Effect of Vehicle Traffic on Dall Sheep Migration in Denali National Park, Alaska

Photo by Jeff Keay

Putera, J. A. and J. A. Keay. 1998. Effect of vehicle traffic on Dall sheep migration in Denali National Park, Alaska. U.S. Geological Survey, Alaska Biological Science Center. Final project report, 33pp.

ABSTRACT:  The effect of vehicle traffic on Dall sheep (Ovis dalli) seasonal migration was investigated in Denali National Park during 1995-1997.  The park road intersects traditional migration corridors where sheep travel up to 10 km through valley habitat, 2 to 7 km away from escape terrain, to reach seasonal ranges.  Aerial surveys, daily observations, and infrared triggered cameras and time-lapse cameras were used to document migration.  Sheep-vehicle interactions at the road were also recorded.  Observed spring migration attempts from the primary study area, Primrose Ridge and Mt. Wright, occurred as early as May 10 and as late as July 7.  Group size ranged from 1 to 62 individuals.  Sheep migrated as ewe groups with or without lambs, ram groups, and on 6 occasions adult rams traveled with ewe groups.  Ewe groups with lambs initiated migration attempts June 12, 1996 and June 18, 1997.  Unsuccessful or partially successful attempts by groups to cross the road were directly observed on 4 occasions in 1996 and on 5 occasions in 1997.  Trail monitor results indicated a sixth thwarted attempt, and monitors along with direct observation suggest that some sheep made up to 3 unsuccessful attempts prior to successfully crossing the road in 1997.  Five ewe-likes and at least 10 rams are known to have remained on the winter range following thwarted migration attempts.   Each year all ewe-likes and all lambs attempted to migrate from the primary study area.  The majority of rams remained on the primary study area each summer.  Successful migration attempts by groups occurred from 5 to 16 days after initial thwarted attempts.  Eighty-five percent of the sheep in 36 % of the groups that migrated or attempted to migrate were near the park road between 1000 and 1900 hours.  Sheep were observed near the road between 0400 and 0630 hours following repeated failed migration attempts in 1997.  Successful groups averaged 16.8 minutes (SD = 11.7) to cross from 100 meters north to 50 meters south of the road, and crossed with or without vehicles present.  Unsuccessful groups waited an average of 62.0 minutes (SD = 62.5) as vehicles passed and stopped before retreating back to escape terrain.  Observed fall migration began as early as August 23 and extended as late as mid-October.  Groups size ranged from 1 to 28 individuals and ewes and rams migrated separately.  Sixty-one percent of all sheep in 39 % of the groups were near the road between 1700 and 2100 hours.  No unsuccessful attempts to cross the road were observed, although 4 of 7 ram groups were delayed by the road from 2.1 to over 7.5 hours due to traffic.  As a result, 2 groups remained near the road after dark.  Ewe groups (n = 16) averaged 16.9 minutes (SD = 11.8) to cross the road during fall.  Seven of these groups crossed the road with up to 5 vehicles present.