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There are now about 200 to 300 in the area, the only population of the east coast of the South Island.
The festival's theme was "Geraldine's gone batty''.
The North and South Island long-tailed bat was confirmed in 2018 as being one species and it has the highest ranking - nationally critical.
South Canterbury supports the only known long-tailed bat population on the east Coast of the South Island and they are limited to a small area of Peel Forest, south through the foothill gorges of the Orari, Waihi and Te Moana rivers, Geraldine, and the Kakahu and Opihi rivers.
On the willow-lined Opihi, bats have been reported from Arowhenua and inland to the gullies of The Brothers and the Opuha Gorge.
The core of population is centred on forest remnants near Hanging Rock.
Geraldine is one of the few places in New Zealand where it is possible to see long-tailed bats.
They flit at dusk like butterflies as they emerge from giant matai and totara in Talbot Forest, a 26ha forest reserve on The Downs in Geraldine.
The Department of Conservation says threats to the population include clearance of lowland forests, cutting of age-old trees, predation by introduced animals including cats, possums, rats and stoats, and exclusion of bats from roosts by introduced mammals, birds and wasps, and human interference.
DOC's bat (pekapeka) recovery goal is to conserve all bat species and establish new populations.
It protects priority populations with predator control.
DOC researchers have been collecting information about bats at Hanging Rock, where the population is down to around 100 and falling.
Bats depend on age-old trees with cavities with the right conditions for breeding.
Residents can help by planting native trees to provide insects for bats, not cut down old trees as bats often live in dead and dying trees (the advice is to check with DOC first before cutting them down), use less pesticide, and keep cats inside at night.