Monitoring the

Effectiveness of 

Bat Compatible

Mine Gates’

by Michael Herder,

Arizona Strip Field Office /

Grand Canyon - Parashant

National Monument

Background

Abandoned underground mine

workings pose serious threats to

human safety. In an effort to protect

the public from the hazards of abandoned mines, public land managers

have implemented large-scale closure

efforts, often at significant expense.

The most economically feasible

mine closure methods include blasting, plugging, backfilling, and other

permanent solutions. Recent studies

have shown that numerous wildlife

species, including bats, use these

artificially created habitats. As much

as 80 percent of the mines in the

Western U.S. show some evidence of

bat activity. Permanent abandoned

mine closure methods have not only

resulted in destruction of roosting

habitat, but have also caused direct

mortality of bats by entombing

them within the sealed mine.

Many agencies have installed

wildlife-passable gates at mine openings in an effort to mitigate the loss

of animals and their habitat resulting

from permanent closure. Gates allow

animals to pass through openings

too small for most humans, while

maintaining air flow patterns crucial

for internal habitat conditions.

Unfortunately, many early gate

designs impeded bats in flight,

allowing predators to take them

easily. In some areas, bats have abandoned historic roosting areas despite

the addition of bat compatible gates.

Gates are effective in protecting

the public from the dangers of

abandoned underground mine

workings; however, few long-term

monitoring studies have been conducted to determine if the gates

impact bats or their behavior. Those

studies done to date have typically

neglected pre-closure bat population counts. Post-gating monitoring

has been mainly limited to exit

counts, where observers count bats

as they leave the roost. Exit counts

are time and labor intensive, often

extremely variable, and may be

biased by environmental factors and

observer presence.

This resource note summarizes interim results of a study to evaluate the

effectiveness of bat compatible gates

at abandoned mines at Silver Reef,

in southwestern Utah. Over 140

abandoned underground mines have

been closed with bat compatible

gates or grates at this site. Pre-closure

surveys found extensive bat use in

abandoned mines here, including

maternity colonies of Townsend’s

big-eared bat (Corynorhinus

townsendii) and fringed myotis

(Myotis thysanodes). Study objectives

include evaluating and ranking the

effectiveness of infrared event counters, infrared video, and night vision

equipment for counting bats as they

pass through mine gates. The end

result of the study will be an evaluation of bat roost monitoring techniques with development of protocols for effective methods. This study

was a cooperative effort with the

Utah Division of Oil, Gas, and

Mining; Southern Utah University,

and the St. George Field Office of

the BLM. Funding was provided by

grants from the National Fish and

Wildlife Foundation and the Keck

Foundation.

Methodology

Prior to closure, four internal bat

surveys were conducted at Silver

Reef, two each during the warm and

cold seasons. These surveys were

used as a baseline for determining

long-term changes in bat numbers at

gated mines and were used to help

select six gated and four un-gated

sites for evaluation. Sites were

selected based on availability of

pre-closure count data, presence 

of colonies, and presence of two

or fewer openings. All gates

selected were constructed of 3.8 cm

manganol steel bars, oriented horizontally, and spaced 11.4 - 14.6 cm

apart. Vertical bars were welded

into place approximately 61 cm on

center.

From June 1997 through January

1998, exit counts using night vision

goggles were used to count bats

emerging from abandoned mines.

Mist nets, harp traps, and acoustic

surveys were used to identify bats to

species. In January 1998, batterypowered active infrared event

counters were secured inside the

opening of each mine. Each event

counter includes a floor-mounted

transmitter which shines an infrared

beam to a receiver mounted on the

ceiling (back) of the mine. Each

time the beam is broken the counter

records the event and time.

Infrared video cameras were installed

in up to four locations per site to

verify the accuracy of events recorded by the counter and monitor bat

behavior as they passed through the

gate. Limitations of video systems

included short tape and battery life,

and low resolution. Battery life limitations were resolved by using

infrared bullet and box cameras in

place of standalone video camcorders. We were able to maximize

tape and battery limitations and

minimize the effect of observer presence on counts by using a video

transmitter to send the signal to a

monitor and recording unit at least

100 m away from the gated mine.

Interim Results and

Discussion

Exit counts alone provided a biased

sample of the number of bats using a

RESOURCE NOTES

NO. 18 DATE 06/01/00

Range

Wildlife Biology
Range

site. Observers had no means of

verifying the accuracy of counts or

determining which species were present. Observer notes were the only

permanent record available for later

review. Other sources of bias included observer limitations such as

fatigue, inability to reliably count

numbers in excess of 30 at once, or

count swirling bats (exiting and then

re-entering the roost). The presence

of an observer sitting outside the

mine may have been sufficient disturbance to cause bats to seek

another exit or remain within the

mine. Numerous observations were

made of bats that clearly detected

observers, even those that remained

still and quiet throughout the count.

Active infrared event counters

proved effective for counting bats

entering and exiting mines.

However, event counters underestimated the number of bats present

when multiple bats broke the

infrared beam at one time or when

individuals avoided the beam

entirely. Conversely, event counters

overestimated the number of bats

present when a single individual

repeatedly triggered events by circling the infrared beam. Numerous

observations were made of bats circling the overhead receivers.

Infrared video cameras provided

the most information with the

highest degree of flexibility of the

monitoring systems evaluated.

Visual data stored on video tape

serves as a permanent record

which may be retrieved, analyzed,

and edited at any time at a fraction

of the cost of quality night vision

equipment. However, reviewing

video data can be very time-intensive

without the use of costly electronic

video editing tools.

Infrared time event recorders consistently provided higher counts than

exit counts when conducted simultaneously. On at least two occasions,

observers left the site after two hours

when declining counts indicated that

the emergence was complete. Within

two hours following observer departure, the infrared event counter

recorded close to 300 bats. While

this appears to support the idea

that observer presence can significantly bias exit counts, it should be

noted that the event recorder was

unmanned and therefore could not

be verified. Bats could have moved

into the area from another location

to use the mine as a night roost,

or individuals may have circled

the infrared beam, triggering

numerous events.

Having video images of each triggered event proved invaluable in

verifying the accuracy of the data. In

most cases, counts made from video

were identical to those of event

counters. Where video counts were

higher, we verified that one or more

bats had missed the event counter’s

infrared beam. Where the event

counter recorded more hits than the

video, we were able to verify that

bats were circling the event counter,

triggering multiple hits. In one

instance, the counter recorded over

1,000 events. Later review of the

video showed that no bats had exited, but that a wood rat had rested

on the floor mounted transmitter,

triggering multiple hits.

Future efforts will focus on developing a qualified index for sampling

with event counters alone, determining if bats habituate to event

counters, reducing video review

time by capturing short sequences

triggered by video motion sensors,

and determining the effect of

observer presence on exit counts.

Contact

Michael Herder

USDI BLM, Arizona Strip 

Field Office

Grand Canyon - Parashant

National Monument

345 E. Riverside Drive

St. George, UT 84790

(435) 688-3239 Office

(435) 688-3258 Fax

michael_herder@blm.gov Range

Range

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experience of the author and has not been peer-reviewed. Conclusions

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