Linda Butterfield developed an enthusiasm for Water Voles when she volunteered with the Trust. Inspired by this and local success with the species growing over the seasons, Linda undertook a Masters project to inform future Water Vole monitoring guidance. Here, in this guest blog, Linda shares how the project developed and what her findings were.

The Lea Valley is a multi-use, nature-rich corridor in which Water Voles are bucking the trend of continued national decline. This charismatic species demonstrates remarkable resilience when their riparian habitat is well maintained and predation by American Mink is effectively controlled. 

I first learned to survey Water Voles under the expert guidance of the Wildlife Trust’s Water Vole Officer, Josh Kalms. In the Spring and Autumn, Josh and a team of volunteers don wellies or waders, part the vegetation with long sticks and peer into the undergrowth of rivers, lakes and ditches. We are looking for piles of vegetation cut by water voles with a typical 45^o angle or accumulations of greenish tic-tac shaped droppings known as latrines. Tennis-ball sized round holes in the top or edge of soft banks or woven rugby-ball sized nests made of stripped sedge show that water voles have made a home where we are searching. My enthusiasm for this species and our local success grew over the seasons. In collaboration with Josh and my Masters Project Supervisor, Jim Littlemore at Anglia Ruskin University, I undertook a project to inform future water vole monitoring guidance. 

Water Voles will pull themselves up onto naturally formed tussocks, mud banks or fallen vegetation to rest, preen, feed or defecate, so placing artificial platforms within their habitat meant I could observe the voles through field camera footage or record feeding or latrine evidence left behind. It is quicker to monitor and less disruptive to the vegetative bankside cover, meaning more water courses can be monitored and the habitat remains in better condition for Water Voles and other riparian inhabitants. 

Twenty rafts (20 x 20 x 7cm), secured with bamboo canes on different banks across four sites gave a 94% accuracy of Water Vole presence when compared to traditional bankside searches. One of four water courses which had not recorded presence in the previous five bankside searches, also indicated presence with droppings left. I also developed a Habitat Suitability Index (HSI) for Water Voles in riparian habitats, and strongly related water vole presence (number of latrines) to suitability value.

Next, I used pairs of rafts observed by field cameras in four ditches at Silvermeade, part of the Lee Valley Regional Park in Broxbourne. Placing fresh carrot on one of each pair daily, demonstrated that Water Voles will visit food baited rafts significantly more frequently than their empty partners, even when the carrots were switched to the opposite raft. 

However, repeating this experiment with fresh local vegetation had little effect; whilst they normally feed primarily on sedges, they did not eat the pieces I placed on the raft for their enjoyment!

I used fresh rafts in the same setup to determine whether the surface of the raft affected the number of visits. I compared rafts covered in dried reed (Phragmites australis) stems or leaves with the bare rafts, but the water voles were unaffected by the surface; video evidence showed them following regular pathways over one or both rafts as they patrolled their territory.

The final experiment estimated population density with latrine counts. I placed a raft every 10m along a 100m stretch of the New River in Hertford and counted latrines on these and the adjacent banks. The Water Vole mitigation guidance (Dean et al 2016), gives latrine counts of six to nineteen per 100m of bank to indicate a ‘Medium’ density of Water vVles in Autumn. There were eight latrines on rafts and another eight on the bank, so combinations agree a medium density population in this habitat.

This project demonstrated that small floating rafts are helpful in determining Water Vole presence in a quick and controlled manner. Applying the HSI to unmonitored sites indicates which new areas are most likely to yield presence data, so where a raft should be placed. Carrot bait may increase the likelihood of Water Voles visiting rafts but field cameras would need to be used to ensure it is Water Voles eating any removed food, and Water Voles also visited rafts without bait. Density estimates can be categorised by counting latrines on rafts alone, although this may lead to an underestimation more latrines will be found on the bank and the number of rafts provided may also affect the number of latrines counted.

The first rafts used in these experiments belonged to The Wildlife Trust, so these and some extras I commissioned along the way have been cleaned and redeployed, to extend presence records and enhance the overall picture of Water Vole success in the Lee Valley. 

Find out more

Video footage from this project has been compiled, available at https://www.youtube.com/@lindabutterfield and you can find out more about Water Voles in Hertfordshire and Middlesex, by reading this blog.