Winter Severity Index for deer is average, so far
The Minnesota Department of Natural Resources (DNR) annual Winter Severity Index (WSI) for white-tailed deer in northeastern Minnesota indicates 2007-2008 is average compared with the long-term. Two significant December snowstorms left 16 to 24 i...
The Minnesota Department of Natural Resources (DNR) annual Winter Severity Index (WSI) for white-tailed deer in northeastern Minnesota indicates 2007-2008 is average compared with the long-term. Two significant December snowstorms left 16 to 24 inches of snow across northern St. Louis and northern Lake counties. Temperatures, through the second week in February, have also been about average.
DNR wildlife managers have conducted the WSI for the past 40 years to statistically analyze our winters and the effects on deer and other wildlife. The WSI is measured by combining the number of days below zero with the number of days with 15 inches or more of snow on the ground. These measurements are taken throughout northern Minnesota and compiled at DNR wildlife offices. An average winter in the Tower area would total about 120 WSI points by the end of April.
As of Feb. 12, Tower has 68 WSI points, Isabella 90, Snowbank Lake 75, Brimson 62, and Eveleth 60. Currently, there are 14 to 16 inches of snow across most of the Tower DNR wildlife work area. Five measurements are taken from a mature hardwood stand of birch or aspen and averaged at these five weather stations. There is a band of deeper snow from Isabella up to the Gunflint Trail. Wildlife managers will continue to monitor deer populations and WSI as the winter progresses.
Last year at this time Tower recorded a WSI of 46 with 14 inches of snow on the ground. The total WSI for the winter of 2006 was 88. Historically, the winter of 1995-1996 had the highest WSI ever recorded in the Tower area with 202.
Research has shown that deer have the ability to withstand very cold temperatures. Deep snow is the most critical factor with 15 inches as the threshold. Extended periods of deep snow combined with cold temperatures, especially in mid to late winter, lead to deer mortality. It is estimated that about 10 percent of the deer herd typically dies in an average winter.
Fawns (less than 1 year old) are the first to starve or be killed by wolves or other predators, as they are the most expendable part of the herd. As the WSI increases to a level of 140 or higher, increased adult buck mortality is expected. Adult does are the best prepared to withstand a severe winter in the Northland, one of nature's way of perpetuating the species.
Currently, wildlife managers are seeing deer using good winter "thermal" cover such as conifer swamps and they are mostly restricted to established trails within cover.
The lack of any significant snowfall in January and February has really decreased the severity of this winter. Snow conditions have firmed up and mobility has improved since mid-December. Three out of five weather stations are reporting less than 15 inches of snow.
Field staff have checked the bone marrow of some road-killed and wolf-killed deer. Body fat reserves are good, which indicates that deer have not been overly winter-stressed thus far. If the winter of 2008 continues to be average, deer populations will not be significantly affected. However, the WSI could drastically change with one major snowfall and prolonged cold temperatures. What happens from now through April will determine how deer fare this winter.