Living with Wildlife in Anchorage:
A Cooperative Planning Effort
Appendix B - Methods Used to Estimate Numbers of Wildlife in the Anchorage Area

Black bears and brown bears. Accurate and reliable estimates of bear populations are difficult and costly to obtain (Miller et al. 1997). Bears are typically wide-ranging, low-density species that are difficult to observe directly in most areas. An accurate technique developed in Alaska uses a standard capture-mark-resight technique. A search area is selected containing representative proportions of different habitats used by bears throughout a year. Bears are captured by darting them from helicopters and fitted with radio collars. A year or more later an aerial survey determines the number and identity of radio-marked bears present in the search area in early summer. At the same time, an independent visual search using fixed-wing aircraft (usually Piper Supercub PA-18) determines the number of marked bears among the total number of bears observed in the search area. The flight pattern is designed to maximize the likelihood of seeing bears — usually flight patterns are large circles in forested and tall shrub habitats, straight lines in open tundra or low shrub habitats, and along elevation contours in steep terrain or narrow drainages. The entire search area is usually searched on a single day to minimize the possibility that unmarked bears would be counted more than once. These flights are replicated on other days.

This technique has not been used in Anchorage due to the difficulty in sighting bears in the heavily forested terrain found in most of the Anchorage lowlands, and the expense (Miller et al. 1997). However, the technique has been used for black bears in the middle portion of the Susitna River drainage and on the Kenai Peninsula, and for brown bears in several locations in southcentral Alaska (Miller et al. 1997). The estimate of brown and black bear numbers in the Anchorage area is based on a subjective extrapolation from density estimates in similar habitats in southcentral Alaska (Miller 1993, Miller et al. 1997). Because the Anchorage estimates are based on extrapolations of short-term studies in other areas, it is impossible to determine annual population fluctuations, except in a subjective sense based partly on public calls about nuisance bears and other bear sightings.

Moose. Moose populations are estimated using a census technique developed in Alaska (Gasaway et al. 1986), accompanied by trend counts. In the Anchorage area, only Fort Richardson (including the upper Ship Creek drainage) and Elmendorf Air Force Base are censused. Using a modified Gasaway technique, the two military reservations were divided into 14 survey areas using natural terrain features. As soon as possible after the ground is covered with fresh snow, these survey areas are flown by pilot and observer teams using fixed-wing aircraft (usually Piper Supercub PA-18). The flight pattern is designed to maximize the likelihood of seeing moose — usually flight patterns are straight lines in forested habitats and along elevation contours in steep terrain or narrow drainages. All moose seen are circled to identify sex and antler size and search for other moose, especially calves. Moose are differentiated by adult/calf, bull/cow, and small/medium/large bulls based on body size and antler presence and size. Each survey area is searched on a single day to minimize the possibility that moose would be counted more than once. Immediately after a survey area is censused, a small, predetermined portion of the area is resurveyed much more intensively by flying tight, overlapping circles with the goal of seeing every moose. This allows a statistical estimate of the percentage of moose missed in each of the 14 survey areas, which is used to calculate moose population size and confidence limits. Usually both military reservations can be censused in two to three days.

Trend counts are conducted in predetermined drainages. Survey areas are selected each year based on funding level and management interest. Areas with a higher density of moose and more hunting pressure have the highest priority. In Anchorage these survey areas include the drainages of Peters Creek, Knik/Hunter Creek, upper Campbell Creek/Anchorage Hillside, and the Twentymile/Portage/Placer rivers. Other areas are surveyed as time and money allow. In Anchorage these survey areas include Eagle River, Bird Creek, Glacier Creek, and Kincaid Park. Trend counts use the same methods as the Gasaway technique; however, small areas are not resurveyed to determine a sightability correction factor. Instead, the average sightability correction factor for the Fort Richardson/Elmendorf census is used to calculate an estimate for all of the trend count survey areas.

