Acoustic Bat Research at Lake Vermilion – Soudan Underground Mine State Park in 2018

Abstract

The Soudan Mine at Lake Vermilion – Soudan Underground Mine State Park (LVSUMSP) is the largest known bat hibernacula in Minnesota. In 2012, the fungus that causes white-nose syndrome (WNS) was detected in Soudan Underground Mine, and the first bats at this hibernaculum with WNS symptoms were observed during the winter of 2015–2016. The appearance of WNS, observations of dead and dying bats by park staff, and no northern long-eared bats being observed in 2017 winter surveys was cause for concern. In this report we summarize bat call data collected at the Alaska shaft and the No. 8 mine shaft at the Soudan Mine in the spring and fall of 2018. We also include a subset of the bat call data collected in 2017 in this report, as well as data from the 2004 acoustic detector work (Nordquist et al. 2006).

A consistent pattern over the past two years of acoustic detection at the LVSUMSP is that the little brown bat is by far the most common species present at the mine shafts. The second most common species is the northern long-eared bat. Little brown bats had begun emerging from the mine shafts in mid-April when we first deployed detectors. There was a linear trend in increasing numbers of bat call files recorded / night until the end of May when detectors were removed. There were more little brown bats recorded / night at the No. 8 shaft in 2004, while currently there are more bat call files recorded / night at the Alaska shaft. In the fall the number of little brown bats recorded remained constant at about 5,000 calls / night at the Alaska shaft until mid-October. As in the spring, there were fewer bat calls / night, in contrast, at the No. 8 shaft. Calls of other bat species are identified in relatively low numbers, less than 1% of the total number of calls except for the hoary bat.

The timing of spring emergence and fall swarming was similar in 2004, 2017, and 2018, but there seemed to be more bat call files recorded in 2004. The biological reason is that WNS is causing mortality of some bats. There could also be some differences caused by changes in acoustic detectors and call analysis programs. We required consensus agreement of two different programs, which was a conservative method and resulted in only 20% of the bat calls being identified to species. However, species composition was consistent with expectations and was also consistent with results of other acoustic detection and mist-netting projects in northern Minnesota.