Minnesota's moose population has dropped by half in the past five years, and officials say the iconic animal could be gone from the state in 20 years.

"It's clear that moose are continuing to go down, down, down,'' said Department of Natural Resources researcher Mark Lenarz. "I'm very pessimistic.''

An aerial moose survey this winter showed a population of 4,230, down 14 percent from last year. The population was 8,840 as recently as 2006.

No smoking gun has been found to explain the decline despite years of research, including the radio-collaring of 150 moose. "I don't think it's a single thing,'' Lenarz said. "The majority of mortality appears to be related to disease and parasites.''

Despite suggestions from the public that wolves -- or hunters -- are to blame, Lenarz said his research shows those factors definitely aren't driving the downward trend. "When we started the project in 2002, our first mortality was a wolf kill, and we assumed we'd see that on 90 percent of our mortalities,'' he said. "But we didn't.'' Of the 119 collared moose that died, just 11 were confirmed to be from wolf predation.

"We found most carcasses intact; they hadn't been touched by wolves,'' he said.

But what about moose calves, which haven't been radio-collared? "No one disagrees wolves are killing calves,'' Lenarz said. "That's exacerbating the problem, but it's not the whole problem.''

The cow-calf ratio is considerably lower today than it was in the mid-1990s, though it increased this year to 36 calves per 100 cows. But even if those numbers were boosted back to 1990s levels, it wouldn't stem the population slide, Lenarz said. The bottom line: Based on his research, even if there were no wolves preying on calves, the moose population would continue to fall.

"The key problem is adult mortality,'' Lenarz said.

Hunters also aren't to blame, he said, and closing the hunting season won't reverse the downward spiral. Hunters killed 53 moose last fall. Still, the DNR announced on Thursday that it will decide in coming weeks whether to hold a 2012 moose season.

"If we stopped hunting tomorrow, it wouldn't stop the decline in the population,'' Lenarz said. "We're harvesting only bulls.'' If so many bulls were killed so that cows weren't getting bred, that could affect the population. "But we're certainly not seeing that,'' he said.

As with deer, one male moose can and often does breed several females.

"As long as all the cows are being bred, the harvest of excess bulls won't affect the rate of increase or decrease in the population,'' Lenarz said.

Last fall, the DNR cut the number of moose-hunting permits from 213 in 2010 to 105. The state's moose-management plan has triggers for closing the hunting season. One is if the bull-to-cow ratio drops below 0.67 bulls per cow for three consecutive years. While that ratio fell to 0.64 bulls per cow in 2011, it went up this year to 1.08.

Meanwhile, Lenarz said he believes the warmer temperatures that northeastern Minnesota has experienced in recent decades also may be a factor.

"I still believe there is a link between climate change and the mortality we're seeing,'' he said. Even slightly warmer temperatures, he said, may be making moose more vulnerable to disease and parasites.

"It's not that moose are dying because of heat stress on a hot day; it's likely a cumulative process. Perhaps their immune system is compromised.''

Research has shown that cattle, which, like moose, are ruminants, will stop eating and produce less milk when exposed to hot temperatures. "If they stop eating, they stop growing and they stop putting on fat reserves,'' Lenarz said. The immune system could be weakened.

"If you have an impaired immune system, you're more vulnerable to disease and parasites,'' Lenarz said. "If it's happening in cattle, it seems logical it's happening to moose as well.''

That moose at similar latitudes in New England and North Dakota seem to be fairing better than Minnesota's moose could be due to regional differences, said Lou Cornicelli, DNR wildlife research manager. "There could be some other variables there that makes them less susceptible to temperatures,'' he said.

Doug Smith • dsmith@startribune.com