• Steam rises off of the colorful Mammoth Hot Springs Terraces. Photo courtesy Jacob W. Frank


    National Park ID,MT,WY

Late Winter Bison Population Estimate Released

Subscribe RSS Icon | What is RSS
Date: March 27, 2008
Contact: Al Nash, 307-344-2015

National Park Service
U.S. Department of the Interior

Yellowstone National Park
P.O. Box 168
Yellowstone National Park, WY 82190
March 27, 2008     08-020   
Al Nash or Stacy Vallie (307) 344-2015


Yellowstone Late Winter Bison Population Estimate Released

Yellowstone National Park has completed the 2007-2008 late winter bison population estimate.  The late winter population is estimated to be 3,000 bison. 

The estimate is based on a late winter aerial survey.  It takes into account the 2007 late summer population estimate of 4,700 bison, known brucellosis risk management and hunting removals, and scientific estimates of over-winter mortality rates.

Under the Interagency Bison Management Plan (IBMP), the park works with other agencies to conserve a viable, wild bison population while cooperating to protect Montana’s brucellosis-free status.  That means keeping bison separated from cattle present on land outside the park.

Hazing bison back inside the park continues to be the first step of adaptive management of bison in the Stephens Creek boundary area. When hazing is no longer effective or safe, capture operations may be conducted.

This winter, 1,087 bison have been captured and shipped to slaughter from the Stephens Creek and Horse Butte capture facilities.  State licensed and tribal hunters removed another 166 bison from the population this year.  Seventy-four sero-negative bison calves have been provided for the ongoing quarantine feasibility study.

An estimated population size of 3,000 bison serves as an indicator to guide implementation of risk management activities under the IBMP. 

Yellowstone National Park had already decided to test most captured bison for exposure to brucellosis, instead of sending all captured bison directly to slaughter.  This change allows the park to hold some bison for a short period in the Stephens Creek corrals for spring release back into the park.  Eighty-five bison are currently being held for spring release, which is expected to occur in mid-April.

Park wildlife biologists continue to closely monitor the Yellowstone bison population.  If future risk management actions were to reduce the population to 2,300 bison, the option to consider more conservative management actions is triggered.  The IBMP agencies would look at adaptive management strategies which could increase the implementation of non-lethal management measures.

Similar previous population reductions have not impacted the herd’s reproductive capability. The historical rate of the herd’s annual summer population increase ranges from 8 to 12 percent.  This demonstrates the robust and resilient nature of the Yellowstone bison herd and the abundance of natural forage in the park.

The five cooperating agencies operating under the IBMP are the National Park Service, the U.S. Forest Service, the USDA Animal and Plant Health Inspection Service, the Montana Department of Livestock and the Montana Department of Fish, Wildlife and Parks. 

This is the eighth winter the IBMP has been used to guide brucellosis risk management actions.

- www.nps.gov/yell -


Did You Know?