Mountain lions likely trimmed island sheep herd to lowest number in decades

Three now-dead cougars that found their way onto Flathead Lake’s Wild Horse Island handily hunted down its bighorn sheep herd to as few as 35 of the prized Rocky Mountain ungulates.

Although an initial sum, it’s the lowest sheep count on the isle in at least 20 years, according to Montana Fish, Wildlife and Parks.

About 130 sheep were tallied on the island state park in 2019, with a couple dozen later removed by the department to help reestablish a herd in southwestern Montana.

All told, the three adult mountain lions — which either swam to the island or accessed its shores via ice-bridge during recent winters — appear to have killed some 70 of the vulnerable bighorns before being killed by local wildlife officials over the past roughly two months.

Sheep might now have to be moved to the island that has served as a prime source for nearly 600 disease-free bighorns to Montana, Washington and Oregon over the last half-century.

If approved, it would be the first time in more than three decades that bighorns have been transplanted to Wild Horse Island to augment the herd and maintain a healthy genetic pool.

An on-the-ground count estimates that 35 sheep remain on the island.

“That’s what groups of people have seen; we haven’t done a flight (survey) yet,” Neil Anderson, regional FWP wildlife program manager, said of the more thorough method of counting the island’s population.

The department announced last week that one adult female and two adult male lions were killed over the winter to protect park visitors and to shield bighorn sheep lacking escape on the nearly 3.4-square-mile island.

“I think what people don’t really realize is that Wild Horse Island has almost no escape terrain for bighorn sheep,” Anderson said of cliffy, rocky areas that the agile sheep hoof to avoid threats.

“So, they’re pretty vulnerable to predation,” he said. “To have that many mountain lions on the island, they could do a pretty good number on the sheep.”

An adult male mountain lion’s home range typically spans more than 100 square miles, with that of a female’s being up to 60 square miles, according to the Forest Service.

Robust in Montana, the big cats mostly eat deer but are known to eat anything from grasshoppers to moose, according to the Montana Field Guide.

Wildlife officials plan to conduct a flight survey this spring to assess the cougars’ damage to the prized sheep herd.

Two bighorns were initially introduced to the island in 1939, having since thrived remotely with limited predation and no documented diseases.

Ten total sheep have been transplanted to the island over the years, the last time being in 1987, when two bighorns were moved there from Lincoln County, according to Montana State Parks.

Wildlife officials believe the three cougars — known as cunning, elusive creatures — either swam to the island or more likely accessed its shores in recent winters atop ice.

In 2019, Flathead Lake nearly froze over for the first time since the early 1990s, though lake waters surrounding the island could have frozen more recently for the cats to gain access.

Wild Horse Island lies just off the western shore of Flathead Lake, within the Flathead Reservation. FWP collaborated with the Confederated Salish and Kootenai Tribes to remove the big cats.

Wild Horse Island has produced some of the largest rams in the world, according to FWP.

The department typically aims for an island population of between 100 to 120 bighorns to limit the environmental impact to the land, Anderson said.

He said a 2019 island survey showed about 130.

FWP removed 26 sheep last year to help reestablish a herd in Southwest Montana’s Tendoy Mountains and maintain a sustainable herd for the island.

That’s about when sheep numbers began dropping.

“It started going down pretty significantly after that,” Anderson said, also noting there remains no evidence showing the sheep dying off from disease. “The logical explanation is predation.”

Bear and lion have been documented on the island, “but what people were starting to see is lions being more accustomed to some of the housing out there,” he said of nearby landowners.