Questions & Answers about Bison Management

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2 minutes, 14 seconds

This video explores answers to a frequent question about bison management.


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Who manages Yellowstone bison?
Five federal/state agencies and three tribal entities make decisions about Yellowstone bison in a cooperative effort called the Interagency Bison Management Plan (IBMP). These include the Animal and Plant Health Inspection Service, the Confederated Salish & Kootenai Tribes, the Inter Tribal Buffalo Council, Montana Department of Livestock, Montana Fish, Wildlife & Parks, the National Park Service (Yellowstone National Park), the Nez Perce Tribe, and the US Forest Service (Custer-Gallatin National Forest).

Why are bison killed every winter?

  • Bison are killed because they do not have enough room to roam. Yellowstone encompasses a limited amount of habitat and Montana allows very limited numbers of bison in small areas adjacent to the park.
  • Allowing the bison population to grow indefinitely could cause overgrazing in some areas and possibly mass starvation of animals in Yellowstone, as well as larger migrations and greater conflict outside the park.
  • Yellowstone bison reproduce and survive at relatively high rates compared to many other large, wild, mammal species. The bison population currently increases by 10 to 17% per year.
  • Currently, predation by bears and wolves has little effect in reducing the bison numbers. Bison are massive animals that defend themselves as a group, making them more difficult to attack than animals such as elk.
  • Hunting outside the park has not been effective at limiting bison numbers because concentrated hunting pressure along the park boundary often causes bison to return to the security of the national park where hunting is prohibited.

How many bison were killed in winter 2019/2020?
A total of 834 bison (17% of the population) were removed during winter 2019-2020, which was within the range of 600 to 900 agreed to by managers. Removals included 548 bison captured at the Stephens Creek facility with 105 entered in brucellosis-quarantine for transfer to the Fort Peck Tribes, 442 sent to slaughter, and one bison dying during handling; 284 bison harvested outside the park in Montana; and two bison shot outside the conservation area.

What is planned for the population in winter 2020/2021?
For the winter of 2020/2021, the NPS recommended removing 500 to 700 bison to slightly reduce the population to 3,850-4,040 at the end of winter and 4,500-4,730 animals after calving. Removals should only occur near the northern park boundary where animals from the central and northern herds intermix. Managers should track migrations and focus removals on the northern herd. If initial removal targets are met, up to 200 additional adult males could be harvested or captured in late winter.

Is Yellowstone’s bison population healthy?
Yes. Yellowstone’s bison population has grown steadily over the last 50 years: from 500 animals in 1970 to more than 4,600 today. Yellowstone bison represent a unique source of genetic diversity. They are one of only a few bison populations with no evidence of interbreeding with cattle.

Aren’t bison protected as our national mammal?
No: that designation did not provide any special protection to bison. Read more about the National Bison Legacy Act.

Why do bison leave the park?
Bison migrate to lower elevations where food is more available (less snow) just like bighorn sheep, elk, mule deer, pronghorn, and many other animals. They return to the park’s higher elevation grasslands to feed during summer.

Why do states treat bison differently than other wildlife when they leave the park?
Bison are not allowed to move freely outside Yellowstone due to fears they might transmit brucellosis to cattle (see below for more info), and out of concerns about competition with cattle for grass, human safety, and property damage. Elk are also infected with brucellosis, but their movements outside the park are not restricted. State governments control the management of wildlife outside Yellowstone (unless a species is federally listed as threatened or endangered).

Why don't you force surrounding states to change their policies?
Outside Yellowstone, states manage wildlife and Yellowstone cannot force them to tolerate more migrating bison.

Why don’t you just ignore the IBMP and let the bison go?
Doing nothing is not a realistic option. Allowing the bison population to grow indefinitely will cause overgrazing and possibly mass starvation of animals in Yellowstone, as well as larger migrations and greater conflict outside the park.

Why don't you withdraw from the IBMP?
Yellowstone isn't big enough to manage a migratory species like bison on its own. Bison conservation is a shared responsibility that crosses many political boundaries. Withdrawing from the IBMP would make it more difficult to collaborate with the land management agencies and tribes that share an interest in what happens to bison when they leave the park. Additionally, Yellowstone’s superintendent is not the signatory of the original IBMP. The agreement was signed and approved by the Secretary of the Interior, who would need to make any decision about withdrawing.

How are bison captured?
On behalf of the IBMP, bison are captured near the north boundary of Yellowstone as they migrate out of the park. Fences guide them into enclosures, so they wander in on their own or are sometimes guided in by riders on horseback. For more information on the capture and shipment process, take a video tour of the Stephens Creek facility or browse photos in our Stephens Creek album on Flickr.

How are bison killed?
We transfer captured bison to tribes who transport them to slaughterhouses for processing.

What happens to the meat?
Our tribal partners distribute the bison meat and hides to their members.

Does Yellowstone profit from shipments of bison?
No: we receive no payment for bison shipped from our capture facility.

Are animals abused at Stephens Creek?
No. The safety of people and animals is our top priority at Stephens Creek. The Humane Society has evaluated the operation twice, and each time we've adopted their recommendations. During 2020, we received expert training and worked to improve low-stress handling of bison during handling and processing. Two old photos are often used to falsely depict our handling practices at Stephens Creek. One shows a bison being held with a nose ring during brucellosis testing, a practice we haven’t used in many years (we now have a hydraulic chute that holds the animals relatively still during testing). The second shows a bison being carried by a front-end loader: a photo that was taken outside the park, most likely after a bison was shot by a hunter or hit by a car. Adult bison can be very large and heavy and often times require heavy equipment to move carcasses from vehicle accident scenes. For a detailed view of the capture and shipment process, take a video tour of the Stephens Creek facility or browse photos in our Stephens Creek album on Flickr.

