An evening survey for bats in the Rees Valley

By day Nicole Sutton is a Jobs for Nature Delivery Manager based in the Whakatipu-wai-Māori / Queenstown DOC Office. But for one night in November Nicole was a volunteer long-tailed bat surveyor.

Twice a year, near the small town of Glenorchy on the shores of Lake Whakatipu, a keen bunch of nocturnal volunteers carry out surveys to get an indication of numbers of long-tailed bat populations in the Dart and Rees River valleys. The first survey was done way back in 1995 and then annually since 1998, resulting in an impressive data set from over 20 years of surveying.

Long-tailed bats, known also as pekapeka or by their scientific name of Chalinolobus tuberculate, are classified as nationally critical. Long-tailed and short-tailed bats are found nowhere else in the world and are New Zealand’s only native land mammal, which makes them very special critters indeed.

Thus, I was excited to be able to join in with the recent 2021 survey. And so it was that on an overcast November Tuesday I headed from Queenstown to the Glenorchy DOC Office for the briefing at 8pm, via the Glenorchy Hotel for a fortifying dinner.

Ten or so people were gathered outside the DOC Office, mainly locals and a handful of off duty DOC staff, discussing outdoor gear preferences and how many times they’d done the bat survey, until Sandra and Helen moved to the front of the group to begin the briefing.

Not really sure what to expect I had brought with me about three days’ worth of warm gear and my best tramping boots, so I was relieved to find that the monitoring transects were along roads and I could leave nighttime off road flitting to the bats. Bats inhabit forest edges and feed along forest margins, so the roads in the wider Glenorchy area make perfect survey sites.

Experienced DOC Ranger Helen Clark showed us the bat detector units and reiterated to make sure they stayed on, or very near, a frequency of 40KHz. Bats use echolocation to navigate around obstacles and find their prey in the dark. They continuously “shout” loudly and use the returning echo to “see” the objects in their paths. The bat detectors convert their “shouting” into audible lower frequency noises that humans can hear as a series of repeated clicks. We were then treated to a round of bat click imitation noises so that we’d know what to listen for.

The bats are counted whilst walking along 1km sections of road transects with a hand-held bat detector. DOC biodiversity Ranger Sandra Barnaba filled us in on how to keep safe while we were working and told us what we’d be recording and how the survey would be carried out. Each team would consist of two people, each with a bat detector, walking one of the five transect lines. Every team would drive together to the beginning of the transect. We were instructed that the first person would then walk slowly but steadily along the first section of transect, while the second person drives 1km to the start of the second transect section, leaving the vehicle for their counterpart to pick up and drive to the next one, and so on until all 1km sections of the transect had been surveyed. We were told we’d start at about 9pm and likely be done by midnight.

Fellow volunteer surveyor Kirsty and I chose the Rees Valley transect. Just before 9pm we jumped in Kirsty’s 4wd ute for the 20-minute drive out to the road end, which is also the beginning of the Rees-Dart walking track, to get started.

Once Kirsty had set out to complete her first section, I drove along the winding narrow gravel road to the start of my section and managed to find a parking spot enough off that road that if another vehicle happened by it’d be able to get past. After rechecking the handbrake was on several times, reasoning that it would be embarrassing for the vehicle belonging to the nice lady I’d only just met to roll off the cliff into the Rees River, I jumped out with my detector and paperwork. We’d been instructed to record start and end times, weather conditions, and insect abundance, along with how many bat passes we heard or saw.

Bat activity can be highly variable and is influenced by numerous factors including temperature, weather conditions, time of night, time of year, habitat type and how much of a flying insect buffet there is around for them. The surveys are done when the bats are most active, that is on warm, fine and calm nights in November/December and February/March, within two to three hours of sunset. Generally, two surveys are carried out each year, one in spring before breeding and one in summer when the young bats have started flying.

Back in the Rees Valley I was three sections down and no sign of a flit or a click from a bat. I was envious of Kirsty who’d already heard three bats. At about 11pm tiredness was starting to seep in, the handle of coke I’d had at the Glenorchy Hotel with dinner was no longer carrying me through. One more section to go. I was strolling along at the recommended dawdling pace with my head lamp off because the star light was enough to see the road by, listening to the roaring of the Rees River running parallel to the road, and contemplating my potential failure as a bat surveyor, when click click click there one was! Instantly more awake, I was happy to report my one detection as I jumped back in the ute with Kirsty to head back to base.

There had been a couple of stream crossings we’d driven through on our way up the Rees Valley Road, which I’d said I’d be happy to walk across in my tramping boots. To work it so that I’d be the one doing the crossings on foot we had concluded that Kirsty should do the first section. We had concluded wrong of course, and Kirsty now had wet sneakers from fording the aptly name Invincible Stream.

Back at the Glenorchy Office we put our gear in the allotted bucket, as we’d been instructed to do, and headed to our respective accommodations. The next day I caught up with some of the other teams. Turns out the Rees transect is always a quiet one, some years there have been no bats detected there. In contrast, on the Routeburn and Dart transects, over 20 bats were detected in just one section.

The good news is that data from these surveys is showing the long-tailed bat population appears to be stable in this area, and even hints at a slight upward trend in the number of bat passes detected since 1998. Bats roost and raise their young in tree cavities, which essentially amounts to living in a hole in a tree trunk with only one way in and out, so they are very vulnerable to introduced mammalian predators such as stoats, possums and cats cornering and killing them.

The community have banded together to carry out introduced mammalian predator control to help protect bats and the other native flora and fauna that can be found in this stunning landscape of meandering braided rivers and beech clad mountains. The Department of Conservation’s Tiakina Nga Manu Battle for our Birds operations has been undertaken on large tracts of Public Conservation Land in response to beech masting events. Air New Zealand have been providing funding for predator trapping efforts on both Public Conservation Land and private land, and the Routeburn Dart Wildlife Trust has traps along the Dart and Rees Rivers and valley roads.

All in all, a stable population of long-tailed bats in the Dart and Rees River valleys is definitely something to celebrate. I’m honoured to have contributed to the collection of data for the November 2021 survey, while being able to gain a night-time perspective of this special area.