Dall's Sheep in Alaska's Arctic Parks May Be at an All-Time Low

Dall's sheep (Ovis dalli) in Noatak National Preserve and Gates of the Arctic National Park and Preserve may be at their lowest numbers since the first park-wide surveys in the early 1980s. The two parks were estimated to hold about 12,000 sheep, about 9,800 of them adults, in 2010-2011. Severe winter weather in 2013 and 2014, among other factors, then reduced populations by 50 to 80 percent in some areas. Kumi Rattenbury and Joshua Schmidt of the National Park Service describe what the surveys show.

A bad year for lambs

Lamb numbers were very low in 2013 in several mountain ranges in Alaska and Canada, after a colder-than-normal winter and record-cold temperatures in May, when lambs are born. Whether fewer lambs were born or they did not survive the cold spring, very few were seen in July, when most surveys are made. Sheep throughout Alaska were affected, but the recent decline appeared more pronounced in the north. In Noatak and Gates of the Arctic it affected all age classes, and lamb recruitment was low from 2012 to 2015.

How the sheep are counted

Biologists with the parks and the Arctic and Central Alaska Inventory and Monitoring Networks run aerial distance-sampling surveys in Noatak and Gates of the Arctic, and also in Denali, Lake Clark and Wrangell-St. Elias national parks and preserves. Pilot and observer teams fly transect lines that follow the mountain contours and record sheep groups on the uphill side of the aircraft. Bayesian statistical models then estimate total abundance, the sex and age makeup of the population, and survival rates of lambs, ewes and rams.

Dall's sheep survey areas in Noatak National Preserve and Gates of the Arctic National Park and Preserve, 2011-2016 (figure from the Park Service's article).

What the surveys found

The ratio of lambs to ewes was higher in 2015 than in 2013 and 2014, but the 2016 results show that those lambs did not add much to the Itkillik and Anaktuvuk populations. Recovery in Gates of the Arctic and Noatak will be slowed by that multi-year lag in lamb recruitment.

Dall's sheep survey results for the western Baird Mountains in Noatak National Preserve, 1986-2016: minimum counts from 1988 to 2009, distance sampling in 2011, 2014, 2015 and 2016, and partial surveys in 1986, 1987 and 2005-2007 (chart from the Park Service's article).

Earlier crashes, hunting limits and what comes next

Sheep populations fluctuate naturally. Dall's sheep are adapted to alpine habitat, relatively non-migratory and sensitive to environmental change, and many factors can affect them: weather events, long-term environmental change, nutrition, predation, hunting, development, parasites and disease. Large-scale declines have been linked mainly to severe winter weather such as deep snow, extreme cold and icing, which may reduce access to forage and increase vulnerability to predation. Climate change could matter through more variable winter weather or shifts in the timing or makeup of vegetation, especially in Noatak, where the small Baird and De Long Mountains populations sit at the northwestern edge of the species' range and are fairly isolated from one another and from the larger Gates of the Arctic population.

This is not the first crash. In 1991 adult sheep in the western Baird Mountains fell by 50% after two high-snowfall winters; state and federal hunts were closed across Noatak for several years and then reopened under limited quotas. The population returned to its pre-1991 levels by 2009 after milder winters but has declined since. Noatak has been closed to sheep hunting since 2014. In the south of Gates of the Arctic, federal hunting rules have become more restrictive, and park staff encourage communities to curb the harvest of ewes while numbers are low. Dall's sheep are an important subsistence species for local residents, particularly when caribou are scarce, and they are highly valued by sport hunters and wildlife enthusiasts.

The parks plan to continue annual surveys in both areas, to track the effects of the decline and to inform harvest management and discussions with local residents and other agencies about sheep conservation.

Sources

• Kumi Rattenbury and Joshua Schmidt, National Park Service, Declining Sheep Populations in Alaska's Arctic Parks, Alaska Park Science, Volume 16; rewritten in hubnx's own words.