From: http://www.nps.gov/yell/parknews/0870.htm

Yellowstone National Park has completed the 2008 summer bison population estimate.

The summer population is estimated to be 3,000 bison. The estimate is based on a series of aerial surveys conducted in July.

The park expected a high level of winterkill; and in late winter estimated the herd at between 2,100 and 2,300 adult and yearling bison.

The winterkill was lower than estimated, and the herd now includes 2,500 adult and yearling bison, and 500 calves of the year, which are equally distributed between the Central Interior and Northern Range herds.

The population was estimated at 4,700 bison last summer. The peak population estimate of 4,900 bison was recorded in the summer 2005.

The summer population estimate is used to inform adaptive management strategies under the Interagency Bison Management Plan (IBMP). Specific management actions may be modified based on expected late winter population levels as corroborated by the summer population estimate.

The IBMP is a cooperative plan designed to conserve a viable, wild bison population while protecting Montana’s brucellosis-free status.

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