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Biologists suspect low lamb survival for Big Sky bighorn sheep

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Bighorn sheep graze near Highway 191 east of Big Sky. PHOTO BY JULIE CUNNINGHAM

By Jessianne Castle ENVIRONMENT AND OUTDOORS EDITOR

BOZEMAN – Despite intensive restoration efforts nationwide, bighorn sheep populations remain fragile across much of their home range. The wild sheep that grace Big Sky travelers on Highway 191 and Lone Mountain Trail, which are known as the Spanish Peaks herd, are no different.

This spring, Montana Fish, Wildlife and Parks conducted routine flight surveys as a part of ongoing monitoring for the Spanish Peaks and Taylor-Hilgard herds. While many of the sheep sporting GPS-satellite collars survived the winter, area biologist Julie Cunningham reported seeing few year-old lambs. Surveys were conducted prior to the 2019 lambing, so lambs referenced from this count refer to those born in 2018.

The lack of 2018 lambs, Cunningham said, is likely due to several causes. Some of the decline could be attributed to hunting, as FWP has offered a moderate number of ewe hunting tags in recent years, resulting in approximately 22 ewes harvested from the Spanish Peaks herd in the last three years combined.

Another factor likely at play was the overall harsh winter. Cunningham said last winter didn’t start out very intense, but by March the snow was deep and temperatures were very cold. It ended up being particularly unforgiving because of the timing.

“They don’t have a bite of nutritious food once grass turns brown [in the fall],” Cunningham said over the phone after releasing a report from her spring surveys on June 17. “Even in a mild winter they are going to have way lower fat reserves by March.”

She added that in a good year, the grass is on the brink of greening in March as snow begins to melt, but as was the case of early spring 2019, the cold and snow finally hit at a time when fat reserves had been depleted. Cunningham said lambs are especially vulnerable because their energy in the summer and fall goes toward growth, not storing fat reserves.

While admittedly a brutal winter, there has not been evidence of large-scale mortalities from the winter. In other words, there have not been many reports of sheep carcasses found at the wintering grounds.

“It is likely the true population has not dropped as sharply as counts suggest,” Cunningham wrote in her report. Instead, Cunningham thinks a large factor in her low sheep counts was observability.

“After bad winters like this [year’s], they aren’t as likely to move away from the helicopter. They don’t feel good. If animals are in bad condition, they are less likely to flush out,” she said. “Did I observe 50 percent or 80 percent of the herd?”

Overall, during April and May helicopter flights, 97 bighorn sheep were counted in the Spanish Peaks herd and 120 were counted in the Taylor-Hilgard herd that winters in the Madison Range southwest of Big Sky.

Amid the report of low counts, Cunningham did note that one ewe captured and marked with an ear tag in the Taylor-Hilgard herd in 2016 was spotted near Deer Creek in Gallatin Canyon last fall, where she joined the Spanish Peaks herd for the winter. This is the first known connection between the two populations.

Given the uncertainty of the actual population sizes, and in the face of a likely decline based on low lamb survival, Cunningham is proposing to reduce the number of 2019 ewe hunting licenses during the upcoming season-setting process.

For the Spanish Peaks herd, she said she’s proposing one tag for 2019—a drop from 10 given last year—and will consider eliminating the hunt in 2020. The ewe hunt was opened up in 2016 at a time when the herd was experiencing population growth beyond the carrying capacity of 150 sheep in an effort to prevent over-browsing of winter range. Cunningham said they will maintain the seven ram licenses this year, but may consider reducing that number in the future.

While hunter harvest can be adjusted to meet the needs of a population, Cunningham said poor forage and traffic collisions continue to pose a challenge for bighorns. The Spanish Peaks sheep, she says, are threatened by noxious weeds that have taken hold on winter habitat.

According to Jennifer Mohler, the executive director for Gallatin Invasive Species Alliance, weeds got a foothold during early development in Big Sky. Since 2010, the alliance has been working to improve the forage on the hillside west of Highway 191 and north of Lone Mountain Trail by treating noxious weeds and planting test plots of native plants. Mohler says they’ve seen impressive results but are still struggling with cheatgrass and spotted knapweed in some areas.

“Every winter range is really critical for wildlife, that’s what they survive on. [Noxious weeds] directly impact the amount of available forage,” she told EBS last summer.

Cunningham says these efforts can have a huge impact. “The footprint of winter range is small. The percentage of good habitat makes a difference.”

She added that drivers on Highway 191 in Gallatin Canyon should always be on the lookout for sheep, no matter the time of year. Sheep come down and lick residual salt on the road from the winter, which often collects in the rumble strip along the center line.

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