An estimate for the entire Anchorage area is calculated by totaling estimates from the Fort Richardson/Elmendorf census, all trend counts, and subjective extrapolations from survey areas not counted (based on comparing population trends in other survey areas with the most recent counts in unsurveyed areas). It is possible to determine trends in annual fluctuations in the Anchorage moose population. The surveys cost approximately $5,000 each for flight time.

Dall sheep and mountain goats. Dall sheep and mountain goat populations are estimated by aerial counts. Dall sheep are relatively easy to see because they are white against the neutral or dark background of alpine slopes (Nichols 1970) and experienced observers can count over 90% of adults and nearly 90% of lambs (Lawson and Johnson 1982). Goats are more scattered than sheep and tend to inhabit more broken terrain. They also spend the warmer midday on snowfields or in shrub habitat and tend to hide from planes by flattening against cliff faces or under overhangs. Therefore, they are more difficult to see than sheep.

Dall sheep surveys are flown every summer in the Anchorage area, if the weather permits. After most of the snow has melted in the Chugach Mountains (late June–early August), a survey is flown by a pilot and observer team using a Piper Supercub PA-18. The flight pattern follows elevation contours above treeline. All sheep are circled to accurately count individuals in groups (especially lambs among groups of ewes) and identify horn length. Sheep are classified into adult rams (categories include ½ to ¾-curl horns, ¾ to full-curl, and full-curl or greater), “ewe-like” sheep (includes all ewes and yearling rams and some 2-year-old rams with less than ½-curl horns), and lambs. The survey takes about three days and costs about $3,000 for flight time. Because almost all the sheep are presumably seen, the total count usually serves as the population estimate.

Mountain goats are counted annually during sheep surveys. However, most of the goat population inhabits Lake George, Twentymile River and Glacier Creek drainages and these are not included in sheep surveys (because they have little or no sheep habitat). A mountain goat survey is flown in these drainages every two to four years to monitor population trends. This survey is flown in August because goats tend to be found at higher elevations than sheep, where the snowpack lasts longer. The survey is also flown late in the evening when goats tend to be more active and visible. A pilot and observer in a Piper Supercub PA-18 follow elevation contours above treeline. Goats are classified into adults and kids. The survey takes about two days and costs less than $2,000. The total count in recent surveys has been 500-600; however, a higher population estimate is obtained by adding a correction factor of 25-50% to account for missed goats and unsurveyed drainages. Population trends can be determined for both sheep and goats.

Wolves and wolverines. Wolves and wolverines can be counted from the air during winter using a method developed in Alaska (Becker 1991, Becker et al. 1998). Wolves and wolverines are not trapped or hunted in Chugach State Park or the Anchorage Bowl; therefore, monitoring population levels is not a high priority. One aerial survey using this technique was conducted in the Anchorage area in 1995 (Sinnott 1996). The survey area included all potential wolf and wolverine habitat in the Municipality.

The survey was conducted by two teams of a pilot and observer using a Piper Supercub PA-18. Potential wolf and wolverine habitat was partitioned into 3x3-mile square sample units. Sample units were grouped into strata depending on the presumed likelihood (high or low) of observing a fresh wolf or wolverine trail after a snowfall. Stratified random sampling selected a greater proportion of units with high likelihood than low. The aerial census was conducted on 23-25 February, beginning about 24 hours after a snowfall. Most of the sample units were censused in the first two days. When fresh tracks were found in a sample unit they were backtracked to the point where they were no longer considered fresh, and then followed forward to the animal(s). By using stratified random sampling and noting the number of animal groups, the number in each group, and all the sample units that the fresh tracks intersected, this method allows an accurate population estimate with confidence intervals. The survey cost approximately $4,000 for flight time. Since 1995, the wolf population estimate has been adjusted slightly based on trapper sealing records, trapper reports, and other incidental observations.

