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DULUTH. Minn. - Wolves, parasites, infections and other health problems continue to be the largest killers of moose in northeastern Minnesota, according to the most recent data from the state Department of Natural Resources.

DNR researchers have compiled an updated assessment of data from a four-year-old study of GPS-collared moose in northeastern Minnesota.

The collars, when they work properly, allow researchers to find dead moose within hours so an accurate cause of death can be made in most cases.

Among 57 dead moose where the cause of death was confirmed, wolves killed nearly one-third. But eight of those 18 moose eaten by wolves, 40 percent, were sick before wolves attacked.

"They were health-compromised in a way that probably made them more likely to be caught by a wolf," said Dawn Plattner, moose field biologist for the Minnesota DNR. "But we still count the ultimate cause of death as wolves."

Parasites were close behind, killing 30 percent of the moose, including a brainworm that's fatal to moose and passed from whitetail deer. Other parasites include liver flukes and winter ticks.

Another 21 percent of the confirmed deaths were caused by bacterial infections, often infected wounds or injuries.

Another 11 percent died from undetermined health factors. Two moose died from accidents (one fell through the ice, one was hit by a car) while one was shot by a tribal hunter and another died while giving birth.

An underlying cause of death in several categories - before the final factors actually killed the moose - was poor physical condition, often caused by malnutrition, officials said. But it's often not possible to determine what shape moose were in before they became diseased or killed by wolves.

The DNR study is the largest effort so far to find out why the state's last remaining moose herd across St. Louis, Lake and Cook counties has declined by more than half over the past decade.

Of 173 moose that were captured and fitted with GPS-transmitting collars from 2013 to 2015, here's what happened to them:

• 28 moose are still alive with collars that are working.

• 53 are believed to be alive but their collars have stopped working.

• 23 are presumed to be still alive but their collars fell off and their status is unknown.

• 12 died immediately after being collared so were not part of the mortality study.

• 57 died with working collars and are the basis for the mortality study data - the moose where cause of death is known.

Plattner said researchers have been disappointed by not only the number of collars that have fallen off, but by the many that have stopped transmitting. Some have stopped and then started again, allowing researchers to recover them. Backup VHF radio transmitters also have failed in some cases.

"We continue to fly periodically (to locate the) 53 missing moose, but it appears their VHF batteries have also failed because we can't hear them during our flights," said Michelle Carstensen, wildlife health program supervisor for the DNR, in an e-mail Friday.

Researchers in coming months will trigger the remaining 28 collars to fall off while they are still transmitting so the collars can be found and the data recovered. When the collars sit idle for more than six hours they are supposed to send out a signal to researchers.

But more than one-third of the original collars have either stopped working or fallen off prematurely.

"It's frustrating. It's disappointing. But it's still a developing technology. Everyone who uses collars like this has issues. There's a lot that can go wrong," Plattner said.

The final 28 operating collars should be jettisoned by year's end, she noted, ending what has been an unusually controversial wildlife health study. The DNR received extensive criticism from some animal rights activists for the capture-related deaths that occurred early on in the study, so much so that Gov. Mark Dayton ordered the agency to stop all moose collaring efforts in April 2015, an order that is still in effect.

A population estimate based on aerial surveys in January showed about 3,710 moose remaining in Minnesota, about the same as the estimated 4,020 in 2016 but down 58 percent from the 8,840 moose estimated in 2006.

While the big, annual population declines of a few years ago seem to have slowed, there's no sign the population can rebuild itself to traditional levels, DNR scientists said.

Scientists for the past decade have been trying to find out why moose declined so rapidly in northeastern Minnesota. That decline has followed the complete collapse of the state's northwestern moose herd 15 years ago. They have focused on several key factors, including the effect of warmer temperatures on moose nutrition and moose parasites. Moose don't eat on warm days, while more days without snow on the ground can spur an increase in some parasites. Warmer winters with less snow also favor more deer, which spread the brainworm that is fatal to moose.

In another update on moose research results, released Monday, June 26, Glenn DelGiudice, the DNR's moose project team leader, said moose are feeling the effects of poor nutrition when they get into the winter months, making it a major factor in the population decline in northeastern Minnesota since 2006.

Meanwhile, only one in three moose calves born in spring 2016 made it through to the January aerial survey, he said. Most are being killed in their first few weeks by wolves and bear.

That poor calf survival will make it "very difficult" for the state moose population to begin any rebound, DelGiudice said.

Other scientists are looking at moose habitat, especially what food is available, noting moose numbers have actually gone up significantly in a few areas where big forest fires occurred in recent years. The highest moose numbers in the Boundary Waters Canoe Area Wilderness now are found where the Pagami Creek, Ham Lake and Cavity Lake forest fires occurred in the past 11 years. Those fires regenerated the forest, and that new growth makes for perfect moose food. That has spurred efforts to cut and burn more large tracts of forest on purpose to create better moose habitat.