DNR: Deer population stressed out by prolonged winter

MADISON (WITI) -- Although winter did not arrive in some areas until well into February, it has persisted longer than normal and the impacts on individual deer are being seen in some areas.

“Hard winters are not something new to Wisconsin’s deer herd,” says Kevin Wallenfang, state big game ecologist with the Department of Natural Resources. “Winter severity is a variable, natural process that can reduce deer numbers in some years. Our herd handles it well in most years, but we are seeing stressed deer in all areas of the state, including direct losses due to starvation in farmland areas where you wouldn’t normally expect it.”

DNR wildlife biologists annually monitor the effects of winter weather on the deer herd using a winter severity index, which uses a combination of cold temperatures and deep snows to gauge winter stress levels and the ultimate survival of deer.

Measurements are recorded annually at 43 stations spread primarily across the northern third of the state as well as several east-central counties, and are recorded from December 1 through April 30.

“Each day that the temperatures fall below zero degrees Fahrenheit and/or the snow depth is more than 18 inches, the conditions are noted for each station.” Wallenfang explained. “For example, a day with 20 inches of snow and a temperature of five-below-zero would receive two points for the day.”

Winter conditions are considered mild if the station accumulates less than 50 points, moderate if between 51 and 80 points, severe if between 81 and100, and very severe if over 100.

At the end of March, two stations were in the severe category (both in Iron County), six were moderate, and the remaining stations were considered mild. However, several stations are likely to move into higher severity classifications after the April data is tallied.

“While it’s a good indicator, the winter severity index doesn’t always tell the full story,” says Wallenfang. “That may be what we are seeing this winter. The readings alone are saying winter in most locations isn’t all that bad, but add in other factors that the index doesn’t acknowledge like hard crust, compacted wet snow, or long-term snow cover and the impacts can be deadly to individual deer even when temperatures are moderate.”

Another factor to be considered is the duration and timing of winter conditions. “The later winter arrives and the longer it hangs on, the greater the impacts can be. Deer have burned through their fat reserves by late winter and are in critical need of food,” Wallenfang said. “Most reports of stressed or dead deer have occurred during the past few weeks.”

DNR staff have been in the field investigating reports of noticeably stressed or dead deer, and monitoring habitat conditions.

In the northeast portion of the state, DNR’s Wildlife Health Team recently investigated an area in Door County with extremely high deer numbers where the natural winter browse has been depleted and dead deer have been reported.

“The situation was very indicative of malnutrition,” according to Jeff Pritzl, DNR Northeast District wildlife supervisor. “All but two of over 20 carcasses were last year’s fawns. It’s symptomatic of a deer population out of balance with its habitat.”

Each of the state’s 136 deer management units has a target over-winter population goal. “As hunters, we all like to see lots of deer, but unfortunately this year in some farmland areas we’re seeing the downside of high deer numbers,” says Pritzl. “We’re not only seeing direct losses of deer, but the damage they are doing to their habitat will take years to recover, and that’s assuming less browsing pressure by fewer deer. It’s very rare that we see deer starvation problems in farmland units, but that’s where we are hearing of a significant number of cases.”

In northern units, local biologists are also seeing stressed deer and have had reports of some starvation losses. In general, deer appear to be faring somewhat better because deer densities in the northern forest tend to be more in line with what the habitat can support, or in many units are still below where they could be. However, going into late April the snow continues to fall in significant amounts and direct losses can be expected until snow melt.

Although this will not be considered one of the more extreme winters since the department began recording winter severity and its impacts on the herd, varying amounts of mortality will occur throughout the state due to starvation and increased predation of weakened animals. In addition, reduced fawn recruitment and survival can be expected as some does will have come through the winter in poor physical conditions and absorb their fawns.

State biologists have already considered winter’s effects on survival and recruitment when developing 2013 harvest quotas which will be released in early May. With more snow in the forecast, they are continuing to evaluate the situation. According to Wallenfang, already conservative quotas may be reduced even further in some areas of the far north.