Timing Issues

Proper timing is essential for both assessments and closures. Mines used by bats in the winter, for example, may not be identified as such in summer surveys. Assessments conducted during each season will more accurately characterize use by bats and other wildlife. Mines that are not regionally critical for bats may still have a small resident colony, which should be considered during the closure process. It is best to avoid closures during the winter, when bats may be hibernating, or during the maternity season, when pups may be present. If a complete visual inspection cannot be made immediately before the closure, contractors should perform pre-closure exclusions to ensure that no bats are trapped inside.


Ease of access may be an important factor in choosing closure options, since assessments, closure, and post-closure monitoring and maintenance sometimes require moving large amounts of equipment and materials. Easily accessible sites can accommodate any type of equipment and structure. Remote wilderness sites may require helicopter access or hand-carrying supplies, which may limit your options. Also, easily accessible sites may get more public visitation, making them greater risks than remote sites.

Hazardous Nature of the Mine

The default option for abandoned mine features is usually closure, but if a mine is short and safe, a case may sometimes be made for monitoring only. Many mines are used for education, recreation, storage, and wildlife hbitat. However, specific hazardous elements — poisonous gases, radiation, water, rockfall, falling danger, rotting timbers, corrosive environments, explosives, etc. — constitute major human health risks and increase the need for closures.

Cultural Importance

In addition to biological resources, abandoned mines are often important repositories for historical artifacts, and many mines and mining districts are listed on the National Register of Historic Places. Resource managers should be sensitive to cultural resources when making closure decisions and should, and at the very least, thoroughly document these resources.

Mine Density

The presence (or absence) of nearby mines that are suitable as alternative roosts may influence closure decisions. Suitability should be evaluated for local species, and on a mine-by-mine basis, since even mines that appear similar may have very different microclimate regimes that make some less useable than others. In areas with many available mines, however, at least a few may be suitable for bats and other wildlife.

Active/Renewed Mining

Continued mining activity may — or may not — cause excessive disturbance to wildlife, but it can result in the alteration or destruction of established habitat. When considering choices for mine closure or protection in active mining areas, you should assess the likelihood of the mine's long-term availability, and whether alternate roost areas may be available or created by mining activity.


Limited funding for mine closures usually influences decisions on design and construction. Nonetheless, a gate's structural integrity and biological transparency should not be compromised. We encourage hiring experienced mine gate designers and builders who can anticipate and avoid expensive problems.

Monitoring and Maintenance

After a gate is constructed, follow-up monitoring is required to ensure that the installation has no negative impacts on the resources it is designed to protect, that it remains structurally stable, and prevents human access. Vandalism and erosion can both cause gates to fail prematurely, once again causing liability issues. If a gate fails for any reason, repairs should be undertaken promptly to prevent additional damage.