Beavers. An aerial survey was conducted in the Anchorage area by a pilot and observer team using a Piper Supercub PA-18 in October 1995 (Sinnott 1997). The survey attempted to locate all beaver colonies in the Anchorage Bowl and on Fort Richardson and Elmendorf Air Force Base. Streams, ponds, and lakes were searched for dams, food caches, lodges, and freshly cut trees. Lower Ship Creek (below Post Road) and lower Campbell Creek (below Campbell Airstrip) and lakes and ponds in west Anchorage were searched on foot in late October and early November because the low-level, looping survey technique conflicted with air safety near the major airports. Beaver colonies were counted if dams and lodges included fresh material and fresh cuttings were observed, and an average of 5 beavers were assumed to live in each colony. The aerial survey cost approximately $700. Since 1995 several other colonies have been found in the Anchorage Bowl.

Feral rabbits. No one has attempted to count feral rabbits in the Anchorage Bowl. The population estimate is based on observations of one to several dozen rabbits at numerous sites on the Anchorage Hillside — but also at the Clitheroe Center in west Anchorage and several sites in midtown-and homeowner complaints to the Department of Fish and Game.

Bald eagles. Eagle nests are monitored annually. Active nests are usually reported to the Department of Fish and Game by Anchorage residents. The rough population estimate includes two adults for each active nest plus eaglets and older juveniles.

Mallards, pigeons, and ravens. Every winter, usually in late December, the Anchorage Audubon Society attempts to count as many birds as possible in a day and within a 7.5-mile radius of downtown Anchorage and Eagle River. These “Christmas bird counts” have been conducted by volunteers for several decades and are reported in American Birds magazine and on the Internet (http://birdsource.cornell.edu/cbc). Although many birds are presumably not counted, mallards, pigeons, and ravens are large and relatively easy birds to see in urban areas in winter. The population estimates for these species assume that half to one fourth of the birds are counted. Population trends can be determined from these counts.

Literature Cited

  • Becker, E. F. 1991. A terrestrial furbearer estimator based on probability sampling. Journal of Wildlife Management 55:730-737.
  • ------, M. A. Spindler, and T. O. Osborne. 1998. A population estimator based on network sampling of tracks in the snow. Journal of Wildlife Management 62:968-977.
  • Gasaway, W. C., S. D. Dubois, D. J. Reed, and S. J. Harbo. 1986. Estimating moose population parameters from aerial surveys. Biological Papers Univ. Alaska No. 22. 108 pp.
  • Lawson, B., and R. Johnson. 1982. Mountain sheep, Ovis canadensis and O. dalli. Pages 1036-1055 in J. A. Chapman and G. A. Feldhamer. Wild mammals of North America: biology, management, and economics. The Johns Hopkins Univ. Press, Baltimore.
  • Miller, S. D. 1993. Statewide brown bear population estimates. Memorandum dated June 9. Div. Wildlife Conservation, Alaska Department of Fish and Game, Anchorage. 4 pp.
  • ------, G. C. White, R. A. Sellers, H. V. Reynolds, J. W. Schoen, K. Titus, V. G. Barnes, Jr., R. B. Smith, R. R. Nelson, W. B. Ballard, and C. C. Schwartz. 1997. Brown and black bear density estimation in Alaska using radiotelemetry and replicated mark-resight techniques. Wildlife Monographs 133:1-55.
  • Sinnott, R. 1996. Wolf and wolverine surveys on Fort Richardson and surrounding areas: final report. Unpubl. report. Div. Wildlife Conservation, Alaska Department of Fish and Game, Anchorage. 8 pp.
  • ------. 1997. Beaver surveys on Fort Richardson and surrounding areas: final report. Unpubl. report. Div. Wildlife Conservation, Alaska Department of Fish and Game, Anchorage. 6 pp.
  • Timmermann, H. R. 1993. Use of aerial surveys for estimating and monitoring moose populations – a review. Alces 29:35-46.