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Deep snow, subzero nights working against northern deer again

Winter severity index numbers are creeping up across northern Minnesota.

deer in deep snow
The winter severity index is creeping up across parts of northern Minnesota, with deer in some areas struggling through more than 20 inches of snow on the ground. The next few weeks will be critical for both deer survival and determining fawn productivity in spring.
Steve Kuchera / 2013 file / Duluth News Tribune
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DULUTH — A pile of snow on the ground and a string of cold temperatures have combined to push northern Minnesota’s winter severity index up, meaning the region’s white-trailed deer are likely to feel the impacts of yet another harsh winter.

So far, the index numbers appear mostly headed into the moderate range. But with another six to eight weeks of snow cover possible, those numbers could go well into the severe range, especially in the snowiest locations of the Northland.

The Minnesota Department of Natural Resources uses a formula that awards one index point for every day with more than 15 inches of snow on the ground and another point for every day the temperature falls below zero. Under a new index measuring system put in place in 2020, the DNR considers index readings of 50 and below as a mild winter, with readings from 51 to 119 considered moderate and any 120 or above considered severe.

This winter has seen widely different snowfall totals across the region, with some areas already over 100 on the index scale and many areas still below 50.

wsi_dpa_map.png
The winter severity index measures how many days Minnesota deer must put up with both deep snow and bitterly cold temperatures. In some areas the winter, especially deep snow, is starting to impact deer.
Contributed / Minnesota DNR

“As of now, I’d say it was setting up as a moderate winter. But it’s already teetering on going into that severe range in some areas," said Penny Backman, acting DNR wildlife manager in the Tower area. “We aren’t seeing any deer mortality yet, either for radio-collared deer or from deer observations. … But they are probably starting to have a hard time of it right now.”

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That’s what Minnesota Conservation Officer Troy Fondie reports from the Orr area, too.

“Deer appear to be struggling as we approach close to two feet of snow on the ground, and a lot of winter left," Fondie wrote in his report this week. “Wolf activity is plentiful.”

Biologists say the deep snow is more of a deer killer than cold, with deer unable to move very far to find better food and less able to escape the jaws of wolves. The worst winters for deer are those with deep snow on the ground well into March and April, when a whitetail’s fat reserves are already used up.

“So much depends on what happens from now on, for the rest of winter. It could end just like that, and (winter mortality) wouldn’t be a problem. Or, it could drag on," Backman noted.

Parts of the Arrowhead, away from the south-facing North Shore hillside, already are approaching a severe winter, especially parts of Cook County. Some parts of north-central Minnesota also have 20 or more inches of snow on the ground.

The DNR estimates that about 10% of deer in the north perish in a normal winter, with that number going up significantly in severe winters. Not only do some deer die outright from malnutrition, but does are less likely to have twin fawns in spring, or any fawns at all, if they are stressed from winter. That makes it hard for northern deer populations to rebound, especially with several severe winters in recent years.

Across much of Northeastern Minnesota, especially north of Duluth, the winters of 2018-19 and 2019-20 were harsh for deer, while the winter of 2020-21 was more mild. For example, Tower-area deer permit area 176 saw an index of 176 in 2019-20, compared to the long-term average of 115. As a result, deer perished, few fawns were born and antlerless deer permits were curtailed for hunters. But the same area saw an index of less than 50 last winter, leading to fewer deer dying and much better fawn production this year.

The impacts of winter appear to be less where deer have access to agricultural crop leftovers to help boost nutrition and where snow melts off unshaded fields faster than it does in the woods.

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The good news is that deer populations are highly resilient due to their high reproductive potential. Annual pregnancy rates average 90% for yearlings and are near 100% for does from 2.5-15 years old, with more than half of those does having twins. If given three to four normal or mild winters in a row, deer have the reproductive capacity to rebound quite rapidly. That's what happened after severe winters in the mid-1990s dramatically reduced deer numbers. By the early 2000s, after a string of mild winters, the region had its highest-ever deer populations.

For more information on winter's impact on deer survival, go to dnr.state.mn.us/mammals/deer/management/wsi.html .

John Myers reports on the outdoors, natural resources and the environment for the Duluth News Tribune. You can reach him at jmyers@duluthnews.com.
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