Minnesota's wolf population likely has fallen in the past five years, state wildlife officials said Tuesday.

Opponents of hunting and trapping the once-endangered species pounced on the news, vowing to increase pressure to cancel this fall's hunting and trapping season, which would be the second regulated hunt in the state's history.

But officials with the state Department of Natural Resources vowed that the hunt, scheduled to start in November, would go on, although the number of wolves that can be killed likely will be lower than the 413 killed this past fall and winter.

Officials declined to discuss specifics of the hunting and trapping season, which will be announced within a month.

DNR wildlife researchers emphasized the population "remains strong." The likely decline, they said, is probably the result of a combination of factors, including a 25 percent decrease in the wolves' primary food, whitetail deer, in the wolf range.

Additional factors include the killing from the hunting and trapping season and the mild winter of 2011-12; the predators do better in harsh winters, when deer are more vulnerable.

The DNR concluded the average wolf pack size has fallen -- from 4.9 animals to 4.3 -- and the average territory of each pack has grown, while the overall range of the wolf in the state has increased only slightly.

After a winter-long survey, DNR researchers estimate there are 2,211 wolves in Minnesota, down from 2,921 in the last survey in 2008.

The debate over wolf hunting and trapping might focus on those figures, but scientists cautioned against such exact numbers. The DNR's estimate is based on sampling.

In fact, from a statistical standpoint, it can't be concluded that the population has fallen at all, according to the DNR's lead author of the estimate and a wolf researcher unaffiliated with Tuesday's announcement.

The two estimates carry margins of error that mean they actually overlap.

"There's a reasonable chance there was no difference in the population," said David Mech, a Minnesota-based senior scientist for the U.S. Geological Survey and one of the world's foremost authorities on wolves.

DNR research biologist John Erb acknowledged, "That's true: One can conclude that there's no statistical difference."

But, Erb said, a combination of indicators led him and his colleagues to conclude the wolf population has fallen, although he said there was no way to quantify the decline with scientific confidence.

"There's always subjectivity involved, and we just felt that when you look at all the pieces -- bigger territory per herd, smaller pack size, the reductions in deer herd, the harvest -- and the decrease in wolf depredation actions, we think the population has declined," Erb said.

This year, federal and state officials have authorized the killing of fewer than 20 wolves to protect livestock; last year at this time, the number was closer to 130.

For wolf-hunting opponents, Tuesday's announcement was the latest call to action.

"We consider this a really significant decline, and we feel the Department of Natural Resources has no choice but to close the season," said Howard Goldman, Minnesota director for the Humane Society of the United States.

The Humane Society has sued the U.S. Interior Department over its 2011 removal of Great Lakes wolves from the federal endangered species list, which allowed Minnesota's hunt. Goldman said he believes the DNR's decision to continue the hunt in the face of a declining population will strengthen his group's case.

Maureen Hackett, founder of Howling for Wolves, which opposes the hunt, said she thinks Tuesday's news will mobilize the public.

"I want to appeal to the better nature of our governor and our (DNR) commissioner to say, 'Let's stop this,' " she said.

Mech, who has studied wolves in northern Minnesota since the 1960s, said that even if the actual number of wolves declined by a quarter in five years, that wouldn't necessarily be troubling.

"Wolf populations vary. They're just all over the map," said Mech, citing his own research showing that populations varied by more than 100 percent in one 13-year stretch. "That's what wolf populations do."
