Deer mortality will increase daily until spring green-up”

Tom Rusch

REGIONAL—The area’s whitetail deer population is feeling the effects of months of deep snow cover, which has substantially increased the winter severity index (WSI) across much of northern St. Louis and Lake counties. That’s according to Tower DNR Area Wildlife Manager Tom Rusch, who noted that the conditions could cause substantial deer mortality this winter, despite the mild temperatures the region has experienced.
DNR wildlife managers use the WSI as a means of assessing winter mortality of whitetail deer. The index includes a point for every 24-hour period with a below-zero temperature reading and a point for every day with 15 inches or more of snow on the ground.
“Snow depth continues to be the main driver for winter severity this winter,” said Rusch. “Early snowstorms in late November and early December pushed our area over the 15-inch threshold on Dec. 9 in Tower.” That totals 87 days as of Wednesday with snow depths of greater than 15 inches. “That’s a severe winter,” noted Rusch. “Deer mortality will increase daily until spring green-up.”
The extended deep snow had pushed the WSI in much of the Tower and Ely area to over 130 as of this week, with potentially weeks of deep snow and below-zero weather remaining. Depending on weather in March and April, that could push the WSI readings to well in excess of 150. The WSI reaches about 115 in an average winter in the Tower area. Whenever the WSI rises above 100, it increases the likelihood of starvation or malnutrition in whitetails. That can result in higher-than-average mortality from predators and can significantly reduce fawn production the following spring.
Despite above-freezing conditions in late February and early March, which started to compress the snowpack, conditions remain difficult, noted Rusch. “Current snow depths are three to five inches lower than last month but still deep for white-tailed deer,” noted Rusch. “Each day a deer has to endure trudging through chest deep snow is a significant drain on its physiological condition.”
As of Feb. 28, the DNR was reporting snow depths ranging from 19 inches in Ely and Orr to 35 inches in Isabella. Other reported snow depths included Babbitt (28”), Tower (26”) inches, Greaney (24”), Snowbank Lake (22”), Cook (20”), and Kabetogama (20”).
The good news for deer is that the deep snow has not been combined with cold temperatures this year. As of this past week, the area had only experienced 48 nights with below zero temperature readings, well below a typical winter in the region. While recent warm temperatures have helped to reduce snow levels somewhat, which is good news for deer, the temperatures also allowed for the development of a crust, that could further exacerbate the stress on whitetails. “This is a real game changer and tips the balance in favor of predators,” said Rusch. “Wolves, bobcats and coyotes can now stay on top while deer break through the crusted layer. With an increase in sun angle, snow conditions now change day-to-day but deep snow favors the predators.”