GRAND PORTAGE, Minn. — For moose, this year’s winter-long deep freeze across the Upper Midwest is truly ideal weather. The large, gangly creatures are adapted to deep snow: Their hollow fur insulates them like fiberglass does in a house. And the prolonged cold helps eradicate pests that prey on moose, like ticks and meningeal worm, or brain worm. Yet moose in Minnesota are dying at an alarming rate, and biologists are perplexed as to why.

In the 1980s, moose numbered about 4,000 in the northwest part of the state; today, there are about 100. In Northeast Minnesota, the population has dropped by half since 2006, to 4,300 from more than 8,800. In 2012, the decline was steep enough — 35 percent — that the state and local Chippewa tribes, which rely on moose meat for subsistence, called off the moose hunt. The mortality rate rebounded slightly this year, but moose continue to die at twice the normal rate to sustain a population. Researchers elsewhere, along the southern edge of moose territory in New Hampshire and Montana, are also beginning to notice declines in the animals’ numbers.

Seth Moore, a wildlife biologist in Grand Portage, theorizes that recent years of warmer, shorter winters and hotter, longer summers have resulted in a twofold problem. The changing climate has stressed out the moose, compromising their immune systems. And warmer temperatures have allowed populations of white-tailed deer, carriers of brain worm — which is fatal to moose — to thrive.

Still, “I’m not necessarily convinced that brain worm is the silver bullet that’s killing all of the moose,” Dr. Moore said. “There are a number of different issues.”