15.3 Trends in Dall's sheep in mountain ecosystems

Last Updated: 
June 9, 2015

Dall's sheep

This indicator tracks population and condition indices of a large herbivore in NWT's mountain ecosystems: the Dall's sheep.

Population indices include population densities and the number of lambs per 100 ewes. The condition index is the annual average horn growth in rams. These indicators may help monitor fluctuations in wildlife disease and parasite loads, and changes in carrying capacity due to climate changes in NWT mountain ecosystems.

Dall’s sheep demographics are measured using aerial and ground surveys performed at irregular intervals1. Additional data on demographics are obtained from annual outfitter reports and hunter questionnaires2. Horn growth rates are obtained from data collected with the help of hunters in both NWT and Yukon3. The interpretation of this indicator is enhanced by the local knowledge of hunters and users of NWT’s mountain ecosystems.

NWT Focus

Dall's sheep are found in Alaska, the Yukon, the western NWT and extreme north-western British Columbia. They are a hunted species. Sheep share NWT’s mountains with other large herbivores, including mountain caribou, moose and in some areas, mountain goat.

Current view: status and trend

Demographics

Northern Richardson Mountains

The northernmost population of Dall’s Sheep in the NWT are in the Richardson Mountains, where numbers have declined from nearly 1600 in 1997 to 500 in 20144.

Mackenzie Mountains

The number of Dall’s sheep in the Mackenzie Mountains is estimated at 15,000-26,000. Sheep have not been systematically surveyed in most of their range in the NWT, so trends in density are unknown. Densities are relatively low5 compared to Yukon, and vary between 20 and 50 adults per 100 km2.

Sheep density per 100 square kilometres

The number of lambs per 100 ewes has changed little since 1995. Source: Density (ENR); Ratio of lamb/100 ewes from N.C. Larter and D. Allaire, hunter's observations; Condition horn index from D. Hicks, pers. comm. Detailed Information available.

Condition

Horns in male Dall's sheep grow each year. The growth rate differs with age and with the general condition of the animal. Looking at the difference between actual horn growth and average horn growth expected at each age gives insight into the general condition of rams in an area. Their condition or health is a result of a combination of food availability, weather and previous year's condition. The mean deviation in horn growth in rams in the Mackenzie Mountains shows a cycle of good and bad years. Low growth occurred in 1990-1991 (probably also earlier), and again in 1997-2000 (Hicks, pers. comm.). High growth occurred in 1993-1996.

Muscleworm (Paralephostrongylus odocoilei) and lungworm (Protostrongylus stilesi), parasites known to infect mule deer and sheep, have been found in populations of Dall’s sheep in the Mackenzie Mountains (2001). These parasites can severely affect the lungs and muscles of infected sheep and reduce their survival. The life-cycle of the parasites involve another species: the parasite larvae are shed in the feces of the sheep, invade a terrestrial snail where they develop over more than one year into other larvae stages, and infect other sheep when they feed on vegetation that carries the infested snails6. The length of the growing season affects the availability of these parasites to sheep. Transmission of the parasites occurs in the fall, and a warming climate may increase the time period in which the larvae in the host snails are available for infection. It can also increase the number of larvae in infected snails6. A changing climate may result in more parasites in populations already infected and in expansion into populations not yet affected7,8.

Looking forward

Northern herbivores are well adapted to harsh climate and to large changes in their environment. Large fluctuations in climate and quality of foraging habitat in NWT’s mountain ecosystems are not surprising. Dall’s sheep populations in the NWT have adapted to these fluctuations. The incidence of muscular and respiratory pathology due to parasites and diseases in Dall’s sheep is being tracked using samples collected by hunters and sent for analysis by ENR.

Looking around

Similar fluctuations in deviation of horn growth from the mean have been noted in Yukon. Over 31 years (1969-1999), Yukon data showed a 10-year cycle in horn growth3. Possible causes for these annual differences in ram condition include regional fluctuations in climate, such as precipitation and temperature cycles, which in turn may influence abundance or timing of availability of food for sheep.

Interestingly, fluctuations in numbers of Dall’s sheep lambs have been correlated to the snowshoe hare cycles in Sheep Maintain, Yukon8. Low lamb numbers occur about 1-2 years after the crash in hare numbers every 9-10 years. This may occur because generalist predators, such as golden eagles that prey on both hares and sheep lambs, may heavily hunt lambs when hares are less abundant (the "Alternative prey" hypothesis)8. It is not known if such a phenomenon occurs in the NWT.

Find out more

  • To find more on Dall’s sheep, visit the ENR website.

Other focal points

Technical Notes

  • Estimated of number of lambs per 100 ewes are obtained from hunter questionnaires provided from each of the eight licensed outfitters for big game harvested in the Mackenzie Mountains each hunting season.
  • Estimated volumes for annual horn growth increments are based on measurements of annual horn segment lengths and their annual base circumferences.

 

Found an error or have a question? Contact the team at NWTSOER@gov.nt.ca.


References:

Ref. 1. Larter N.C., and D.G. Allaire. 2005. Sheep surveys of the Liard Range, Nahanni Range and Ram Plateau in the Mackenzie Mountains, August 2003 , GWNT, ENR.

Ref. 2. Larter N.C., and D.G. Allaire. 2014. Mackenzie Mountain Non-resident and Non-resident Alien Hunter Harvest Summary 2013.

Ref. 3. Hik D.S., and J. Carey. 2000. Cohort variation in horn growth of Dall sheep rams in the southwest Yukon, 1969-1999 .

Ref. 4. Davison. T. 2014. Northern Richardson Mountains Dall’s Sheep Survey June 16 to 23, 2014, Field Summary. ENR -GNWT.

Ref. 5. Weir J.N., S.E. Morrison, D.S. Hik. 2008. Linking foraging behavior to population density: An assessment of GMM models for Dall sheep. Ecological Modelling 211:396-402.

Ref. 6. Jenkins E.J., A.M. Veitch, S.J. Kutz, E.P. Hoberg, and L. Polley. 2006. Climate change and the epidemiology of protostrongylid nematodes in northern ecosystems: Parelaphostrongylus odocoilei and Protostrongylus stilesi in Dall's sheep (Ovis d. dalii). Parasitology 132:387-401.

Ref. 7. Kutz S. et al. 2009. The Arctic as a model for anticipating, preventing, and mitigating climate change impacts on host.parasite interactions Veterinary Parasitology 163:217-228.

Ref. 8. Wilmshurst J.F., R. Greer, J.D. Henry. 2006. Correlated cycles of snowshoe hares and Dall's sheep lambs. Canadian Journal of Zoology 84:736.