The water vole is Britain’s fastest declining mammal. Two national water vole surveys carried out in 1989 and 1996 found that the UK population had declined by 89% over this period. The main reasons behind this decline are thought to be loss of habitat and through predation by American mink, an invasive non-native species introduced to the UK in the early 20th century.
In 2001 the Chilterns Chalk Streams Project (CCSP) working with local organisations, landowners and volunteers, undertook a series of surveys to find out the distribution of water voles along the Chilterns chalk streams. Surveys in 2001 and 2002 revealed that water voles were present on only two rivers, the River Chess and Ewelme Brook, despite anecdotal evidence that they had been present on most Chiltern rivers in the recent past.
In 2003, following a series of sightings of mink along the Chess valley, a repeat survey was carried out by the CCSP and the Berks, Bucks and Oxfordshire Wildlife Trust. The results revealed that the water vole population had dramatically declined by 97%. A comparison of the two surveys identified that no significant changes in the quality of habitat had occurred between the two surveys and so habitat loss could not account for the sudden population decline. After careful consideration the CCSP, BBOWT Water Vole Recovery Project and the Environment Agency decided to work in partnership to implement a strategic mink control scheme on the river using the newly developed Game Conservancy Trust mink rafts. The scheme would monitor the river for this introduced species and then remove them once they were found.
With the assistance of riparian owners, mink rafts were deployed along the Chess at 1km intervals and monitored regularly for signs of mink. Between September 2003 and March 2004 eleven mink were caught and dispatched.
In addition to the mink control scheme, the CCSP working with landowners, initiated a series of habitat enhancement schemes to help increase the amount of habitat suitable for water voles and to link areas of good habitat together. Some of this enhancement work involved fencing of the river to enable bank-side vegetation to grow and provide a home for water voles and other wildlife.
In 2005 a follow-up water vole survey carried out by BBOWT revealed that that the population decline had been halted and had, indeed increased slightly to 18% of its 2001 size. The survey also showed that water voles had begun to colonise some of the recently enhanced river sections, including the section pictured above.
The river was surveyed again in 2007 and the results revealed a further increase in the water vole population back to 38% of the 2001 population size and also identified a hitherto unknown population on the river in Chesham town. A survey in 2009 showed that the population had further recovered to 87% of its 2001 size.
By 2011 the water vole population had recovered to the level found in the original 2001 survey. In fact numbers were slightly higher! By 2015 however, the population had declined again and signs of mink had again been found. A reinvigoration of the mink control scheme followed with new rafts being deployed and new volunteers recruited to check them. As a result, water vole numbers have begun to increase again. Habitat improvement work including fencing and creation of new marginal habitat in the Latimer area have also helped water voles spread further up the river into Chesham.
The results of the water vole survey show clearly how mink control combined with habitat enhancement can rapidly bring about a recovery in water vole numbers, particularly when applied at a catchment scale. The success of the R. Chess water vole recovery scheme has been largely a result of the commitment of landowners and local volunteers who have supported the water vole surveys and mink control as well as helping to improve habitat throughout the life of the scheme. The R. Chess water vole recovery scheme demonstrates the effectiveness of local projects to reverse the decline of water voles and serves as a model for other projects.
Find out more about the latest survey: 2017 Survey report