Spring game surveys have been revealing a troubling trend in one area - a sharp decline in the Thompson Falls bighorn sheep herd that is mostly due to vehicles on Montana 200.

While spring surveys of deer and elk are aimed at showing trends in offspring survival rather than being population counts, bighorn sheep surveys are considered population counts because the sheep are exposed in open, rocky habitat.

The Thompson Falls herd numbered 270 animals in 2008, but this year's survey came up with just 52 animals, said Bruce Sterling, the Montana, Wildlife and Parks biologist for the Thompson Falls area.

Over the last four years, Sterling said, the herd lost 107 sheep to vehicles on Montana 200. Most of them died in two one-mile stretches that are adjacent to rocky terrain above the highway.

"Vehicle kills are now a huge issue," said Jim Williams, the state's regional wildlife manager. "Every ewe mortality is especially significant."

The situation prompted Fish, Wildlife and Parks to cut the number of sheep permits offered for the area from 10 to four for this fall's hunting season.

And plans are being developed to augment the herd with sheep transplanted from Wild Horse Island on Flathead Lake sometime this fall.