Winter Severity on deer

[Lee Borgersen]

 

                 
    Winter Severity Index (WSI) and deer management


  ..........
Winter 2013-2014 has turned out to be a doozie for a considerable portion of the state. As of this writing, nearly all of northeastern Minnesota has been classified as having experienced "severe" winter conditions for white-tailed deer, with the winter severity index (WSI) having reached 180 by April 2, 2014. Some areas have exceeded the WSI level recorded during the tough winter of 1995-1996.

The WSI is a general measure of winter conditions based on the premise that prolonged cold temperatures and deep snow can reduce overwinter survival for deer. In Minnesota, the WSI is calculated by accumulating a point for each day with an ambient temperature less than or equal to 0 degrees Fahrenheit and an additional point for each day with a snow depth greater or equal to 15 inches.

What does this mean for deer and our deer management? Long-term DNR research has demonstrated the relationship between WSI and deer survival. In our research, increasing WSI, and specifically snow depth, had a significant decreasing effect on deer survival. In adult females, the average winter mortality over a 15 year period was less than 10%, but it ranged from 2-30% depending on winter severity. It is important to note that despite similar winter conditions in 1995-96 and 1996-97, the second severe winter had much lower over winter mortality (29% versus 9% observed in the study). This was likely related to lower competition for food, initial mortality of the more vulnerable deer, and the strong condition of non-lactating does entering the second winter. This means that the same WSI value in different years may have a different impact on the population. As with many ecological relationships, it's complicated.

That said, those winters had a significant impact on the population. However, through fairly restrictive harvest and a few moderate winters (as well as the high reproductive potential deer have), the population and resulting harvest rebounded in just a few years. In 2014, we anticipate taking a similar, conservative harvest approach in areas hardest hit by this winter. DNR staff will be reviewing population and harvest trends and making season management decisions over the next couple of months. Information on our plans for season management will be available this summer.

[deadeye]

I remember well the winter of 1996.  When the snow finally subsided, I counted 10 -12 winter killed deer on my property.  I believe that winter was harder on the deer because of a very late snow melt.  I remember Easter Sunday (April 7th) and not being able to walk my land due to deep crusted snow.  I did a drive to the area and actually saw deer running on top of the crusted snow until they hit a swamp and broke in.  I hope I'm right in predicting less winter deer kill this year than 1996 due to looser less crusted snow all winter.  This is despite an equal or higher WSI.  I'm talking about central MN not the norther part.