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Water voles, Dartford Fresh Marshes, Kent

Background

Applied Ecology Ltd was commissioned by GlaxoSmithKline (GSK) to prepare and implement a water vole mitigation strategy to support a development proposal to construct a new link road from an existing roundabout off the A206 in Dartford, to GSK’s Dartford manufacturing site to the south.  The presence of water vole within the GSK-owned Dartford Fresh Marshes, alongside which the new road would be constructed, was identified by Applied Ecology Ltd as part of an ecological baseline assessment of the GSK landholding.

Water vole feeding

Approach

A total of 118 water vole burrow holes, and 47 latrines were recorded by Applied Ecology Ltd along the entire length of the ditch to be lost. In order to minimise the risk of killing and injuring water vole, and to maintain the current population of water vole within the Dartford Fresh Marshes site, a mitigation and compensation approach based on the capture and re-location of water voles to a new ditch network constructed within the same hydrological catchment as the ditch to be lost, was agreed with the Environment Agency (EA) as part of the planning application process.  Applied Ecology Ltd also prepared and agreed a ten year management plan for the receptor ditch network with the EA.

The network of new water vole receptor ditches (450m in total length) was constructed 350m to the west of the ditch to be lost, and were planted with submerged and emergent aquatic plants a year in advance of water vole capture to give the newly planted vegetation an entire growing season to become established. The new ditches were separated from the existing ditch network by water vole and cattle-proof fencing.

Receptor site location

Receptor ditches immediately after aquatic planting:

Receptor ditch before

Receptor ditches in the autumn after one growing season:

Receptor ditch after

Security fencing along the eastern boundary of the GSK site was erected to minimise the risk of third party inference/vandalism, and the ditch from which water voles were to be captured was enclosed with water vole-proof fencing to contain water voles.

Large Sherman mammal traps, baited with apple, were used over the period mid-March to May to capture water voles.  Traps were set and checked on a daily basis, and all captured water voles were relocated into soft-release pens within the receptor site.  These provided water vole with immediate cover, bedding and food, and water voles could easily burrow out of them. A total of 17 adult water voles were captured and re-located.  Local foxes quickly cottoned onto the presence of water vole in the Sherman traps, and measures had to be put in place to fox-proof the traps by placing them in pipes (see below).  This approach proved successful in preventing foxes damaging the traps but still allowed water vole entry into them.

Fox-proofing the Sherman water vole traps by placing them in pipes

Once water vole capture rates had dropped to zero after a number of consecutive trapping days, and water vole field signs had diminished within the trapping ditch, a phased destructive search involving vegetation removal and hand excavation of individual water vole burrow holes was completed by Applied Ecology Ltd.  Following this, a new ditch channel was dug alongside the trapped ditch and the flow diverted.  The water vole trapped ditch was then pumped dry and infilled using excavated soils from the new adjoining channel.  Finally, the water vole-proof fencing around the receptor ditch network was removed, allowing water vole movement across the entire Dartford Fresh Marsh site, and livestock fencing was retained to prevent cattle trampling the recpetor ditches – a problem on the wider site for water vole.

Monitoring

Applied Ecology Ltd water vole monitoring of the receptor ditch network and the wider site by has been ongoing since the translocation took place, and has shown immediate and long-term occupation and breeding use of the new ditches by water vole.