The winter of 2008-2009 was rough for everyone in the southern Yukon, with record levels of snow and cold temperatures that kept the snow on the ground deeper and longer than usual. But some had it rougher than others.
In March, Yukon Environment employees Shawn Taylor and Lorne LaRocque did a helicopter survey, looking for Dall sheep wintering areas. They searched an area east of Haines Junction, from the Alaska Highway north to the Nordenskiold River near Division Mountain.
The snow was noticeably deeper than usual, says Taylor. In fact, Yukon Environment’s snow-depth records show the snow pack was above normal throughout the southern Yukon, well into March.
“Sheep range that would normally be snow-free in March was still covered in snow as deep as a sheep’s belly.”
The effect on the sheep was soon apparent. In a single day of surveying, Taylor and LaRocque spotted five dead sheep, all killed by predators in deep snow.
“We found three fresh sheep kills within a few hundred metres of each other on one hillside,” Taylor says.
Tracks in the snow told the story of each kill. One adult sheep had been killed and partially eaten by wolves. A dead lamb, still untouched, had been killed by a golden eagle. Another adult sheep had been killed by a lynx just before the helicopter passed over, and the lynx was still on the body.
“On the same hillside we saw a ewe being stalked by a pair of wolves. This ewe was far from escape terrain and in deep snow with little likelihood of escaping,” says Taylor.
Steep rock slopes, bare of snow, are a sheep’s friend, but belly-deep snow on the lower, gentler slopes near the treeline—where the sheep were forced in search of forage—gave all the advantage to the predators.
Later that day, Taylor and LaRocque come upon a kill made by a wolverine.
“We also found another sheep with five golden eagles feeding on it. Tracking showed that it had been killed by eagles.”
In March, golden eagles migrate north from their wintering grounds in the western United States. Sheep are likely a regular part of their diet on the way north, but this year the heavy snow made the sheep easier prey than usual. Taylor and LaRocque spotted eagles near or directly above sheep several times during their flight.
“When eagles were nearby, sheep were often tight against large rock formations, likely to protect themselves against these aerial attackers,” Taylor says.
Sheep weren’t the only animals struggling with the deep snow. In late winter moose surveys in the Haines Junction area, Taylor and LaRocque found that snow depth and crust were affecting where the moose looked for food. Several were spotted along the Dezadeash River, at the valley bottom. That’s a habitat the moose don’t use much in years with less snow and crust.
“It definitely appears to be a lot more important to moose during a winter like this,” says Taylor.
Moose normally avoid snow deeper than about 70 centimetres, but this year cow and calf moose were browsing in areas with a metre of snow.
“Their sheer size and long legs allow moose to browse shrubs above the snowpack, which gives them advantage over species like sheep, caribou and bison that paw through the snowpack to forage at ground level.”
Snow depth can be a serious limitation for some species, including bison, he adds. Yukon bison probably haven’t had to deal with deep snow since their release in the territory in the 1980s, mainly because their range is one of the driest parts of the Yukon. However, this year even the bison were in trouble.
“We heard comments from hunters that reported bison to be thinner than normal, and other reports of bison having a tough time travelling in the deep snow,” Taylor says.
In other places where bison live, snow depth is a factor in what range they use and whether they expand into new areas. One tough winter isn’t likely to have much impact, but a long-term change in temperature and precipitation could affect the range and numbers of Yukon bison too.
By Claire Eamer
One tough winter isn’t a threat to Dall sheep either. Although they spotted an unusual number of predator kills, Taylor and LaRocque also counted 395 sheep and discovered some new wintering areas on their March flight.
And a number of golden eagles, lynx, wolves, and other predators may have begun the breeding season better nourished than usual, thanks to the good hunting.
For more information about the wildlife of the southwest Yukon, go to www.environmentyukon.gov.yk.ca/viewing.
This column is co-ordinated by the Northern Research Institute at Yukon College with financial support from Environment Yukon and Yukon College. The articles are archived at www.taiga.net/yourYukon.