A pair of bull moose pause while feeding in the Umbagog National Wildlife Refuge in Wentworth's Location, New Hampshire. A study by University of New Hampshire researchers finds that winter ticks are posing an increasing threat to the region's embattled moose population. Credit: Robert F. Bukaty | AP

CONCORD, New Hampshire — Moose calves are dying at unprecedented levels in parts of New England, mostly because of the hordes of winter ticks — as many as 90,000 on one animal — that latch onto their bodies and drain their blood.

Those are the findings from a new study in the Canadian Journal of Zoology that looked at moose health over several years in New Hampshire and Maine. The study identified 125 calves that died from 2014 to 2016 in the two states and found that tick infestations caused nearly 90 percent of those deaths. Ticks were also impacting female adults, resulting in fewer calves being born and a decline in overall calving rates.

Researchers found overall mortality rates of 70 percent among calves over that period, compared to about 15 percent two decades ago. The only bright spot came in 2017, when mortality rates were about 30 percent; researchers attributed that to a September drought and earlier snowfall resulting in fewer ticks.

“The deaths were directly related to heavy loads of winter ticks,” said Peter Pekins, a University of New Hampshire wildlife ecologist and one of the study’s authors, adding that ticks can extract most of a calf’s blood in two to three weeks. On average, they found 47,000 ticks on a calf.

“For a calf, that is just death,” he said. “That leads to acute anemia. The analogy is that someone cuts your arm off and you didn’t stop the blood flow. Physiologically, you can’t deal with that. You have to replace that blood.”

Maine’s moose population is thought to have fallen from 76,000 five years ago to between 60,000 and 70,000 today. New Hampshire’s numbers have reduced nearly 50 percent, to about 5,000 animals. Vermont has also seen a decline to about 1,700 animals.

“Normally, you wouldn’t see a major population impact because the population can respond quite easily to one or two years,” Pekins said. “But this is documented in four of five years, and our suspicion is we have had this happen in six and maybe seven of the last 10 years.”

Frigid weather and extended snow cover once kept the ticks in check. But with climate change resulting in winters starting later and less snow in some places, winter ticks have more time to find their hosts.

That may help explain why the deer herd in the northern Maine study area is faring far better, with the winter mortality of calves being much lower than in Maine’s western study area or in New Hampshire: Winters in northern Maine are still quite severe, and that may make life more difficult for the ticks.

Lee Kantar, the Maine Department of Inland Fisheries and Wildlife’s moose biologist and a co-author of the study, said the paper doesn’t include data from Maine’s northern research zone, which along with a western zone will be studied for the next two years.

“I’m trying to strike a balance here between concern for moose in parts of the state and then the idea that in other parts of the state, in northern Maine, the population appears to be quite stable,” Kantar said. “And we don’t know what the next 20 years is going to bring us. We’re trying to do our due diligence in understanding the moderating climate, winter ticks and moose densities. And that’s what Pete and his grad students and that research was all about.”

Earlier this year, the DIFW unveiled its big-game wildlife management plan. For moose, the stated goals were to :

— Maintain a healthy, sustainable moose population while providing hunting and viewing opportunities.

— Continue researching the relationships between moose and parasites, habitat condition, climate and management.

— Ensure public satisfaction with Maine’s moose population and increase the state’s understanding of moose biology, ecology and management.

Moose lack the grooming strategies of elk and deer that help remove the parasites. These winter ticks, compared to deer ticks, also never leave their host and live several feet above the ground, a perfect place to catch a ride.

And the younger calves suffer much more than the adults, since they are smaller and feeding more often — which exposes them to the tick-filled trees and underbrush. That means more ticks on moose calves and a continued threat to the population’s long-term health.

Still, experts expect that moose will survive this parasitic onslaught and that their numbers will reach the point where there just aren’t enough animals to provide a stable food source for the ticks — thus causing the parasite population to fall and the moose numbers to rebound.

But not everyone is willing to let nature take its course. There is a vibrant debate within the conservation and hunting community over the role hunting can play in stabilizing the population. New Hampshire has reduced the number of hunting permits of late in a bid to bolster its moose population.

Vermont, meanwhile, went another route in the early 2000s. Over the years, the state has intentionally reduced the moose population by issuing more hunting permits to improve reforestation and cut down on car collisions with the animals, said Scott Darling, the Vermont Fish & Wildlife Department’s wildlife program manager. The state is glad it did, as winter ticks took a toll on moose, believing that with few moose, there would be fewer host animals for the ticks and thus a healthier population.

“It probably gave our moose somewhat of an advantage. But I will confess it’s not as much of an advantage as we had hoped,” Darling said, adding that the calf mortality rate was still 40 percent in 2017, primarily because of winter ticks and 52 percent this past winter.

BDN Outdoors Editor John Holyoke contributed to this report

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