In an attempt to reverse the decline of Rocky Mountain bighorn sheep in the Taylor River area north of Gunnison, the Colorado Division of Wildlife is exploring whether augmentation might be used as a tool for herd recovery efforts.
The habitat in the area appears sufficient to support the local bighorn herd. But because of ongoing respiratory disease problems, the number of animals in the Taylor River bighorn herd has declined to about 30 individuals since 2000. Bacterial and viral pathogens infect the respiratory system of susceptible animals and cause pneumonia. While some adult animals seem able to live with these diseases, most newborn lambs become infected soon after birth and die within a few months. With very few new animals surviving, the herd is slowly dying off, explained Scott Wait, senior terrestrial biologist for the DOW's southwest region.
In late March, the DOW transplanted three bighorn ewes into the area from a herd on the east side of the Sangre de Cristo mountain range. The ewes were treated with long-lasting antibiotics and vaccines that biologists hope will prevent infections. Blood tests showed that the ewes were pregnant and they are expected to give birth in May. By early fall, DOW biologists will know if the lambs born to these ewes have survived.
The ewes were also equipped with radio collars so that their movements and survival can be tracked.
"This is just a small trial to see if this type of augmentation might work," said J Wenum, area wildlife manager for the DOW in Gunnison. "The new ewes might become infected, so we understand that we're taking a risk with these animals. But we can't determine it's a potentially viable approach without taking a small risk."
If the introduced ewes survive into the fall, and particularly if their lambs survive into the fall, the DOW will consider that information in planning future management of the Taylor River herd. In addition to the augmentation, DOW is considering options for culling sick sheep that are observed in the herd.
Problems with lamb survival are not unique to the Taylor River herd. Over the last several years, various combinations of vaccination, medication, nutritional supplementation, and other approaches have been tried in bighorn herds throughout Colorado in an attempt to identify an effective means of reversing declines in other affected herds.
"To date it is yet unknown if we have been successful with mid-winter treatments and supplementing herds with new animals from other areas of Colorado," said Wait. "However, with some new tools and treatments we are willing to keep trying. At this point we can only wait and see how this works."