More comprehensive survey yields smaller estimate or original population

MADISON — Preliminary analysis of new aerial surveys of the number of white-tailed deer in the area where a fatal brain disorder was discovered in Wisconsin last year indicates that landowners and hunters made significant progress in reducing deer populations in the area.

Department of Natural Resources wildlife officials, who were able to conduct a more comprehensive aerial survey following late winter snowstorms, say that based on the new estimate they believe hunters removed about 40 percent of the herd in the Eradication Zone during the fall and winter hunts.

“We’re still analyzing the data, but we estimate the deer population in the chronic wasting disease Eradication Zone was actually between 16,400 and 17,900 animals in the fall of 2002.”

Based on those numbers, biologists say that the more than 7,200 deer hunters shot during the fall and winter hunts would be about 40 percent of the herd in the Eradication Zone.

Officials say the total deer removed from the 411-square-mile Eradication Zone since the disease was discovered is 9,287, which includes nearly 2,000 deer shot during spring surveillance hunts and special summer landowner hunts.

“Looking ahead to this fall, after factoring in expected fawn births this spring, we expect the population in the zone to be down about 25 percent from what it was last fall.”

Sharply reducing deer populations in the infected area is a major element in the state’s attempts to eradicate the disease, and the updated population estimates for the Eradication Zone provides new insights on the progress being made, says DNR wildlife biologist Bill Vander Zouwen.

Taking into account both the hunt and other types of deer mortality, biologists estimate the current population at between 8,700 and 10,200. Fawns born this spring are expected to bring the population to between 12,100 and 14,300 deer this fall.

Following some late February and early March snowfalls, biologists conducted an aerial survey and counted deer in 100 one-square-mile sample blocks within the eradication zone. The sample blocks were randomly selected to maximize the probability that they represented the entire zone.

Biologists had originally estimated the pre-hunt deer population in the eradication zone to be between 25,000 to 30,000. That estimate was based on a much more limited flight conducted in March 2002 that surveyed 63 square miles centered on the original three deer that tested positive for CWD.

“While the original count yielded accurate numbers for that small area, it didn’t represent average deer densities on the larger eradication zone,” says Tom Hauge, director of the DNR Bureau of Wildlife Management. “Thanks to a couple of late-winter snowfalls, we were able to conduct a much more thorough aerial survey of the deer population in the zone than we were able to for the initial population estimate.”

Wildlife researchers emphasize that well-designed surveys are the key to producing good population estimates. The March 2003 helicopter survey used a random sample design based on three levels of habitat quality designated as low, medium and high with all sample blocks in a class having an equal chance of being counted. Even with this design, the resulting population estimate is affected by a variety of factors.

Aerial observation of deer is affected by many factors, including moving deer that are difficult to count, varying amounts of vegetative cover that hide deer and the amount of snow that contrasts animals against the landscape. These factors produce a level of statistical variance that is reflected in the population estimate range.

Vander Zouwen emphasized that “it will likely take several more years to have the kind of disease control effect we’re hoping for.”

The herd reduction totals in the eradication zone are:

Spring surveillance shooting (Feb 1, 2002 – April 7, 2002), total = 423 deer Summer hunts (one week each in June, July, August and September) total = 1,515 deer Fall hunting seasons, total = 6,683 deer Winter hunting (Feb 1, 2003 – March 31, 2003) = 666. TOTAL, Feb 28, 2002 to March 19 = 9,287 deer