Over 400 bighorn sheep have been hit by cars, and more than 50 have been hit by trains on Highway 200 near Thompson Falls MT. These sheep experienced a sharp decline in the late ’80s, early ’90s, corresponding with vehicle mortality. Besides bighorn sheep, high numbers of mule deer, white tail deer, elk, and black bear are also hit along this stretch of highway. The sheep are drawn down to the roadside where they lick “salt” off the side of the road, and off of the road itself. This behavior has been observed in declining bighorn sheep near the mouth of American Fork canyon in Utah, and in deer on Highway 6 and highway 39 in Utah. Preference sampling on highway 39 in Utah has shown that the deer there are going after magnesium chloride, not just salt. The magnesium chloride is a deicing agent used on the highways in the winter time. It tastes bitter unlike salt, and is probably needed due to a mineral deficiency.

Thompson Falls lies within the Lolo National Forest. The use of herbicides for weed control in the forest was ceased in the early 1980s. In 1992 the use of herbicides in the forest was brought back, including aerial spraying. Thompson Falls sheep were in decline in the mid 1980s, but were starting to come back up again in the early 1990s. In 1994 their numbers again began to decline.

Looking at mile marker 64.5 on highway 200 from Google Earth, several things stand out. At this point you have railroad lines, highway, and power line right of ways all next to each other. All of these courses get sprayed with pesticides to clear vegetation from them. The association between highway and power line spraying, and wildlife congregating nearby to lick deicer, has been seen in Utah on highways 39 and 6, and in other Western states. 

Mile marker 64.5 is where the majority of fatalities happen.

Large amounts of vegetation were removed from the sides of Hwy 200 near mile marker 64.5 in 2006. This corresponds with an increase in sheep numbers in 2006. The population ultimately collapses a few years later though. Spraying of highway and power line right of ways on Utah’s highway 39, corresponds with increased vehicle mortality of deer and antelope. Utah’s highway 6 has higher vehicle mortality of deer, elk, and moose. It also has several large power line right of ways, and more miles of highway right of way spraying as well. The deer on both of these highways “camp out” on the road side licking deicer.

There was a large timber harvest in the area during 2005, these have occurred on the mountain above highway 200 since the early 1990s. Most of these occurred during the early ’90s as the sheep population started to decline. Over the last 20 years the use of pesticides on timber harvest have increased. This has been seen in the forests where hoof rot is found in Washington elk.

Many of the pesticides used for right of way spraying have been shown to cause thyroid disruption. Deer along Utah’s highway 39 have exhibited signs of congenital hypothyroidism in the form of under bites. Hypothyroidism has been associated with magnesium deficiencies. The magnesium chloride sprayed as deicer is a soluble form of magnesium available to those animals that are consuming it. Ruminant animals that are deficient in minerals have been shown to seek them out. This is the case for whiskey mountain big horn sheep seeking selenium, and mule deer in Utah seeking copper and selenium.

Below are before and after images of vegetation removal from the sides of highway 200, in 2006.