Derbyshire County Council (DCC) has Government funding to open a tunnel as part of a new multi-user trail for walkers, cyclists and horse riders along a former railway line, and to form an extension to the existing Monsal Trail. This route would also form part of the proposed White Peak Loop, a high profile initiative to link up off-road cycle paths across the Peak District.
The entrances are currently blocked, but vent shafts are present along the top of the tunnel allowing potential access for bats. Woodland, parkland and a river occur nearby, providing an ideal range of foraging habitats.
Roosting and hibernating bats had previously been recorded within the tunnel. Because of this BSG Ecology was asked to provide specialist advice to DCC in support of a planning application for the phase of the route that included the opening of the tunnel. Critical to the success of the study, given the presence of bat roosts and the complexity of the project, was the ability and experience to undertake a variety of specialist bat survey techniques, including thermal imaging and trapping.
Specialist bat survey and advice
The presence of roosts within the tunnel was recognised by DCC as a significant constraint to the proposal, and clear advice was needed in relation to project feasibility. The survey work had to be sufficiently robust to allow a thorough assessment of the tunnel’s importance to bats, and also to identify with a high degree of certainty whether mitigation for bats could be designed that would allow the tunnel to be opened without adversely affecting their conservation status.
During summer and autumn 2015, thermal imaging cameras were used to locate roosts and search for swarming activity. Trapping, using harp traps and mist nets, was also carried out to confirm the range of species using the tunnel. Remote bat detectors were deployed to identify patterns of activity and to provide further information on species present. DNA analysis of collected droppings was also carried out to confirm the species using summer roosts. The tunnel was resurveyed for hibernating bats in January and February 2016 to update the previous baseline.
The thermal imaging cameras were particularly effective in identifying summer roost sites within the tunnel walls and roof cavities, many of which would otherwise have been difficult to detect. They were also used to differentiate bats’ swarming behaviour and the behaviour of foraging bats emerging from established roosts. Thermal imaging was particularly appropriate on this project as swarming activity can occur late into the night when conditions are too dark to use the naked eye to assess bat behaviour.
The data obtained allowed us to confirm that the tunnel supports a Daubenton’s bat maternity roost in the summer, and that it is used by small numbers of Daubenton’s, Natterer’s bats and brown-long eared bats for hibernation. It also confirmed that the tunnel is unlikely to be an important site for swarming.
Impacts on bats using the tunnel are an important design consideration for the project as it evolves. The survey data and advice provided by BSG Ecology has enabled an informed and positive consultation with Natural England through their Discretionary Advice Service and this has informed the development of the mitigation strategy. This will be needed to ensure that the favourable conservation status of bats using the tunnel can be maintained.
Top image: A tunnel entrance with a harp trap in position.
Bottom image: A thermal image of the tunnel end wall. The red circular areas (which are emitting heat) indicate the position of Daubenton’s bats within the hollow tunnel end wall.