Water voles were once among the most common small mammals in Britain. Between 1989 and 1998, the population collapsed by 90 per cent — one of the fastest declines of any native British mammal in recorded history. The primary cause was predation by American mink, an invasive species that escaped from fur farms and spread along river systems throughout the country. Habitat loss was a secondary factor: the loss of bankside vegetation left water voles with nowhere to hide.
In 2021, our Wildlife Guardians team began preparing a two-kilometre stretch of the River Brue on the Somerset Levels for reintroduction. The preparatory work took two years: clearing invasive Himalayan balsam, planting water-vole-friendly vegetation including reed sweet-grass and purple loosestrife, installing nest boxes, and — most critically — establishing a mink control programme across the wider catchment.
Mink control is not a one-off operation. American mink are territorial, and removing them from an area creates a vacuum that is quickly filled by animals dispersing from adjacent areas. Our volunteers maintain a network of mink rafts — floating platforms with a clay pad that records footprints — checked weekly throughout the trapping season. If a mink is detected, a live trap is set. The pressure is constant.
In April 2024, the reintroduction took place. Forty-two water voles, sourced from a licensed captive breeding facility and genetically screened for compatibility with the local population, were released at five points along the prepared stretch.
The autumn 2024 population survey recorded 61 individuals — evidence of successful breeding in the first season. A second survey in spring 2025 recorded 78. The population is self-sustaining.
This is the fourth water vole reintroduction Alder & Otter has completed. All four populations are currently self-sustaining. In every case, the critical factor was not the reintroduction itself but the years of preparation that preceded it.