Bighorn sheep numbers in Southwest Colorado are stable, but the population isn't growing.
Catastrophic die-offs caused by respiratory illness in the 1990s and early 2000s decimated some herds and in some areas, the numbers have stayed low, said Scott Wait, a senior terrestrial biologist for the southwest region of Colorado Parks and Wildlife.
"There's no doubt overall distribution is less than it was," he said.
In the southwest region, the bighorn are spread across the Rio Grande, San Juan, Gunnison and Uncompahgre watersheds. It is difficult to sample bighorn sheep populations because they are usually accessible only by helicopter. But over the last 10 years, Parks and Wildlife estimates around 2,000 sheep are living in 32 herds.
The sheep were hurt by pasteurella, a respiratory disease that can wipe out 50 to 75 percent of a herd. It is carried by domestic sheep, who are immune, and die-offs are an ongoing phenomenon in other parts of the country.
This spring, a die-off in Montana reduced a herd of 90 animals down to 18, the Associated Press reported.
Southwest Colorado has seen even greater reductions in some areas. In the Gunnison National Forest, the Taylor River herd of only 25 bighorn, once numbered around 200 until the herd was exposed to the disease. It has not recovered since hitting its low point in 2006, Wait said.
It is likely the numbers remain low because once a herd is infected, some animals remain carriers for the disease after the die-off.
The lambs become infected before their immune system is mature enough to fight off the disease, said Mike Miller, a Parks and Wildlife veterinarian.
If biologists do not intervene, sometimes herds recover naturally after 10 or 20 years, Wait said. But sometimes, they don't.
To help herds recover faster, biologists have tried different techniques. They can kill the remaining animals in a herd after a die-off and reintroduce animals from another area. But this is controversial, and it comes with a risk.
"You're just hoping they are going to learn how to migrate. That's dangerous," Wait said.
Another option is to vaccinate all the remaining animals in a herd. Scientists have not had any success with this, Miller said. But there is ongoing research into a vaccine that would protect the herds and a practical way to administer it in the field.
Right now, biologists are aiming to keep the region's herds stable by keeping domestic sheep away from bighorn herds.
But pasteurella is not the only disease the wild sheep face.
One local herd near the Pine River around Vallecito struggled with a mysterious disease that was never identified.
Starting about five years ago, mostly male carcasses began turning up. About 20 to 25 were found, but there was no way to take samples of what had killed the sheep, Wait said. The carcasses have not been found for about two years, so it is unlikely that the disease outbreak is on-going.
In the long term, an opportunity exists for local populations to increase because lots of unoccupied bighorn sheep habitat is void of sheep and has been for 20 to 50 years, Wait said.
Most herds aren't natural pioneers. But there has been an encouraging expansion of the herd above Pagosa near the Upper San Juan River, where the herd has been expanding its range for about 12 years.
"In order to grow the overall population, in many cases, we need to have them occupy new areas," he said.
