Are we exacerbating moose declines in severe winters?

Alaska’s big game animals, including moose, caribou and Dall sheep, are well adapted to survive severe winters, having evolved over thousands of years in the far north, where deep snow and low temperatures occur during much of the year. Moose have large, blocky bodies and thick hair to conserve body heat, and long, thin legs to wade through deep snow. These help them survive harsh conditions, but there are limits that, when exceeded, result in starvation. During the worst winters, widespread mortality from starvation triggers major moose population declines. If severe winters occur often, they can outweigh habitat improvement, restricted hunting and predator control as primary factors regulating moose numbers.

Early in my career as a moose biologist, I learned that a person on snowshoes in snow deeper than four feet could walk down an exhausted moose after a short pursuit. In deep snow, moose struggle and leave a trench as their belly drags through it. The energetic cost of moving through deep snow is many times that of normal movement, and deep snow covers plants that moose eat, preventing them from gaining adequate energy and resulting in starvation.

In Southcentral and Interior Alaska, biologists have many decades of information on big game die-offs caused by deep snow. In some cases, severe winters precipitated population crashes that were accelerated by other factors including hunting and predation. For example, the Nelchina caribou herd north of Glennallen declined from about 80,000 to 8,000 after a series of severe winters in the 1960s and early 1970s. The Tanana Flats moose population south of Fairbanks declined from about 23,000 to 2,800 during the same period, with record setting deep snow in the winter of 1970-1971. It took many years of restricted hunting for these populations to recover.

In more recent times, during winter 1989-1990, an estimated 4,400 moose starved to death in the Matanuska-Susitna valleys. In addition, 1,200 moose were killed by motor vehicles and 749 were killed by trains as moose exhausted by wading through deep snow refused to leave plowed roads and train tracks. I have vivid memories of driving between Willow and Talkeetna that spring and seeing many carcasses of starved moose within sight of the road. But most striking was the widespread damage to trees and shrubs produced by desperate moose stripping bark containing few nutrients. Few trees escaped.

What conditions prevail during severe winters, and how often do they cause moose die-offs? Severe winters feature deep snow that comes as early as October and lasts as late as early May. Anchorage’s record total snowfall occurred in winter 2011-2012, when 134.5 inches (11.2 feet) fell, in contrast to the long-term average of 74 inches (6.2 feet). Snow that persists and is deeper than five feet on the level during severe winters puts moose in jeopardy. Weather records indicate that deep snow winters occur fairly often, on average 1-3 times per decade, and tend to occur even during periods like those of recent years when generally mild conditions prevailed. Winter 2019-2020 is looking like it will be severe in several areas of the state, including the Nelchina Basin (Game Management Unit 13), one of our most important moose hunting areas.

Wildlife managers in state and federal agencies can’t control winter weather, but they can act to reduce the chances of moose population crashes when severe winters occur. However, in recent years, state managers at times acted to worsen the effects of severe winters by keeping moose numbers high. Alaska’s intensive management law allowed the Board of Game to adopt a policy of managing for high moose populations, often as high as the habitat could support during mild winters. As a result, during severe winters, many moose occupied marginal habitat and starved, and competition for food in good habitat caused some moose to die that otherwise would have survived.

In the Nelchina Basin during the past 50 years, there have been persistent cycles of moose population increases during normal winters followed by die-offs during severe winters. Rather than trying to keep moose numbers at moderate levels and thereby avoid high losses caused by deep snow, managers approved wolf and bear reduction programs to build moose numbers back following declines. As a result, predator control became perpetual, applied to increase moose numbers after declines, and continuing after moose were again abundant, thus setting the stage for future declines.

If this winter continues to be severe in areas of Southcentral and Interior Alaska, expect to see starving moose, increased motor-vehicle collisions with moose and reduced hunting opportunities as moose numbers decline. Biologists have the necessary knowledge to avoid such problems. Managers just need to apply it.

Vic Van Ballenberghe lives in Anchorage. He is a moose and wolf biologist and a former Game Board member.

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