Rapidly rising hunter success has started taking its toll on Idaho's famous trophy-bull elk herd, and state Fish and Game officials are beginning to look at possible ways of increasing the herd's chances of survival during the fall season.

"The elk hasn't changed, but hunter numbers and technology have," said department biologist Ruth Gale.Mild winters have helped the elk herd grow. But the number of hunters has been skyrocketing as well, and with easy access to most of the elks' favorite haunts, the percentage of bulls being bagged has soared.

Nowhere is it more apparent than in in the Big Bend area of Island Park, where a three-year radio tracking study produced dramatic results.

"We sent 16 bulls into the open hunting areas . . . during the general season and none of them came back alive," Gale said. "In all the Sand Creek units, 33 radio-collared bulls went in and only six came out."

Such success by hunters threatens to eliminate all older bulls in many of these areas, a trend that would hurt the quality of a hunt already plagued by crowding. What has allowed the big trophy bulls to survive has been selected sanctuaries where the lack of roads or hunting closures have kept hunters away.

These retreats include the Centennial Mountains, Yellowstone National Park and Harriman State Park. In the Centennials, four of nine radio-collared bulls survived three hunting seasons, Gale said.

"We can produce high numbers of elk with restrictive hunting seasons," said Ted Chu, game manager for the region. "We do have concerns about the rate of harvest of yearling bulls from the Sand Creek elk herd."

The department's surveys have shown a trend of overharvest of yearling bulls, particularly in the Big Bend area. Sixty-five percent of the elk shot there over the past three seasons have been yearlings.

The yearling kill is significant because elimination of older bulls could lower the reproductive success of the entire population. Cows might be impregnated later, resulting in younger calves less prepared to handle the winter.

The percentage is also important because the deparment's management plans call for action to restrict the hunt once the harvest averages over 60 percent during a three-year period.

Many of the other hunting units are also hanging dangerously close to that 60 percent level.

To preserve the general hunt, Fish and Game officials are working with the U.S. Forest Service to study methods of increasing the odds of survival for older bulls.