National Park Service officials are taking a closer look at lethal management of the Yellowstone bison herd after a recent population estimate showed the herd has declined by more than 36 percent this winter.

The recent population estimate showed about 3,000 animals. In summer 2007, Yellowstone officials said the bison herd reached about 4,700 animals. The National Park Service and the Montana Department of Livestock captured and slaughtered almost 1,100 of the 1,700 dead bison for leaving the park’s north and west entrances, a migration that officials say could spread the bacterial disease brucellosis to cattle. Hunters killed another 166 animals.

According to Yellowstone spokesman Al Nash, the Interagency Bison Management Plan, a document that governs bison management in Yellowstone, stipulates that wildlife managers must review lethal control of bison once the Yellowstone population reaches 3,000 animals.

Once that trigger is reached, “we need to look at our management activities to see if we feel there needs to be any change from existing practices in order to accomplish our goals, one of which is to conserve a viable bison population,” said Nash.

Officials would again assess management activities if the herd were to drop below 2,300 animals.

This year, Nash said, state and federal wildlife managers already implemented a management strategy to conserve bison in mid-March when they started using Stephens Creek as a capture-and-hold facility instead of a way station for bison before they get shipped to slaughter. Currently, bison that leave the park through the north entrance are captured and tested for brucellosis. Those animals that show no signs of disease are held until the snow melts in the spring.

“We were ahead of the game because we were looking at green-up,” said Nash.

There are currently 215 bison in the Stephens Creek facility. Sixty five of those animals tested negative for brucellosis and another 131 remain untested. Since testing began, 62 bison have tested positive for the disease and were shipped to slaughter.

If the number of bison did drop below 2,300 animals, park officials could halt testing activities, allowing them to hold up to 400 animals for a short time, said Nash.