Ungulate

Recommended method(s): Total counts

As with mountain goats, mountain sheep often occur in fairly discrete blocks of open upland habitat. Inventories are usually total counts with classification from the air (Burles and Hoefs 1984; Elliott 1985; Stelfox 1990; Steventon 1985; Schultze 1992). Ground counts have been used on some accessible seasonal ranges (Hoefs and Bayer 1983). In British Columbia, seasonal ranges of many thinhorn sheep populations are still being learned, such that surveys are combined reconnaissance and population estimation. The ranges and winter concentration areas of most southern bighorn sheep populations are much better defined (Southern Interior Mountains, Central Interior and Southern Interior ecoprovinces).

4.3.1 Survey methods

Running, weaving sheep are difficult to classify, or even count, from the air. The alternative, ground counts, also present problems when sheep are at a distance, and partly obscured within groups and by cover. Aerial surveys are the only practical method of inventorying remote populations. Aircraft must also be used to survey broken terrain and partially forested slopes. Ground counts may be possible during aerial searches, by landing near large herds. Ground counts will provide much better classification and count information for large groups which congregate on open slopes. Total counts are recommended.

4.3.2 Defining search areas and stratification

Sheep cannot tolerate deep snow. They generally begin leaving summer ranges in September, and virtually all are on winter ranges by November. Open, snow-free slopes form the core winter ranges in all areas. In southern bighorn populations (Southern Interior Mountains, Central Interior and Southern Interior ecoprovinces), those areas are most often within the BG, PP, IDF, or MS zones on warm south aspects where snow accumulation is minimal. A few southern herds also use windswept alpine ridges. Snow free alpine ridges in the AT and SWB zones form the core of winter range for most northern sheep (Northern Boreal Mountains ecoprovince).

Reconnaissance surveys are still required for many northern populations whose winter distribution is poorly defined. Low snow sites used by sheep in winter are relatively easy to recognize. Locating rutting areas may be more difficult. Winter ranges have been defined for southern herds (e.g., Stoddart, Columbia Lake, Premier, Bull and Wigwam herds in the East Kootenay). Low strata search areas should be defined around core winter ranges to locate dispersed groups, particularly rams.

4.3.3 Survey timing

Because of their use of open habitats, sheep are visible at most times of the year. However, they are generally best counted and classified during two time periods. Trophy male bighorn sheep are most successfully enumerated during the rut in November (Stelfox 1990) or December (Southern Interior Mountains). Incidental observations suggest that the rut, in the last half of November, also has potential for inventory of Stone's sheep. The preferred timing for population estimates for Stone's and bighorn sheep is late February through March, when animals are concentrated on winter ranges (Elliott 1985; Stelfox 1990; Schultze 1992). The best period will vary annually with weather and snow conditions. In the Thompson-Nicola (Southern Interior), early spring (late March/April) is best for population estimates when sheep concentrate on green-up areas. However, older males may be underestimated at this time.

Dall's sheep, which are not visible against snow, are usually surveyed in spring before lambing (Nichols 1978), or in early summer (Burles and Hoefs 1984). During six counts of Dall sheep in the Tatshenshini River area (Tatshenshini Basin, Coast and Mountains ecoprovince) between June 1988 and June 1990, the highest counts were in September 1988 (241) and June 1990 (266). Comparable numbers on the other flights were as follows: June 1988 - 98; February 1989 - 126; April 1989 - 133; and September 1989 - 144 (Hatler 1990). The low counts of June 1988 and September 1989 were ascribed to adverse climatic conditions.

Surveys for all subspecies are conducted in June for neonatal recruitment, and July for 2 to 3 month recruitment and summer distribution. Rams will be underestimated then due to their solitary habits and use of rough terrain. April surveys, currently used in the Ashnola, may also be used to estimate 12 month lamb recruitment, before sheep disperse from winter ranges.

4.3.4 In-flight survey procedures

Dall's sheep have been regularly surveyed from small fixed-wing aircraft, especially Supercubs (Nichols 1978; Simmons et al. 1984 ). Nichols notes that helicopters provide "more accurate" surveys, but create more disturbance. Many northern sheep ranges (Northern Boreal Mountains) are in more open and gentle terrain than exists in most of British Columbia. We recommend the use of Jet Ranger helicopters for sheep surveys.

Large groups of sheep should be counted, and the number of lambs determined, while circling high enough so that they do not bunch and run. The proportions of harvestable and younger rams should be determined, based on appropriate horn size regulations for the survey area. That requires a closer approach or, preferably, landing the aircraft for observation with a spotting scope.

4.3.5 Classification

Consistent recognition of some classes of sheep varies with the season. Yearlings are readily distinguished in surveys conducted from winter to about June, but may be difficult to recognize thereafter. The most difficult distinction in all seasons is between young (Class I) males and adult females, particularly in large groups. Most who have attempted to make that distinction from the air have expressed uncertainty about the results (Steventon 1985; Schultze 1992). Judging from a collection of Dall sheep undertaken by Simmons et al. (1984), that lack of confidence is justified. Of 112 sheep shot from a helicopter, that were suspected to be rams, but "...were not clearly identifiable as rams", only 11 (9.8%) turned out to be Class I rams.

Adult males are classified as Class I, II, III, or IV on the basis of horn curl (Geist 1971). When viewed from an aircraft, Class I rams, which have not reached 1/2 curl, do not show their horn tips. Class II rams, between 1/2 curl and 3/4 curl, show the horn tip. Class III, between 3/4 and full curl, show an upward curl from above and little weight at 1/2 curl. Class IV rams (full curl) have horns that do not appear to taper from their base to the 1/2 curl point. Assuming a survey in May in a southern Yukon population, those classes generally correspond to the following age groupings: Class II - 3 to 5 years, Class III - 6 to 8 years, Class IV - 9 years and older (Hoefs and Cowan 1979). Age estimates need to be confirmed in each ecoprovince and region.

For bighorn sheep, yearling rams and Class I rams appear distinct in winter (Bill Warkentin, personal observations). Class II to IV rams may be found with other classes in the winter, from about November through April, but most will move to separate ranges for the rest of the year. Yearling and 2-year-old rams (Class I) are usually in nursery bands and, as noted above, are difficult to consistently distinguish from adult females, especially in winter pelage. In summer, the 2-year-old males are usually somewhat larger and have longer horns and larger horn bases than most females. They also show testicles when viewed from the rear, but it is difficult to attain that view from an aircraft. Adults with yearlings (winter) and/or lambs in summer are almost always females. However, most other adults in a nursery group are most honestly assessed simply as "unclassified adults".

4.3.6 Estimating numbers missed

Like goats, sheep will hide from aircraft. Hoefs and Bayer (1983) enhanced counts of Dall's sheep by reference to marked animals, but the details of that were not elaborated. Simmons (1971) marked large numbers of Dall's sheep with a modified crop spray apparatus on a fixed-wing aircraft, but used the marked animals for studies of movements rather than population estimation. The procedure and equipment used by Cichowski et al. (1991) for marking mountain goats should work for sheep as well. Radio-collared bighorn sheep have been available for use in population studies and estimation of sightability, but no details have been reported from that work (P. Davidson, pers. comm.). Dye marking and the two stage sampling procedure, proposed by Stelfox (1990), appear to be the most economical methods available to improve estimates of sightability in sheep populations. Opportunities to use radio-collared animals for sightability research should be addressed.

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Published by the Resources Inventory Committee