The annual ritual of shipping bison to slaughterhouses began Wednesday, as 15 bison previously set aside for a potential quarantine program were loaded onto trucks at the northern edge of Yellowstone National Park. 

Roughly 400 more are in the corrals, awaiting their own trips to slaughterhouses, and park officials plan to capture more throughout the winter. Park spokeswoman Morgan Warthin couldn't say when more bison would be shipped, but she said that the work will continue into March.

Slaughtering bison has long been controversial, but its beginning had a pronounced sting this year for some bison advocates. A park proposal to quarantine some bison at the Fort Peck Indian Reservation was shelved because of a legal dispute between the state of Montana and the park. 

The 15 bison shipped Wednesday were originally part of a group of 40 that officials wanted to use to establish the program, which advocates hoped would reduce the number of bison that officials try to kill each winter and help boost other bison herds around the country.

Stephanie Adams, the Yellowstone program manager for the National Parks Service Conservation Association, said it was "a shame that we're not seeing that occur this year."

About 5,500 bison live in Yellowstone now, and a 17-year-old management plan fueled in part by fears of the disease brucellosis calls for a population of 3,000 there. Brucellosis can cause animals to miscarry, and livestock officials worry bison will transmit the disease to cattle. No case of bison transmitting the disease to cattle has been documented in the wild.  

Officials try to reduce the population through public hunting and ship to slaughter. They hope to kill about 1,300 bison this year. 

Montana Fish, Wildlife and Parks spokeswoman Andrea Jones said they have confirmed the harvest of 342 bison by hunters, though she said with reporting lags that number is likely closer to 400. About 400 have been captured in the park's trap near Gardiner, and park officials expect to catch more as they migrate north.

Last year, Yellowstone proposed establishing a quarantine program at Fort Peck. Quarantine is a process by which the animals can be deemed brucellosis free. It requires isolation in specifically designed corrals and rigorous testing.  

The Fort Peck tribes built a facility for quarantine, but the plan stalled. State livestock officials wanted the bison to be certified brucellosis free before they left the Yellowstone region — essentially quarantined there before they could leave.

Park officials set aside a group of 40 bison trapped last winter to establish the program. But after the quarantine plans stalled, they planned to send them to slaughter.

Montana Gov. Steve Bullock blocked the slaughter of bison in January until the state and the park could find a way to preserve the bison for Fort Peck. A deal was struck to send 25 of the bison — all male, making them a lower risk for brucellosis transmission — to U.S. Department of Agriculture corrals near Corwin Springs. One of the 25 broke its leg in the Stephens Creek facility and was killed, meaning only 24 bison will be sent there. 

The park didn't have details on when they would be transferred. Those 24 could eventually end up being sent to Fort Peck, but likely not until after they are deemed brucellosis free. That could take as long as a year from the time they are moved to Corwin Springs. 

Adams said she was happy that some bison might still be sent to Fort Peck, but she still feels that not sending bison to Fort Peck now is a "missed opportunity." She said that establishing quarantine could be a way for officials to end the annual slaughter of bison. 

"With opportunities such as operational quarantine, this should be the last year that we waste this genetically unique species," Adams said. 

Warthin said the park doesn't know whether it will set more bison aside for potential quarantine this year, but that talks regarding quarantine are ongoing. 

"That's our longterm goal, but it's going to take a little bit to get there," Warthin said. 

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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