Unfortunately, most mill-tailings ponds are too large to cover, and because ponds on top of heaps are difficult to reach, they also are never netted. Most pregnant or barren ponds, however, are easily netted, though factors such as being torn by wind or loads of snow and ice must be taken into consideration. Data on wildlife deaths are minimum estimates, most notably for bats. Because bats are small and dark, they are difficult to find and identify. Bats drink in flight over open water and so are likely to disappear into the pond, while many kinds of birds and other animals are more likely to die on land where they can be found.
Bats are also of much less concern to most miners than birds.
The Migratory Bird Treaty Act makes it illegal to kill most migratory birds except game. Although it subjects miners to fines and other legal actions, thus far, enforcement has been limited to two cases in Nevada. What data there are on bat deaths seem to indicate that mortality is concentrated in late summer and fall when many bats are migrating. None were reported from January through March. In Arizona, California, and Nevada, the geographic ranges of six bat species on federal or state lists of endangered, threatened, or otherwise protected wildlife have cyanide-extraction gold mines within their jurisdictions.