Hunters armed with shotguns filled the woods of the Blue Hills reservation for the first time in over a hundred years Monday, ready to put a dent in what state officials say is a deer population in overdrive.

It was open season in almost half of the 7,000-acre Milton reservation. According to the state’s Department of Fish & Game, 89 hunters started around 6 a.m. Monday and checked their kills with officials throughout the day.

Animal rights activists plan to rally at the reservation Monday afternoon near the check-in site. They say the deer population can be controlled with contraceptives and there’s no need to do it the old-fashioned way.

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The area where hunters came to have their prey evaluated and measured by the state was made off-limits to the public and the press. A stream of pickup trucks, some with hooved limbs hanging off their beds, came through the check-out station throughout the afternoon. Activists were turned away from the check-in site by Massachusetts State Police.

State deer and moose project leader David Stainbrook said the number of deer living in the park is five times larger than what’s ideal for a healthy forest. That leads to all sorts of problems like an uptick in lyme disease and a whole generation of young trees eaten before they get a chance to grow.

“We want to see those numbers dropped in a reasonable timeframe, so this controlled hunt tends to be a way to balance, to bring those numbers down to where we need them to be,” Stainbrook said.

The state’s Department of Conservation and Recreation decided to allow four days of hunting: Monday and Tuesday of this week and another two days in December. Opposition to breaking the 100-plus-year hunting ban cropped up immediately, with a Facebook group titled Friends of the Blue Hills Deer gathering hundreds of supporters.

“We and many others believe this could collapse the deer population,” a post on the group’s page said Monday. “And these poor animals have been protected for a century. They will have no chance.”