Local wildlife officials released a mid-season Winter Severity Index (WSI) update late last week and the verdict thus far is pretty obvious: There’s a lot of snow out there.
How the northern Minnesota deer herd will be impacted by that will have a lot do with how the rest of the season goes.
The WSI is the tool the Minnesota Department of Natural Resources uses to analyze and compare winter’s impact on white-tailed deer in northern forest habitat from year to year.
They’ve been the same system for more than 50 years.
The WSI index accumulates one point for each day the temperature is below zero degrees Fahrenheit and one point for each day there is more than 15-inches of snow on the ground.
The measurements are taken in mature aspen stands from Nov. 1 to May 30.
An average score for a Tower-Ely area winter is about 115 using data from the last 50 winters.
As of Jan. 31 the winter of 2019-20 has been tough on the northern St Louis and Lake County deer herds, said Tower Area DNR Wildlife Manager Tom Rusch.
In the Tower Deer Permit Area (DPA 176), WSI was at 78 (54 snow points and 24 temperature points). In the Eveleth area (DPA 178), WSI was at 58 (34 snow, 24 temp). And in the Isabella area (DPA 131), WSI was at 80 (64 snow, 16 temp).
As a comparison, at this time last year the WSI index ranged from 41 to 60 in the nine local deer permit areas across northern Minnesota. The winter before that number was between 27 and 52 at this time.
“Snow depth has been the main driver for winter severity this winter,” Rusch said, adding that early snowstorms in late November and early December pushed parts of Lake County over the 15-inch threshold on Thanksgiving weekend. Tower reached that number on Dec. 9 and Eveleth hit it on Dec. 29.
“Current snow depths are historically deep for the first half of winter. Each day a deer has to endure trudging through chest deep snow is a significant drain on its physiological condition,” Rusch said. “Deer are at the northern extent of their range in northeast Minnesota. Snow depth is the biggest limiting factor. Moose are much better adapted to surviving in winters with 24 inches or more of snow.”
Rusch added that very few deer over-winter in the BWCA due to a combination of snow depth, starvation and wolf predation and that some parts of Cook County currently have more than 40 inches of snow.
“If deep snow persists into April like the long winters of 2013-14 and 1995-96, WSI totals could total 150-175,” he added.
According to the Tower area DNR wildlife office, as of Jan. 30, snow depths were greatest on the Iron Range, in northern St. Louis County and in Lake County along the higher elevations of the North Shore.
Snow depths around the region ranged from 20-inches in the Cotton area to about 24-inches in the Eveleth and Cook areas. Orr was sitting at 22-inches while Duluth and Hibbing had 26-inches.
Officials measured 27-inches in Tower, 28 in Babbitt and Greaney, 37 in Isabella and 42 in Finland.
If there is a silver lining to all of this, it is that temperatures have been fairly mild by northern Minnesota standards.
A continuation of that trend would be helpful to the herd.
“With some luck, we will evade the normal, extended stretch of sub-zero temperatures that further drain the fat levels of wintering whitetails which help maintain body temperatures,” Rusch said.
Long, cold winters with deep snow are to be expected in this neck of the woods but when they start early (like this year) and go longer than normal – say into late April and early May – the deer herd starts to feel the impact.
They are only equipped to withstand so much punishment from mother nature.
According to Rusch, malnutrition and starvation are a direct result of when the WSI exceeds 125 and increases incrementally with the WSI index.
It can also lead to higher than normal wolf mortality.
“Deep snow and poor physical condition make deer vulnerable to wolf predation. Fawns are smallest and are most vulnerable. Adult bucks, run down by the fall rut, are also more vulnerable,” Rusch said. “Adult does fair the best but still suffer higher mortality as winter progresses into March and April if snow does not recede.”
He added that fawn production will decline significantly because does in a compromised physical condition give birth to fawns in poor condition that then have high mortality rates.
The end result of a severe winter, Rusch added, is deer populations will decline and hunter success will decrease.