Counting Big Game

By Jon Rachael, Idaho Department of Fish and Game Wildlife biologists from the Idaho Department of Fish and Game will soon take to the skies to assess the size and health of big game herds in a number of areas across southern Idaho. We try to estimate deer and elk numbers in most zones about every three to four years. Using a helicopter allows us to assess population status over a wide area in a relatively short time with little impact on the animals. Beginning in mid-December and running through the end of the month, biologists will focus on assessing buck-doe ratios and annual deer fawn production in units throughout the southwest region. After completing these deer herd composition counts in January, a helicopter survey is scheduled to estimate the size and composition of the elk population of Game Management Unit 39, better known to the hunting public as the Boise River Zone. So how does Fish and Game go about estimating the number of elk in an area? Glad you asked. Estimating the number of elk, or deer, out there is one of our most important and time consuming tasks. Before we even attempt to estimate elk numbers in a unit, we have to do a fair amount of background work. In the Boise River Zone case, the first step is to divide the entire unit into smaller sampling areas, or subunits. This zone encompasses about 3,000 square miles. To make it manageable, we've divided the zone into more than 300 subunits of approximately six to 12 square miles each. Subunit boundaries are ridgelines, streams and roads - things that can be identified easily from the air. Then, based on our previous counts and the knowledge of past elk distribution, we assign each subunit to one of three categories based on the density of elk we expect to find there during winter months. We don't include the highest elevation portions of the zone where the snow is too deep for elk to winter or heavily populated areas around Boise where we wouldn't expect to see elk. To get a good population estimate for the entire zone, we need to survey and count the elk in about 80 subunits. This target number allows us to survey all subunits classified as having a high or medium density of elk, as well as a fair number of subunits in the low density category. After the planning is done and the subunits for survey have been selected, two biologists and a pilot get in the helicopter and begin flying. The helicopter starts at the lowest elevation and begins flying along the boundary of the subunit maintaining a constant altitude, about 200 feet above the ground. When all of the terrain has been surveyed at that elevation, the helicopter makes another contour back across the subunit 300 to 500 feet higher up the hill, while still maintaining the 200-foot altitude above the ground. When an elk or group of elk is spotted, we circle and count the total number of elk, and determine the number of calves, cows, spikes, raghorn bulls and adult bulls. The activity of the group when they were first spotted is recorded along with the amount of vegetative cover and snow cover. This process continues until all of the selected subunits have been surveyed. It takes about 80 flight-hours to complete the elk survey in this zone. Back at the office, this data is crunched through our "elk sightability model." This mathematical computer model will make a correction for animals that were probably missed during the survey. For instance, single elk or small groups of elk bedded in thick cover are less likely to be seen during the survey than larger groups of moving elk in an area with little cover, especially if the ground is covered with snow. The sightability model also expands the estimate to include the number of elk that could have been expected to be in the subunits that weren't sampled based on the number counted in the lower density units that were surveyed. The ultimate outcome is an estimate of the number of elk in the zone. This population estimate, along with the ratio of bulls, cows and calves in the population, is then compared to the population objectives for the zone. This information, together with trends in hunter harvest in the zone, forms the basis for recommendations for hunting seasons in the coming years. For more information about big game winter counts, contact the Southwest Region Fish and Game office in Nampa at 208-465-8465 or the McCall office at 208-634-8137. Jon Rachael is the regional wildlife manager in the Southwest Region.