March 23, 2023 at 11:45 a.m.
Late winter snowfall could affect deer herd numbers
Late winter snowfall could affect deer herd numbers
In the northern parts of the state, this metric is one of the most important factors in helping decision-makers understand whether deer population numbers are likely to trend up, down or stay the same from year to year.
Deer spend the spring, summer and fall building up fat reserves. Those reserves allow them to "coast" through winter, needing less food than they would otherwise. In the early winter, while fat reserves are being used, the deer are still in decent shape, according to DNR wildlife biologist Curt Rollman. But the longer winter lasts, and the more severe the winter, the quicker those fat reserves become depleted. The severity of the winter is calculated using the Winter Severity Index.
The Winter Severity Index is calculated using temperature and the amount of snow on the ground. One point is given for every day temperatures are below zero and one point is given for every day there are 18 inches of snow or more on the ground. It is easy to see, then, why late winter storms, such as those rolling through the Northwoods recently, can increase that index rather quickly.
Points are accumulated from Dec. 1 through April 30. A score of less than 49 is considered a mild year. A score of 50-79 is a moderate year. Winters with a severity index of 80-99 are considered severe years, and those with accumulating 100 points or greater are said to be very severe.
The Winter Severity Index from December 2022 to January 2023 for Vilas County was 27.
For Oneida County, the number was 17. In Iron County, the index was already at 40. By February, all of those numbers had risen, as to be expected.
From December 2023 to February 2023, Vilas County's Winter Severity Index stood at 44. Iron County's had risen to 59, and even Oneida County's index had risen to 28. By the end of February, then, Iron County had already been placed in the "moderate" category for winter severity. While Vilas and Oneida were still in the mild category, continued late winter snow storms could change things substantially by spring.
"This winter was kind to deer up until this point," Rollman said. "What will make a difference will be March and April and that's when we start to see the mortality if we continue to have those 18-inch snow days."
Looking back, 2013-14 had the highest Winter Severity Index in recent memory, scoring well over 140 points on the scale.
As hunters and others know, this had a definite impact on the deer herd in the Northwoods. Even with three years in the "mild" category, many say it took years for the deer herd to rebound and in some places deer numbers have yet to fully rebound.
While deer in better habitats tend to be less effected by extreme winter weather, habitat conditions vary, even within one county. This could mean much more detrimental effects on certain parts of the deer herd when winters become severe.
Rollman said deer at the highest risk for mortality are the fawns going into their first winter.
While severe winters can also cause mortality in mature deer, it is the first year fawns that are especially susceptible to the rigors of winter.
He expected the Winter Severity Index to gain at least one point per day through the end of the month. He said he would not have a final figure, however, until the numbers were delivered to him at the end of the month.
For now, though, he expected the deer herd to fair well through this winter. Should the area receive any more late season storms or unseasonable cold weather, however, that could change the outlook.
More information on the Winter Severity Index and how it is calculated can be found on the DNR website dnr.wi.gov by searching keywords, "deer metrics."
Beckie Gaskill may be reached via email at [email protected].
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