Bats in Greywell tunnel, Hampshire

by Paul Hope

Greywell tunnel is a disused canal tunnel on the Basingstoke Canal in north Hampshire. The 1,124m long brick lined tunnel was constructed in tandem with the canal between 1788 and 1794. The tunnel is approximately 5m high and has a maximum width of 4.5m. It was excavated through an area of changing geology from Hampshire chalk through Reading beds into London clay. There was a partial collapse in the tunnel during the 1930s and a further fall during the 1950s that left the bore of the tunnel blocked about 800m from its eastern entrance and at 130 m from its western entrance. Bat friendly grilles prevent public access to the site.

Greywell tunnel has long been known as an important site for hibernating and swarming bats. It was designated a SSSI for bats in 1985. Hibernation surveys are conducted from a boat by a team of up to five surveyors. As you work your way into the tunnel light from the portal becomes a distant dot. During hibernation counts many of the bats encountered are hanging against the brick lining, however a high proportion are often packed into crevices and openings in the brickwork left during the tunnels construction. Within crevices it’s sometimes a matter of counting ears and feet to get as accurate a count as possible and on occasion it’s not possible to record bats to species.

Natterer's bats at Greywell. (c) Roy Champion

Natterer’s bat is the most commonly encountered species (highest count of 569 in 2013), Daubenton’s bat is also regularly recorded (highest count of 69 in 1986). We also often record whiskered, Brandt’s and brown long-eared bats. The count this January gave some very exciting results. We had our second highest count for Natterer’s bat in the eastern end (564) and our first ever record of a greater horseshoe bat. The horseshoe bat is an excellent record for North Hampshire. Bat Group records show it is the seventh on record; this includes two records for Winchester from the 1940s.

Greater horseshoe bat at Greywell. (c) Roy Champion

Sadly after over 30 years of monitoring at the site, future regular monitoring is threatened by a recent move by the owners Hampshire County Council to declare it a Confined Space. Now surveys can only be conducted once every two years due to the excessive costs of gas monitoring equipment and a rescue team stationed outside the tunnel. We do not feel that this is often enough to provide adequate data resolution for the SSSI condition assessment, but Natural England have said that it will be sufficient.