It always depends on the weather. Hunters and bison managers alike rely on cold temperatures and deep snow in Yellowstone National Park to push the animals out to the north and west, onto lands where they can be targeted by hunters or captured for slaughter. If it doesn’t happen, if the park stays comfortable, they won’t leave.

The fickle balance varies from year to year. This year, following the season that saw the most bison killed in a decade, the balance has swung the other direction — at least so far. No bison have been shipped to slaughter, and hunters have taken fewer than 200.

“We’ve had several hunt teams go over and come back empty handed because the animals just aren’t out,” said Carl Scheeler, a wildlife manager for the Confederated Tribes of the Umatilla Indian Reservation in Oregon.

In November, bison managers set a goal of removing between 600 and 900 bison from the population through both hunting and ship-to-slaughter, but the numbers are lagging so far this year. A winter plan managers signed in December set a goal of removing at least 372 bison by the end of January. As of Friday, estimates from different sources put the total somewhere between 130 and 170.

Tom McDonald, a wildlife manager with the Confederated Salish and Kootenai Tribes, said it’s all because of the weather.

“(This year) has just been one of those years where the migration’s been low,” McDonald said. “We may not hit the harvest goal this year.”

Yellowstone’s bison are killed each year because of a management plan that prescribes population reduction as part of preventing the spread of brucellosis to livestock. The disease can cause animals to abort and is feared by the cattle industry. No case of bison transmitting the disease to cattle has been documented in the wild.

The population was estimated at about 4,800 prior to this year’s hunts. The management plan calls for a population of roughly 3,000, but managers in recent years have set removal goals based on having “a decreasing population.”

Removing 600 would keep the population relatively stable while removing 900 would lead to a slight decrease. Last year, more than 1,200 bison were killed, with slightly less than 500 of those killed by hunters.

Hunting is the removal method preferred by bison managers. Hunters licensed through six tribal governments and the state of Montana take aim at the bison each year in two places — west of the park, near West Yellowstone, and north of the park, near Gardiner.

In a typical year, more bison wander north of the park than west, but this year hasn’t been typical. Until this week, nearly all the bison harvest came in the West Yellowstone area — about 70 animals total.

The balance between the two sides shifted somewhat last weekend, as the weather pushed a big group of bison north into the Gardiner basin. They stayed on the west side of the Yellowstone River and walked past the Stephens Creek Facility, the place where bison are caught for slaughter. No bison had been trapped so far, and Yellowstone officials decided against using the trap during the weekend migration.

“With the number of hunters in the field, we made the decision to let the bison push and get out and give the hunters a chance,” said Tim Reid, Yellowstone’s bison management coordinator. “This was the first real testing and pushing of the boundary by the bison this season.”

The migration coincided with the opening days of the Blackfeet Nation’s first bison hunting season since asserting its treaty rights to hunt the area. Keith Lame Bear, the Blackfeet’s chief game warden, said Blackfeet hunters took 31 bison in the first couple of days of the hunt on the Gardiner side.

A few other hunters apparently managed to be successful there, too. Adam Pankratz, Montana Fish, Wildlife and Parks’ regional warden captain, said slightly more than 40 were killed between Sunday and Monday.

The activity slowed over the following few days, as bison headed back into the park. Prior to that burst, 21 kills had been confirmed in the Gardiner basin this winter.

Despite the muted activity, tribes have been testing a new method of managing hunting pressure on the Gardiner side. Tribal hunters are required to hunt on public land, and hunters have typically flocked to a sliver of land known as Beattie Gulch, which butts up against the Yellowstone border.

Over the years, locals have complained of constant gunfire, unsafe hunting practices and seeing too many trucks on the road. Various efforts to address the problems over the years have failed.

Tribal hunters adhere to their own tribe’s regulations and are policed by their own wardens, but four tribes agreed to a set of common regulations that applied only to those four tribes. It limits the number of their hunters that can be out at one time, prohibits all hunters from shooting at the same time and boosts cooperation between game wardens for different tribes.

McDonald, of the CSKT, which is part of the agreement, said it had led to a more structured hunt and seemed to be working OK but it was tough to judge its success in such a slow year. He said the time to test it is during the larger migrations, but the arrival of a few dozen Blackfeet hunters, who aren’t subject to the agreement, has thrown a wrench in the plan.

“Now was the time to truly vet it, but unfortunately we can’t with other people that aren’t party to it being on the landscape,” he said.

The addition of the Blackfeet hasn’t caused any major problems, according to multiple sources. Lame Bear, the Blackfeet’s chief warden, said they’ve been trying to get along well with the other entities that have been hunting there for years.

“We’ve been following all the rules that they’ve set in place,” he said.

Pankratz said there have been a few complaints about hunters chasing bison and shooting into large groups of the animals. He also said there has been good communication between wardens, but one of the same old issues at Beattie Gulch has remained.

“It’s crowded,” he said. “You got a lot of hunters in a very small area.”

Season’s end comes next week for at least two tribes and Montana hunters. All harvest and any capture for slaughter usually ends in mid-to-late March.

Michael Wright can be reached at mwright@dailychronicle.com or at 582-2638. Follow him on Twitter @mj_wright1.

Michael Wright covers the environment and wildlife issues for the Chronicle.

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