrows of water voles and take their young.

To provide safeguards against these voracious predators, raft traps were placed along the stretch of the river Ver to the west of St Albans, which had been selected for this month’s reintroduction. “These traps were introduced six months ago in order to catch any mink in the area. Thanks to the traps, we now know this part of the river is mink-free and safe for reoccupation,” added Kalms.

Supported by 40 local volunteers and backed with funds from the Debs Foundation and the Linder Foundation, conservationists from the Herts and Middlesex Wildlife Trust then took a large group of captive-bred water voles and readied them for reintroduction. “It took several days to get our pens of voles in place on the riverbank and ready for release. Then on the last day, we fed them up with fruit and vegetables, opened the pens and set them free,” added Kalms.

As a result, 150 animals are now busy making burrows and seeking partners along the banks of the river Ver, where the last recorded sighting of a water vole occurred in 1987.

The next crucial part of the programme will be to look for signs that water voles have settled and made homes. “Water voles leave little piles of vegetation, all cut at 45 degrees, behind them when they have meal,” said Kalms. “That is a telltale sign they have settled in an area. We have a team of local volunteers who will be monitoring the riverbank for signs the reintroduction is working.”

Although water voles do not have such large-scale impacts on the countryside as species like the beaver, they do bring benefits. “They eat over 120 types of plants and create open areas inside a bank of sedge or wetland grasses that allow a wider variety of plants to grow,” said Kalms.

“In addition, old burrows are used by other mammals and amphibians to live in, and, of course, the odd animal is picked up by a kestrel or an owl to provide dinner. In short, water voles are good for the neighbourhood.”