National Park Service managers are mending livestock fencing at a volcanic monument in southcentral Idaho to help a herd of pronghorn antelope along its 160-mile roundtrip migration across the northern Snake River plain and into Rocky Mountains valleys.

Federal, state and private groups including the New York-based Wildlife Conservation Society began cooperating on a GPS study last fall to pinpoint the route of hundreds of these fleet-footed antelope as they travel between the Pioneer Mountains southeast of Sun Valley to near the Continental Divide along the Idaho-Montana border, and back.

In addition to rivers, steep terrain and other natural obstacles, the antelope encounter manmade highways, housing developments - and fences erected for sheep and cattle ranching around the Craters of the Moon National Monument, located 50 miles west of Idaho Falls.

John Apel, a National Park Service resource program manager, says information from the study about the pronghorns' migration has been a guide to removing or modifying fences, to make it easier for the animals to reach summer and winter range used by pronghorns for thousands of years.

"They are going this long distance not for the joyride - it's to get some place to improve their survival," Apel said Friday. "As more and more land is developed with highways and subdivisions, every one of these is cumulative in making that migration a bit more hazardous."

This past year, the National Park Service removed a half-mile of old livestock fence from the northern portion of the monument; Apel is now planning to modify fences constructed of mesh wire.