SOUDAN MINE— For the past several years, state park officials here had hoped that the former iron mine’s bat population might be spared the worst of the effects of whitenose syndrome, which they first detected in the mine in 2013.

Based on the results of a bat survey completed last week, however, those hopes now appear to have been dashed.

“It looks like we’ve seen a 70-percent reduction based on the bat numbers we’ve found on previous counts,” said Park Manager Jim Essig. And that suggests that bat mortality at the mine could eventually reach 90-95 percent. “This does fit the pattern,” said Essig, who noted that significant bat mortality typically does not appear at infected bat hibernacula for a few years. Once it does appear, it typically starts slowly, then peaks in the second and third years. “Usually, the first year [of the die-off], you’ll see a few deaths, then the second and third years you’ll see a huge die-off,” said Essig.

Bat researchers first documented the presence of the pathogen in the Soudan Mine four years ago, but researchers didn’t find evidence of a significant number of bat deaths until last winter. That would make the current winter the second year of the die-off phase. The Soudan Mine has been known as the state’s largest bat hibernaculum, which was previously home to an estimated 10,000-15,000 bats. But it appears the mine is home to far fewer bats now, and based on the experience elsewhere, park officials anticipate the rest of this winter and next winter will continue to show increased bat mortality, potentially as high as 90-95 percent.

While grim, the survey results aren’t necessarily as definitive as they might seem, in part because park officials have not found as many dead bats as the estimated decline might suggest. Essig noted that the survey found a record number of live bats on the twelfth level, near the mine’s Alaska shaft, which suggests that bats are relocating within the mine, presumably in response to the effects of the fungus that causes whitenose syndrome. The bats appear to be congregating in colder portions of the mine, such as levels located near the Alaska shaft, which conducts cold surface air down into the mine. By contrast, bat numbers dropped precipitously in portions of the mine near the Number Eight shaft, where the upward flow of air from the depths of the mine keeps conditions somewhat warmer, and more amenable to the growth of the fungus. At the tenth level, near the Number Eight shaft, Essig said bat numbers were down sharply. The tenth level typically houses the largest known concentration of bats in the mine.

Whether those bats died or moved somewhere else within the mine isn’t clear. Essig said the survey crew that entered the mine last week did not access the tenth level near the Alaska shaft because conditions in that portion of the mine are unsafe. Vast expanses of the underground mine are inaccessible to humans and it’s not possible to know for sure how many bats might have relocated to those areas.

While it certainly appears likely that the mine’s bat population is being hit hard by whitenose syndrome, researchers are learning that bat populations can recover after the initial wave of deaths caused by the disease. “By the fourth year, they are starting to see recovery in some places,” said Essig. But that’s a long-term process, notes Essig, since female bats typically only give birth to a single pup each year. “We’ll have bats here again,” said Essig, “but it could be decades before we see the numbers we used to have.”

The fungal disease known as whitenose syndrome was first identified in the U.S. in 2007, and it has decimated bat populations in eastern North America over the past decade. The disease is named for the fuzzy white growth of fungus observed on infected bats.