Smith: You know last winter was mild. The DNR's winter severity index confirms it.

The Wisconsin winter of 2023-24 will be remembered for its lack of snow and ice.

Snowmobilers largely kept their machines in storage. Organizers of the 2024 American Birkebeiner nordic ski race were forced to hold the event on a 10-kilometer loop over mostly machine-made snow. The ice went out at a record early date on the Wolf River.

Recently-released data from the Department of Natural Resources add further confirmation to last winter's historical nature.

The agency's winter severity index found the winter of 2023-24 wasn't severe at all. In fact, all 72 counties were rated as "mild" and the average WSI in northern Wisconsin was a record-low value of 10.

"(Last winter) was pretty remarkable," said Dan Storm, DNR research ecologist who compiles the annual index. "The values we recorded, especially in the north, were just unheard of."

The DNR uses snow and temperature readings from dozens of sites to form the winter severity index, or WSI.

The gauge was established primarily to help assess the impact of winter conditions on Wisconsin's white-tailed deer herd, especially in the northern part of the state. Deer elsewhere in Wisconsin are rarely affected by winter.

Each site accumulates one point each day the temperature is 0 degrees Fahrenheit or colder and one point each day the snow is 18 inches or deeper. The data are recorded Dec. 1 through April 30.

At the end of the season, if the points total less than 50, it's considered a mild winter; 50 to 79, moderate; 80 to 99, severe; more than 100, very severe.

The DNR recently updated the index for the five months of data collection last winter.

The WSI for northern Wisconsin averaged 10, lowest on record in the 64-year data set.

The previous low was 14 in the winter of 1986-87.

In recent years, 2022-23 was a moderate 69 and 2021-22 was 55.

The winter of 2013-14 was the most severe on record with an average WSI of 143 at northern sites.

However it's important to note conditions vary even within a county. For example even though the winter of 2022-23 was rated as moderate across northern Wisconsin some areas experienced above average losses of deer to starvation and predation.

March and April are especially critical to survival of deer. If severe conditions persist into spring, the animals, with resources depleted over the preceding months, have greater difficulty recovering and die-offs mount. If in a weakened state, deer are also more susceptible to predators.

But last winter severe conditions never developed.

Not only was last winter's average WSI lowest on record, but the uniformity of values across the state was unprecedented, Storm said.

For example the highest values seen in 2023-24 were 12 in the northern counties of Oneida and Vilas but nearly matched by values of 11 in the southwestern counties of Richland and Vernon.

And the WSI values in Richland and Vernon were actually higher than the northern counties of Barron, Bayfield, Burnett, Florence, Lincoln, Price and Rusk.

The main points, however, are these: No spot in the state had severe conditions for wildlife last winter. And when it comes to deer in Wisconsin, winter 2023-24 was as favorable for survival as ever recorded in the state.

The deer herd was helped this spring, too, with an early "green up," or appearance of new growth valuable as food for deer and other wildlife.

Storm said the DNR is looking at tweaking its assessment of winter impacts on deer, including less focus on temperature and more on snow depth and length of winter conditions.

Sawyer County musky study planned

In recognition of declines in muskellunge populations in many Wisconsin waters, the DNR is planning a study of interactions between muskies and northern pike in the Spider Lake chain in Sawyer County.

The chain historically had a naturally reproducing musky population and did not have northern pike. However in 2002 northerns were detected in the waters, according to DNR surveys. The DNR plans to implant tracking devices in muskies and northerns and study interactions between the species "over time and space," said DNR fisheries director Justine Hasz.

The work will be supported in part by donations. On Wednesday the DNR accepted a $100,000 donation from the Duke and Lisa Uihlein Family Foundation to help fund the study. Hasz said additional funds to assist the work, including a $50,000 donation, will be announced in the near future.

This article originally appeared on Milwaukee Journal Sentinel: Smith: You know last winter was mild. The DNR's winter severity index confirms it.