Lethal lick 

Is road salt drawing bighorn sheep to their doom?

The bighorn sheep herd near Thompson Falls has fluctuated over the past 31 years. Montana Fish, Wildlife and Parks has recorded population numbers as high as 432 and as low as 161. Wildlife biologist Bruce Sterling says that's only natural. The sheep have always had a tendency to rebound from mild declines. But the population has never dipped low enough to sound the alarm at FWP.

Not until this spring, anyway.

At latest count, Thompson Falls is down to 52 bighorn sheep. "The bottom just sort of fell out," Sterling says. Now, with only 32 breeding ewes and eight rams, every mortality brings the herd closer to biological instability.

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Bighorn sheep have had a rough time in the West lately. For the past two years, pneumonia has swept through many herds, leaving more than 600 dead in Montana in 2010 alone. The disease—for which there is no known vaccine—has caused significant die-offs in 14 bighorn populations in the state since 1984.

From what Sterling can tell, the Thompson Falls herd is pneumonia-free. Yet its population has declined rapidly in recent years, from 270 in 2008 to 52 this spring. He can't be sure yet, but Sterling's confident that disease is not the problem. "It doesn't fit some of the other disease scenarios that we've seen in other sheep herds," he says. "That's usually a sudden crash of the population. People have not been reporting sick or dying sheep. The sheep we've seen appear to be healthy."

Instead, Sterling points to an alarming number of bighorn deaths on two one-mile stretches of Highway 200 near Thompson Falls. Over the past four years, 110 sheep have been struck by cars and killed. That figure is a minimum, Sterling adds. FWP can't say how many sheep were clipped by vehicles, wandered away from the road and died later as a result of their injuries.

The agency knows why sheep die on highways: salt. Sheep linger on the highway in the winter months, when de-icing efforts turn the pavement into a salt lick. The same is true on windier stretches of Highway 93 in the upper Bitterroot Valley and along Highway 1 west of Butte. In early 2010, a motorist drove his pickup through a herd of bighorn near Anaconda, killing eight sheep, including two trophy rams.

"Drivers are too busy," Sterling says. "They're just not paying attention to what they're doing. They've got a lot of other activities going on, talking on cell phones and doing whatever."

Sterling adds that venturing onto Highway 200 for the salt deposits has likely become a learned behavior among the Thompson Falls herd over several generations. That the grass near the highway greens up faster in the spring than vegetation at higher altitudes only compounds his concerns.

The problem is troubling enough that FWP requested a meeting with the Montana Department of Transportation to discuss possible solutions. Sterling says the discussion, scheduled for May 10, will likely lean on alternatives to salt use, like sand-only de-icing.

However, according to the Montana Department of Transportation, a sand-only alternative would prove less effective in ensuring public safety, and chemical de-icing agents are potentially cost-prohibitive. Similar discussions concerning sheep on Highway 1 near Georgetown Lake several years ago resulted in removal of salt from de-icing procedures. "It's helped deter them," says MDT spokeswoman Lori Ryan. "It has not kept them off the roadway, but [MDT] has taken the salt out of the sand over there."

Ryan adds that there have been no other complaints of salt attracting wildlife to roadways in the state. Her agency keeps rough data on highway mortality counts among individual species, but it's limited "by the fact that many wild animal-vehicle collisions are not reported."

Fencing could be another solution, but given how close the stretches of highway are to rock walls, Sterling believes that option would require road modifications that could take years. Other improvements, such as overpasses and underpasses, may not gain FWP much.

"What's interesting with this scenario is these sheep do not need to, nor do they necessarily, get to the other side of the highway," Sterling explains. "This isn't a crossing zone. They're not getting to another section of their habitat, because the other side of the highway is the railroad tracks and then the Clark Fork River."

Highway 200 already has flashing lights that warn drivers of the possibility of sheep lingering on the road ahead.

The problem in Thompson Falls raises a broader question, however, given that the current herd was reintroduced to the area in the 1950s. An average of 16 of those sheep a year fall victim to highway accidents. Others are killed by trains on the adjacent rail line. Is Thompson Falls really a suitable area for these sheep?

"What happens is a lot of these winter ranges for wildlife—sheep included—are also some of the best areas to put highways and railroad lines," Sterling says. "It's an unfortunate event that occurs for a lot of wildlife."

The highway is clearly an issue for the bighorn herd. But Sterling isn't convinced FWP knows the whole story yet. One hundred and ten sheep killed in four years is substantial. It doesn't explain the other 108 deaths in that time.

"There's something else that's probably at play here to cause this dramatic a decrease," Sterling says. "My guess—and it's purely a guess—is that it's lion predation."

Sterling says FWP will likely augment the herd with sheep from Wild Horse Island later this year.

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