The report, which was made public this week, faulted federal and state officials for not doing enough to stop the slaughter of bison that leave Yellowstone National Park. A record number of bison from the Park have been removed or killed under the plan since February.

Some conservation groups say this report backs their view that the current management plan isn't working. "We just really need to take a hard look at what's going on out here," says Dan Brister, Project Director for the Buffalo Field Campaign. "We've had a year where almost 1,400 bison have been killed. That's a significant portion of this herd, so we really need to look at the science and look at how that's affecting the population structure of the herd."

The bison kills are made to ensure that the animals don't spread livestock diseases to cattle on ranches near Yellowstone. The GAO report determined that the management plan has been successful in that regard. Yellowstone Park officials were unavailable for comment Friday, but have previously said they are determined to maintain a viable and free-ranging bison population.

Since 2000, more than 3,200 bison have been killed under the management plan.