REGIONAL—Three years into the Department of Natural Resources’ adult moose study, researchers say they are getting a clearer picture of why Minnesota’s moose population appears to be on the decline.

In short, say researchers, health problems, more than wolves, appear to be the biggest factor behind an adult moose mortality rate in the state that seems to be exceeding the norm, at least compared to other parts of North America where moose are found. To date, according to the DNR, fully two-thirds of the moose deaths in their study have been health-related.

But the details of those findings suggest a picture that is more complex and subject to more than one interpretation. It also remains unclear whether the current rates of adult moose mortality are high enough to trigger the sharp decline in moose numbers that the DNR has documented through its annual aerial survey.

Researchers note that the findings are preliminary and that the data they’ve gathered at this point is far from conclusive.

The numbers

Between 2013-2015, DNR researchers collared a total of 173 adult moose for their study. Of those, 47 have since died, translating into an annual mortality rate of 12-19 percent. DNR biologists say the average non-hunting mortality rate for moose in Alaska or Canada is 8-12 percent.

According to examinations of adult moose carcasses, DNR researchers have concluded that wolves killed 16 moose, or just over a third of the animals that have died so far in their study. But they say an underlying illness or disease made adult moose predisposed to wolf attack in 25 percent of those cases.

While the DNR has determined that the remaining moose died of health-related causes, it isn’t clear that those numbers are higher than is found elsewhere. While some researchers have pointed to factors like climate change, which they suspect may be behind an increase in parasites and disease, the DNR’s data is far from definitive on that point. Moose can die from complications related to a wide variety of health issues, as the DNR study has revealed. While parasites, such as brainworm and winter ticks, accounted for 15 (32 percent) of moose deaths in the study so far, it is difficult to know whether that represents an increase over levels seen 10-15 years ago, when moose numbers appeared to be relatively stable in northeastern Minnesota.

Other factors, such as infection, have played a significant role as well, accounting for 21 percent of moose deaths. According to the DNR’s findings, four moose died from infections resulting from wolf attacks, while four others died of infection from other types of trauma. Such data could as easily support a conclusion that trauma—ranging from accidents to wolf attack— is the leading cause of moose deaths, rather than poor health.

Indeed, of the 47 adult moose that have died in the study to date, 25, or 53 percent have died directly or indirectly from some form of trauma, resulting from wolf attacks or other accidental injuries that led to infection.

Poor calf survival may be bigger factor

While the latest research has revealed a range of health problems affecting moose in northeastern Minnesota, it isn’t clear whether the health of the moose population is declining overall. Research team leader Dr. Glenn DelGuidice notes that a lack of data from earlier years makes it difficult to answer that question. But he argues that anecdotal information, such as the well-documented collapse of the moose population in northwestern Minnesota and the ongoing decline in the northeast, seem to point to declining health as a major contributing factor.

Perhaps the larger question is whether a moose population with an annual adult mortality rate of 12-19 percent can sustain itself in the long run. In general, says DelGuidice, it’s a manageable rate of mortality, so long as it can be “finely balanced with annual reproductive success and recruitment.”

In recent years, reproductive success has been limited, at least based on calf-cow ratios from the DNR’s annual aerial survey.

Fifteen years ago, calf-cow ratios of 0.65 calves per cow were the norm. From 2006-2015, however, the survey found an average of 0.33 calves per cow.

DelGuidice advises against relying too heavily on that data, however, since the DNR changed its survey methods in 2005, substituting slower-moving helicopters for the fixed wing aircraft used prior to 2005.

Many in the public have pointed to wolf predation as the primary cause of moose calf mortality, and that’s a claim that’s largely supported by the limited data from the DNR’s short-lived calf study. According to DelGuidice, wolves accounted for 68 percent of calf deaths, while black bears accounted for an additional 16 percent during 2013 and 2014.

But DelGuidice said it isn’t at all clear that predation is the full story behind the apparent decline in moose calf survival.

“We know that the impact of primary mortality forces like predator-specific predation can vary dramatically year to year over the long-term due to numerous factors,” said DelGuidice. “But we also know that the health and survival rates of the adults are of critical importance as well, and most likely, of even greater importance to the long-term trend and persistence of moose in northeastern Minnesota.”

Researchers expect to know more about the moose population when the 2016 aerial survey results are released later this month.