Does culling put the population at risk?
No. Yellowstone’s bison population has grown steadily over the last 45 years: from 500 animals in 1970 to 4,680 in 2020. Along with elk, bison are the most numerous large mammals in Yellowstone, and periodic culling will not alter that fact. The sizes of most wild ungulate populations in North America are limited by hunting and/or captures and removals.

Are you catering to the livestock industry?
We are legally obligated to follow the directives of the IBMP. Each member of the IBMP has a different perspective on the conservation of wild bison, so conflict resolution is an integral part of the management strategy. We would like to reduce the need for capture and shipment to slaughter, but that would require changing the current bison management plan and agreement from all members of the IBMP.

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4 minutes, 17 seconds

A step-by-step explanation of how bison are captured, processed and shipped at the Stephens Creek Facility.



What is brucellosis?

  • Brucellosis is a bacterial disease that can induce abortions or stillbirths in infected animals, but does not kill them. For ranchers, brucellosis has an economic impact because it affects the reproductive rate and marketability of their animals.
  • Up to 60% of Yellowstone bison test positive for exposure to brucellosis (depending on age and sex).
  • Brucellosis has been eradicated in cattle herds across most of the United States. Bison and elk in the Greater Yellowstone Ecosystem persist as one of the last reservoirs of infection.
  • Brucellosis was introduced to Yellowstone bison and elk by domestic cattle in the early 1900s.

Is brucellosis really a threat?
Yes. People both understate and overstate the risk of brucellosis transmission. Transmission of brucellosis from bison to livestock is possible because in late winter, bison migrate to low elevation areas outside the park where livestock are concentrated. At this time, bison are late in their pregnancy: the most probable time for infected animals to abort fetuses that might be found by other animals. The fact that there's never been a documented transmission of brucellosis from Yellowstone bison to cattle does not mean it couldn't happen. Rather, it’s a testament to the diligent management efforts put forth by the state of Montana and the National Park Service to prevent commingling of bison and cattle during the time period when transmission is most likely.

Do other animals carry brucellosis?
Yes. Over the last two decades, more than 20 livestock operators in the three states surrounding Yellowstone discovered brucellosis in their animals. In each case, wild elk transmitted the disease. However, the state of Montana allows elk to move freely outside Yellowstone: a freedom that bison deserve, but have been denied.

Why don't local ranchers vaccinate their livestock?
They do, but there is no vaccine that’s 100% effective at reducing abortions due to brucellosis or eliminating the potential of infection from wildlife.

Why can't bison that test negative for brucellosis be allowed to live?
Identifying animals that do not harbor the bacteria is difficult and requires many months or years of quarantine: a single test is not enough. For more details, read the Environmental Impact Statement about our proposal to establish a quarantine program for Yellowstone bison.

Why can't bison be vaccinated against brucellosis?

  • Vaccinating wild bison against brucellosis with existing vaccines would not substantially suppress the disease and could have unintended, adverse effects on the bison population in Yellowstone.
  • There is no easily distributed, highly effective vaccine: current vaccines would only create a 10 to 15% reduction in infection, and immune protection is short-lived.
  • Even if we reduced the prevalence of brucellosis in bison, they could be re-infected by elk.
  • For more details, read the final Environmental Impact Statement that evaluated the potential of remote vaccination of bison.


Are there alternatives to capture and shipment to slaughter?
In the history of North America, only humans, disease, predation, and starvation have successfully controlled bison numbers. Hunting outside the park is the preferred method for controlling the population today, but bison need greater access to land in order to disperse beyond our boundaries and pioneer new areas. Currently, the area where bison can be hunted is very small, and bison often respond to the hunting pressure by returning to the security of the national park (where hunting is prohibited). Our capture and shipment operation makes up the difference when hunting outside the park fails to meet reduction targets set by the IBMP managers.

Why can't Yellowstone ship live bison to other areas?
Right now, it’s against state and federal laws to move wild bison exposed to brucellosis anywhere except to meat processing and research facilities. We’ve proposed a quarantine program for bison and are working with our IBMP partners to implement this program.

How does quarantine work?
To determine if a bison is brucellosis-free requires blood to be drawn and tested regularly for months or years depending on an animal’s age, sex, and reproductive status. During testing, bison must be isolated or quarantined in fenced facilities to ensure they don’t come into contact with other animals. Bison that test positive for brucellosis during this process would be removed from the facility and shipped to slaughter. Animals that eventually pass the testing protocols would be considered brucellosis-free and could be sent to join or start conservation herds on other public, private, or tribal lands. Read more about our proposed quarantine program.

What’s the benefit of quarantine?
Quarantine would reduce the need for capture/slaughter operations and promote the restoration of plains bison to suitable areas of the country.

Why can't hunting control the population?
To date, hunting outside the park has been ineffective at limiting bison numbers. The area where bison can be hunted is very small, and bison often respond to concentrated hunting pressure along the park boundary by returning the security of the national park where hunting is prohibited. For hunting to become more effective, bison need greater access to public lands outside the park, like wild elk and other animals, so they can disperse beyond our boundaries and pioneer new areas.

Why doesn't Yellowstone allow hunting in the park?
Federal law (National Park Protection Act, 1894) prohibits hunting in Yellowstone. Because of this prohibition, Yellowstone offers some of the best wildlife viewing opportunities in the world. This prohibition also supports our goal to manage the park so that human activities have as little effect on natural processes as possible. Allowing hunting in Yellowstone would drastically affect the behavior of animals and change the experience for people who come to watch them.

What about fertility control in bison?
No fertility control methods that are affordable, easily delivered, highly effective, and reversible are currently available for delivery to wild bison spread across a vast landscape. Fertility control could have unintended, adverse effects on the bison population in Yellowstone.


Last updated: December 30, 2020

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