Ashland, Wis. — Every three years, the DNR completes an aerial survey to estimate the numbers of beavers in northern Wisconsin. The most recent survey wrapped up Oct. 30, and although the data is still in draft form, according to John Olson, DNR furbearer ecologist, there are an estimated 9,791 beaver colonies in northern Wisconsin, with 4,980 observed in Zone A and 4,811 in Zone B.
“There are no significant changes from 2008 and 2011, with the population at relatively low levels,” Olson said.
Since 1992, the highest number of colonies observed occurred in 1995. That year, colony numbers were nearly double that of the latest findings. In 1995, 17,268 colonies were observed. Of that number, 9,425 were found in Zone A and 7,843 in Zone B.
The lowest number of colonies occurred in 2008, with 8,256 sightings across northern Wisconsin – 5,063 in Zone A and 3,177 in Zone B.
Based on the current low numbers of beaver colonies, DNR wildlife officials are recommending to maintain or slightly increase the population in upcoming years. In order to accomplish this population objective, the DNR will recommend the beaver trapping season be closed March 30 each year instead of April 30, except on priority trout streams, according to Olson. Fish management officials will take the lead on defining priority trout streams with input from local residents and user groups, he said.
Aerial survey by helicopter was perfected by former DNR wildlife specialist Bruce Kohn, now retired, in the early 1990s. Kohn’s survey technique has been used for beaver population estimates ever since. Prior to that time, beaver registration was required, and those numbers were used to develop population estimates.
Problems occurred because harvest was affected by weather, prices, and other factors, so population estimates were not always accurate, Olson said.
The DNR initially compared fixed-wing results with helicopter surveys and found that they were only able to see 40 percent of activity with airplanes compared with 85 percent of beaver activity with helicopters, after confirming with on-ground observation. The reason for the difference? The design of the aircrafts. With a plane, visibility was difficult and they would have to circle once they saw beaver activity. Helicopters offer better visibility and can slow down, hover, lower, or lift to get better views and confirm beaver activity.
During the aerial survey, the observers look for fresh chips that create “doughnuts” around fresh trees, active beaver dams, food caches, lodges, and chew sticks. Altogether the crews surveyed 41 plots in Zone A and 43 plots in Zone B, totaling 462 square miles. Each plot covers 4 to 6 square miles, and they are able to complete anywhere from eight to 11 plots per day. The team tries to start the estimates as soon as the leaves come off the trees. This past year they were able to start Oct. 16.
While in the air, they document visuals on each colony and record all activity. They also try to use the same biologist each year and return to the same plots every three years, looking for trends in the beaver population. After surveying their designated plots, researchers apply the findings to the 19,934 square miles that make up Zone A and Zone B in northern Wisconsin. Then, knowing that they only see 81 to 85 percent of total colonies in the air, compared with actual on-ground observations, they multiply their findings by that number to reach the overall population estimate.
The negative side of aerial surveys is that it is becoming more expensive to contract helicopters. Olson said it cost $11,000 when they started, and $60,000 to complete the survey this past year. Their helicopter pilot, Don Vollen, is also nearing retirement and has recently been inducted into the Wisconsin Aviation Hall of Fame. Bids from other pilots have been even higher in past years. Because of this, the DNR has been testing new techniques by using imagery from a satellite company. Right now, DNR officials are comparing those satellite images to the data found in the test plots during the aerial survey. The hope is that this new idea will save time and money and they’ll be able to apply the information statewide. Currently, there is no population estimate for the southern portion of the state.