It seems lately that almost every time I turn on the news we have broken another weather record of one kind or another this winter. Long-running weather records are readily accessible; we can look back across decades and compare numbers for snowfall, high temperatures and low temperatures.
We have had cold winters before, to be certain. We have also had snowy winters. This winter, we have had a combination of cold and snow that sets it apart from most in recent memory.
As a biologist, an important part of my job is tracking cold and snow and evaluating its potential impact on deer. We operate the Winter Severity Index (WSI) every year from Dec. 1 through the end of April. Any day that the low temperature gets down to zero or below, we add one point. Any day that the snow depth in the woods is 18 inches or higher, we add one point. On days where we have subzero lows and deep snow, then, we add a total of two points. Winters are considered “mild” when WSI is less than 50, “moderate” if it is between 50 and 80, “moderately severe” if it is between 80 and 100, and “very severe” if WSI exceeds 100. The 30 year average is 67.
When the winter severity index was established in the 1980s, DNR researchers had already been using U.S. Department of Commerce weather data. Using an existing data set gives a biologist the ability to go back in history and do the same calculations to see what WSI levels were in years before we had started our evaluations; these researchers developed data going back to 1960.
This meant we could compare winter severity with other deer population and survival studies performed in Wisconsin. For example, a 10 year study (1965 to 75) showed a strong relationship between winter temperatures, snow depth and deer mortality (deer dying in winter). Researchers learned by counting deer tracks, droppings, and carcasses that approximately 10 percent of the winter deer population died when the WSI was less than 80; up to 15 percent died when the WSI approached 100; and 20 percent or more of the deer population died when the WSI was more 100.
This early WSI gave us information, but it took time to get that data (there was no internet back then) and it was not broken out across the landscape. What eventually resulted was a request to wildlife and forestry staff at 35 locations across the north to take measurements on a daily basis—the low temperature and the snow depth. The information would be sent in at the end of each month, but in years when the severity began to ramp up (like this year) the numbers could be called in weekly. This information plays an important role in our deer population model, the Sex Age Kill (SAK)–and has a direct impact on how many antlerless deer tags we issue for the following fall.
Looking across years of data collected during WSI operation, summer deer observations, and mandatory in-person deer harvest registration, we have learned we can use WSI to predict other outcomes in our deer herd. For example, the numbers of fawns per doe seen have been about 20 percent lower after severe winters than after mild ones. Buck kill is also impacted by winter severity. In seasons after a mild winter, the buck kill has increased about 30 percent. After winters scoring 80 to 100, buck harvest was stable. After very severe winters (100 plus) buck harvest has dropped by as much as 25 percent.
Right now, we are past 80 in our local WSI measurements, and winter shows no sign of relenting. Our deer population was well below our established overwinter goal before deer season. You can expect very few, if any, antlerless deer tags to be issued in this area next fall based on what we are seeing to date.
Jeremy Holtz is a wildlife biologist with the Wisconsin DNR and writes a weekly column in the Star Journal. To contact him, call 715-365-8999.