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MOOSE STUDY

Wolves have killed majority of calves in study

Marshall Helmberger
Posted 9/25/13

REGIONAL—Four months into a study on moose calves in northeastern Minnesota, the data is increasingly grim— and wolves appear to be the largest single contributor to a mortality rate that …

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MOOSE STUDY

Wolves have killed majority of calves in study

Posted

REGIONAL—Four months into a study on moose calves in northeastern Minnesota, the data is increasingly grim— and wolves appear to be the largest single contributor to a mortality rate that researchers acknowledge is higher-than-expected.

“There’s no doubt that the wolves are having the biggest effect,” said lead researcher Glenn DelGuidice. “It’s more than we thought.”

The numbers tell the story. Of 34 moose calves that were collared by researchers last May, and not immediately abandoned by their mothers, only ten remained alive as of this week. That’s a 71 percent mortality rate through the first four months, notes DelGuidice. “To be there this early is a surprise.”

Of the 24 calves killed, researchers believe 16 were killed, or likely killed, by wolves. Another four died from bear attacks, all of those while the animals were still very young. A handful of other causes accounted for the four remaining calf deaths. DelGuidice and other researchers knew that wolves play a role in moose calf deaths. “But we didn’t have any idea of the magnitude,” he said.

The most recent calf death was on Sept. 11, when a wolf pack claimed the second of two twins that were born back in May. Wolves had taken the first twin a few weeks earlier, and DelGuidice said it wasn’t a surprise that they came back for the second one.

While the rate of calf deaths has slowed as the young animals have grown, DelGuidice said he anticipates losing more in the months to come. And that’s even before the effects of winter take their toll on the young animals.

While the data from this year point to wolves as a major factor in poor moose reproductive success, DelGuidice warned that results from a single field season can prove misleading. Researchers prefer multi-year studies that can even out some of the normal fluctuations in results that can occur from year-to-year, especially within very small sample sizes, such as the moose calf study. “I hope people reserve their judgments until we have more data,” he said.

The moose calf study is set to continue, at least for one more year, and DelGuidice is hoping the DNR will authorize sufficient funding for a third year. Current plans call for radio-collaring another 50 calves next spring.

Trying to understand abandonment

In addition to tracking calf mortality, DelGuidice said he’s trying to pinpoint possible factors behind the high abandonment rate by mothers of collared calves this spring. At least nine moose mothers abandoned their calves as a result of the stress of capture and DNR wildlife officials are hoping to find a way to reduce that number in subsequent years of the study.

DelGuidice had theorized that younger or very old mothers might be more likely to abandon calves, but so far, he said, no such pattern has emerged from the data. DeGuidice said the capture team will try working from the ground next year, whenever possible, since it’s possible that helicopter noise may have contributed to some of the abandonments. Abandoned moose calves quickly perish, and they provide no usable scientific data.

Winter survey will provide insights

Whether the high mortality of moose calves in the DNR study reflects the experience of the overall moose calf population isn’t clear at this point. This winter’s annual aerial moose survey, however, should give DNR officials a better handle on calf survival across the region. DNR Tower Area Wildlife Manager Tom Rusch said the calf-to-cow ratio is always a closely-watched statistic, since it gives an indication of likely reproductive success.

As recently as 10-15 years ago, the aerial survey commonly found as many as 70-80 calves per 100 cows. In recent years, however, that ratio has fallen dramatically, and the lack of recruitment of young calves into the overall population is likely a major cause of the dramatic decline in Minnesota’s moose numbers. Just 24 calves were spotted per 100 cows in 2011, although that number improved to 34 per 100 during the 2013 survey, conducted in January.

Given the high mortality among the calves in the DNR study so far, the calf-to-cow ratio is already under 30 per 100, and it’s likely to decline further by next winter’s survey. Whether the aerial survey, which looks at a substantially larger sample size, finds similar results remains to be seen.

Wolves versus moose?

While very preliminary, this year’s moose calf study results are likely to reignite a longstanding debate over the proper balance between populations of iconic species like moose, and the equally iconic timber wolf. “It will be interesting,” said DelGuidice. “People have strong feelings for both animals.”

Rusch said he’s heard complaints from some area residents for years that wolves were taking too big a toll on deer and moose populations. But he noted that the DNR can’t make management decisions based on guesswork. “Before we put the collars on, we really didn’t know,” he said. “Now we’re getting the data.”

But like DelGuidice, Rusch said the results so far aren’t conclusive. “It’s just one year,” he said. “And results can vary a lot from one year to the next.”