BILLINGS, Mont. - The rounding up and shipping to slaughter of hundreds of bison from Yellowstone National Park will have an effect on the population, though it's not yet clear how great that will be, a park spokesman said.

Al Nash said that since last week, authorities have captured 569 bison for venturing too near or beyond Yellowstone's northern boundary. None of the bison are expected to be rereleased into the park, and most are destined for slaughter plants without first being tested for brucellosis. It's the fear of bison possibly spreading the disease to cattle in Montana that helps drive a state-federal management plan that allows for the hazing, capture or slaughter of wandering bison.

Brucellosis can cause cows to abort.

Nash said officials won't be able to judge "adequately" the effects of their actions on the bison population until sometime this spring. But, he said they were confident that their actions, along with any foreseeable natural mortality inside Yellowstone, would still leave a population of well over 3,000 animals in a late winter-early spring count. The management plan gives 3,000 as a target population.

The bison population was estimated at 4,900 before capture operations began last week.

"We don't want to take any actions that would result in our inability to keep a viable, wild bison population," Nash said. The situation surrounding wandering bison is closely monitored, he said: "We're trying to provide ourselves the widest range of management options on any given day."

But Dan Brister, of the activist Buffalo Field Campaign, said officials "seem hell-bent on capturing as many buffalo as they can right now." Brister worries about how the captures might affect bison family units and genetics in the herd, and he dismisses the 3,000 target level as nothing more than a "politically derived" figure.

On Thursday, 45 bison were captured at the Stephen's Creek facility just inside the park's northern boundary, and 36 were sent to slaughter, Nash said. Another was found dead in the capture pen.

Officials were still holding 229 bison at the capture site, as of midafternoon, he said. While most of those are destined for slaughter, Nash said it's possible that some of the calves will end up at a so-called quarantine facility, north of the park, if they test negative for brucellosis.