Minnesota's moose continued their long decline in 2015.

The state's annual aerial survey, taken in January, estimated the state's moose population at 4,020, up slightly from the previous year. But the change was not significant enough to signal a shift in the long downward trend, the Minnesota Department of Natural Resources (DNR) said Tuesday. The population has dropped by more than half since 2006.

Offering a glimmer of hope, moose numbers have stabilized somewhat in recent years, and the number of calves that survived their first year doubled compared to an earlier count.

"It's encouraging to see that the decline … since 2012 has not been as steep, but longer-term projections continue to indicate that the population decline will continue," said Glenn DelGiudice, the DNR's moose project leader.

Moose once roamed all of the northern part of the state — a signature species in the great North Woods that stretch from Minnesota to Maine — but in the past several decades they have been slowly disappearing. Researchers blame invasive parasites, diseases of greater severity and warmer weather — conditions that can combine to make moose more vulnerable to predators.

A separate DNR long-term health study of 134 collared moose found that, among those that have not survived, 64 percent died as a result of health problems and parasites, and the rest from predators.

Large swings in winter severity also may have played a role in recent years, in part because of weather's impact on winter ticks. Ticks, which can weaken a moose when feeding in sufficient numbers, plague the species in winter and then drop off in spring. Harsh winters leave fewer ticks to survive for the following winter.

"I think the back-to-back long winters in 2013 and 2014 really worked to suppress winter tick numbers, which is why we didn't see any of our collared moose die from ticks in 2014 and 2015," said Michelle Carstensen, who leads the moose health study for the DNR.

This year's winter, however, has been exceptionally warm, a bad omen for next year.

"What I don't know is if we're going to see tick mortality back this year, or if it will take another year to really show up," she said.

Moose also have been hit hard by brainworm, a parasite brought into the woods by deer moving north along with warmer winters and human development into territory they never used to occupy.

The January survey did find that the number of calves surviving their first year appears to be higher now than when researchers first started collaring moose in 2013. Then, of the 34 calves that were collared, 76 percent were killed by wolves and bears.

The calf study was called off because collaring appeared to place them at risk, and since then researchers have been counting calves by tracking their adult collared mothers by aerial surveys in November and December. DelGiudice said 44 percent of the calves appear to have survived until early winter this year.

Moose also may have benefited from the massive 2011 Pagami Creek fire in the Boundary Waters Canoe Area Wilderness. For the first time since the blaze, moose have moved into the area to feed. It's an indication that the fire generated the young trees they like to eat, which will help more survive, said Mike Schrage, wildlife manager for the Fond du Lac Band of Chippewa, a partner in the state's research.

"Maybe there will finally be some good habitat for moose," Schrage said.

Josephine Marcotty • 612 673 7394