March 13, 2013 - By: Morgan Sherburne - Petoskey News
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Tunnels from an old rock quarry near Alpena are providing a hibernaculum for Michigan bats. Dr. Allen Kurta, a researcher at Eastern Michigan University, found the hibernaculum in 2009. (Courtesy photo/Allen Kurta / March 13, 2013)
Winter steelheaders may not know it, but just yards above their drifting spawn bags or swinging flies, between 18,000-20,000 bats are resting quietly through the winter.
The structure in which the bats are hibernating — called a hibernaculum — is the spillway of the Tippy Dam.
Usually, bats hibernate in caves or other below-ground structures, said Allen Kurta, a researcher and professor at Eastern Michigan University. The majority of bat hibernacula are in the western Upper Peninsula, where copper mining carved out long, snaking tunnels from the landscape.
But because the Lower Peninsula is coated with a thick layer of glacial till — soil and sediment swiped through the region by glaciers that last retreated 10,000 years ago — very few caves exist here. The kinds of caves that do exist form in what's called "karst topography," said Heather Rawlings, fish and wildlife biologist for the U.S. Fish and Wildlife Service in Alpena.
Karst topography results when water reacts with limestone. Water percolates through the limestone, forming caves. That's how the famous caves near Mammoth, Ky., are formed.
But here, most of the caves are either filled in or have collapsed in a sort of sink hole. So bats have to wing up through the Upper Peninsula to hibernate in the abandoned mines — except for two bat hibernacula, in northern Lower Michigan.
Tippy Dam's hibernaculum is the larger of the two. The dam is unusual in that the spillway — the structure at the top of the dam that controls the release of the water — is hollow. The water that the dam holds back snugs against the spillway and keeps the temperature in the hollow spillway warmer than air temperatures above ground.
"It keeps the air temperature just barely above freezing," said Kurta, who has known of the Tippy hibernaculum since 1994. "It makes it a suitable place for them to hibernate."
The other hibernaculum is housed at Rockport State Recreation Area, north of Alpena.
On the property is an old limestone quarry. When it was in operation, the quarry owners created what are called "surge tunnels," said Rawlings — concrete tunnels about 6 feet high and 6 feet wide. Each tunnel is about 500 feet long.
"Back in the day, they contained a conveyer belt and back in the day of quarrying, as the rock was broken up in the mines, they would dump the finished product in the surge tunnels," said Rawlings.
The tunnels had an open line down the center of their ceilings through which the rock would be dumped and conveyed to waiting freighters.
The quarry was decommissioned in the 1950s, but the tunnels remain — which bats have discovered. Kurta discovered the bats — a group of about 100 — hibernating there in 2009.
Generally, hibernating bats prefer air temperatures to be between 42-45 degrees, said Kurta. Rockport's tunnels are probably around 40 degrees and under, he said.
"(Tippy Dam and Rockport) are the only places within a couple hundred miles (for bats to hibernate)," said Kurta. "I figure the bats will put up with the inconvenience in order to avoid that extra couple hundred miles of migration."
The marginal temperatures of the tunnel as well as the danger of the tunnel has spawned a project to make the tunnels more comfortable for bats and safer for humans. Rawlings said over the decades the quarry has been closed, people have broken into the tunnels and spray-painted graffiti. One tunnel has a pile of rubble over much of its opening, but the other allows access for the tenacious.
"Our first concern is safety," said Rawlings. "You really have to make these things bomb-proof to keep people from getting into them."
Rawlings is working with the Department of Natural Resources, Alpena High School students, the Northeast Michigan Great Lakes Stewardship Initiative, 4-H Youth Programs and the Organization for Bat Conservation to also make the tunnels more bat-friendly.
Alpena High School welding students are making gates that will allow bats to enter and leave the tunnel, but keep humans out, said Helen-Ann Prince, Huron Pines AmeriCorps education coordinator for the Michigan Sea Grant-Great Lakes Stewardship Initiative.
All of the organizations will be working together to move some of the rubble covering the first tunnel in an even layer over both of the tunnels, hoping to eventually place 5-10 feet of fill over the top of the tunnels in order to insulate the tunnels, said Rawlings.
These bats, comprised of species such as the little brown bat, the big brown bat and the tri-colored bat, also known as the Eastern pipistrelle bat, have so far been insulated from white-nose syndrome. The syndrome is a devastating fungus that proliferates in the dank caves bats love to hibernate in. Since it was discovered in 2007 in New York, white-nose syndrome has killed between 5.7 and 6.7 million bats in North America, according to the U.S. Fish and Wildlife Service. Recently, the syndrome was discovered in Illinois.
Kurta calls it the most devastating syndrome to hit North American mammals.
"The latest was discovered in Illinois. It's in Ohio, Ontario, Indiana. We are almost surrounded," he said. "Michigan and Wisconsin are probably the last great bastion of hibernating bats on the continent."
But Kurta is not optimistic that Michigan's bats will remain free of the fungus.
"I don't see any biological reasons why our bats are different," he said. "I believe it is not here yet because we've been protected by isolation to a certain degree. And I don't think bats fly east or west across these lakes."
Should the syndrome enter Michigan, it's likely it will jump from mines in Illinois to mines Wisconsin, then to Michigan's western Upper Peninsula.
But for now, bat populations remain healthy and free of the syndrome. Kurta recently returned from a research trip to the Upper Peninsula.
"For the last 10, 15 years we've been keeping track of them," he said. "Things look good. There is no visible sign of white nose. Populations, some of them up, some go down. Whether those are true changes or whether that is an error inherent in our way of counting, but most populations are fairly stable."
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