BUTTE, Mont. (AP) — Mining companies hope to prevent another massive die-off of geese in the polluted waters of an old open-pit copper mine in Montana.

The Montana Standard reported Friday bird experts worry that with the annual northward migration of geese underway, birds might again be tempted to rest in the water in Berkeley Pit in Butte.

In November 2016, an estimated 3,000 snow geese died when they landed in the acidic, metal-laden water. The pit holds about 50 billion gallons (189 billion liters) of water.

The U.S. Fish and Wildlife Service determined they were killed by severe internal tissue damage that appeared to have been caused by a corrosive substance.

Fish and Wildlife Service, which has jurisdiction over migratory birds, decided not to fine the two companies responsible for the site, Montana Resources and BP-owned Atlantic Richfield Company.

Atlantic Richfield put together a hazing system to scare the birds away, including the use of booming noises and rifle shots.

Local bird experts briefed Montana Resources supervisors and consultants last week on how to identify 42 species of migratory birds that might land on the lake so they can decide best how to frighten them away.

Some species can be left alone because they won't linger, Stella Capoccia told the group.

"A lot won't stay because there's no food for forage," said Capoccia, a biology professor at Montana Tech. "If left alone and allowed to rest, it reduces the stress to the birds."

About 200 birds were in the area early last week, but with the spring migration underway, they numbered more than 1,700 by Thursday, ornithologist Gary Swant told the group.

"Things are changing rapidly," he said.

The site is undergoing a Superfund cleanup. Montana Resources is working on a plan that would eventually treat water from Berkeley Pit and discharge it into a nearby creek.

The goal is to slow down and eventually stop the rise of the water level in the pit.

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Information from: The Montana Standard, http://www.mtstandard.com