BILLINGS (AP) — More than 500 wild bison from Yellowstone National Park have been sent to slaughter, killed by hunters or removed for experimental research this winter.

State and federal officials want to reduce the park’s bison population by up to 900 animals this winter as part of a long-term effort to curb their winter migration into Montana. That equals about 18 percent of the 4,900 bison counted in the park last summer.

Figures released Wednesday show that so far this season, 333 bison have been captured and transferred to American Indian tribes for slaughter.

Ten more captured animals were transferred into an experimental animal contraception program, and three died while being held inside the park’s bison holding pens near Gardiner.

Another 166 have been killed by hunting, primarily by members of four Indian tribes with longstanding treaty rights that allow them to take Yellowstone bison. 

Only 27 state-licensed hunters killed a bison during a season that closed Feb. 15. Members of the Salish and Kootenai tribes killed 110 bison, and the remaining animals were harvested by members of the Nez Perce, Shoshone Bannock and Umatilla tribes.

Wildlife advocates and members of Congress have for years criticized the capture and slaughter program as unnecessary. But supporters in the agriculture industry argue that it’s needed to prevent bison infected with brucellosis from transmitting the animal disease to livestock.

Officials from counties adjacent to Yellowstone say it’s also a matter of public safety and potential property damage, given that bison leaving the park sometimes enter residential areas.

Out of 327 captured bison tested for exposure to brucellosis this winter, 123 animals, or about 38 percent, tested positive, according to state and federal officials.

Montana State Veterinarian Marty Zaluski said Wednesday that he appreciated the park’s commitment to reduce Yellowstone’s bison herds and prevent more of the animals from spilling into Montana.

However, he added that too few bison have been killed to significantly reduce the population. Once this year’s births are factored in, “we’re holding even,” Zaluski said. 

“Ultimately the population is the driver of the majority of the conflict at the boundary of Yellowstone National Park — private property, disease and public safety,” he added. “You’re just not making any headway reducing those issues if you’re population is at nearly 5,000 animals.”