Insights Into Deer Mortality Figure To Improve Hunting

Hunters and predators may take a bite out of the state’s deer population, but Idaho Fish and Game Department studies show the herds rise and fall with summer rains and winter snows.

The agency is in the midst of a long-range study of mule deer in the Owyhee Mountains, Boise River drainage and north of Mountain Home.

And what it finds could smooth out the roller coaster Idaho’s deer population seems to ride so hunters have more choices each fall.

Armed with better, more accurate information about the herds, game managers should be in a position to tailor seasons so sportsmen can get the maximum benefit.

The Fish and Game Commission already held a round of public hearings this fall that drew hundreds of sportsmen worried that too many large bucks were being shot each year. The commission will set the direction for future deer hunting next spring.

For the most part, though, it is the ability of young fawns to find enough browse in the summer and survive drifting snow in the winter that dictates sportsmen’s luck.

“The major cause of mortality has been effects of the winter on fawns,” said Jim Unsworth, wildlife research biologist in Nampa.

Even under normal conditions, Unsworth said, a 50 percent fawn mortality rate would not be uncommon. And in 1992-1993, a hard winter following a summer of drought resulted in the loss of up to 80 percent of the fawns and nearly 30 percent of the older bucks.

But, he added, “if you think about it, 90 percent of does get pregnant and most have twins. We would have deer 10 feet deep across the earth if we didn’t have such a high mortality rate on fawns.”

The heavy toll three years ago, especially in the Snake River Plain, alarmed hunters who feared the “muleys” had vanished. Fish and Game had to impose emergency hunting closures.

This year was the first in several that more deer are being seen, Unsworth said, and “hunters have been pretty happy this fall.”

Does have the best chance of surviving - about 90 percent got through that hard 1992-1993 winter. They expend less energy than fawns that are growing and eat more than bucks to build up protective fat for the winter. Predators, hunters and accidents are much greater threats to does than the elements.

“Bucks compete for does and does compete for food,” Unsworth said. “If we have the wrong type of winter, bucks don’t have much time to catch up from a fat standpoint.”

Biologists have put electronic collars on deer in the three study areas to keep track of them and are using a computer model to refine population estimates from aerial survey statistics.

While radio telemetry has been used on deer for a decade, using the computer model is a new wrinkle in Idaho, following similar work with elk.

The experts also are looking at ways to glean more information about the Idaho deer herd, which could total 180,000 head from hunter check stations during the fall.

With more up-to-date figures, the status of trophy bucks, for example, would be better documented so that hunts could be opened for them if they rebound well during the year.

Better information on the life of deer gives Fish and Game the insight needed to decide whether too few fawns survived the winter and hunting should be curtailed, or that the herd is getting too large and needs to be reduced.

Fortunately, Unsworth said, the state’s deer population is healthy, so there is time to carefully assess long-term trends.