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November 1, 2016

Every so often, wildlife managers in Pinedale, Wyoming, discover carcasses of mule deer floating in Fremont Lake, outside of town. The deer drown while crossing thin ice during their spring migration to the mountains. Exactly where they came from was unknown—until recently.

In winter 2012, GPS tracking collars were placed on mule deer thought to live year-round in the Red Desert, 100 miles south of Pinedale. Tagging was done by Hall Sawyer, a biologist with Western Ecosystems Technology, who serves as an adjunct faculty member at the University of Wyoming.

By spring, the collared deer were nowhere to be found. It took an airplane search across most of western Wyoming to find them. The deer had migrated 150 miles north, on a narrow corridor, to the Hoback Basin, making this the longest-known migration of mule deer anywhere in the United States.

At the 100-mile mark, the mule deer crossed Fremont Lake.

Researchers at the U.S. Geological Survey (USGS) conducted a scientific assessment to determine the top threats along the migration corridor, mile by mile. 

The assessment revealed a bottleneck between Fremont Lake and the growing suburbs of Pinedale. Since it restricted the movement of over 5,000 migrating mule deer, the bottleneck was identified as a top threat.  Historically, the migrating deer moved around the lake, but development restricted the available area, forcing many deer to swim the frozen lake or squeeze through an 8-foot-high elk fence.  The bottleneck centered on private land zoned for a lakeside-home subdivision, which could lead to further restrictions on migrating deer, or “bottlenecking.” 

Mule deer migrate across the half-frozen Fremont Lake, which the Wyoming Unit identified as the top threat to the longest-known migration in the United States. Photograph credit: Mark Thonhoff, Bureau of Land Management
A map shows the location of the Fremont Lake bottleneck property, which the nonprofit Conservation Fund purchased and transferred to the State of Wyoming in 2016. Credit: University of Oregon InfoGraphics Lab/Wyoming Cooperative Game and Fish Research Unit 

With the importance of the corridor made clear, a nonprofit organization, The Conservation Fund, launched a $2.5 million campaign to buy the property. “It is tremendously helpful to have real guidance for our prioritization, and that is exactly what the assessment provided us,” said Mark Elsbree of The Conservation Fund. The public outreach galvanized so much support that fundraising concluded a year ahead of schedule. 

In July of 2016, the policy commission of the Wyoming Game and Fish Department (WGFD) voted to accept donation of the Fremont Lake property from The Conservation Fund to create a new, State-owned Wildlife Habitat Management Area where lakeside homes had been envisioned. 

"Maintaining migration corridors has always been a priority for the WGFD,” said Deputy Director John Kennedy. "We have worked on collaborative migration research with the USGS Wyoming Unit for over a decade." 

"This research has made it clear that these corridors are critical to the performance of big-game herds in the West, and they are increasingly challenged by development and land-use change,” said Jim Lyons, Deputy Assistant Secretary for Lands and Mineral Management at Department of the Interior. “We are now looking at ways to incorporate this new information into how we plan for managing wildlife on our public lands.”

USGS researchers are experts in animal movement, behavior, and population structure, which are the driving forces behind animal migration. Work was done by the USGS Wyoming Cooperative Fish and Wildlife Research Unit (known as the USGS Wyoming Unit), who also founded the Wyoming Migration Initiative team. The Wyoming Unit is studying deer, moose, pronghorn, elk, wolves, and bighorns sheep to understand the influences on these species from human disturbance, habitat conditions, climate change, and energy development. Work that connects science to management helps ensure that migration pathways for our Nation’s cherished wildlife can endure. 

USGS Wyoming Unit student Matt Hayes listens for radio signals from mule deer along their migration corridor. Researchers locate animals in early winter to see who still has a fawn with them. Most are on winter range, but some, like this one, are still migrating. Photograph credit: Matthew Kauffman, USGS
Insights gained from research on mule deer migration could alert managers to when animal movements are threatened by land-use changes. Photograph credit: Matt Kauffman, USGS

For more information, please contact Anne Kinsinger, USGS Associate Director for Ecosystems, at


Read more stories about USGS science in action.



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