ZION NATIONAL PARK — What began as a dozen desert bighorn sheep reintroduced into Zion National Park in 1973 has now become the largest and possibly healthiest herd of bighorn sheep in Utah.
Park officials and wildlife managers want to keep it that way.
"This herd is definitely a valuable asset to the park," said Dustin Schaible, bighorn sheep program manager for the Utah Division of Wildlife Resources. "We want people to see healthy animals."
On Feb. 14, Zion National Park announced a proposal to partner with DWR in managing bighorn sheep in and outside the park to ensure a sustainable and healthy population.
Prior to the 1973 reintroduction, Utah's desert bighorn sheep had been wiped out by various human factors. After decades of recovery, the herd currently boasts more than 500 desert bighorns across roughly 70,000 acres of habitat in and around Zion National Park.
"Zion is really good habitat," Schaible said. "When you have good habitat, populations just grow."
But expanding territory and rising population density pose inherent risks to the herd. The desert bighorn's current range fringes on private land near Springdale, Hilldale and Kanab where there's potential for contact with domestic livestock.
"It is widely recognized that any contact with healthy or non-healthy domestic sheep or goats, which bighorn are behaviorally attracted to, poses one of the most significant risks of disease transmission within bighorn populations," said Ryan Monello, a wildlife biologist and disease ecologist with the National Park Service.
Even without contact with domestic fauna, bighorn sheep can be susceptible to disease-induced mortality due to overcrowding, according to Fred Armstrong, division chief for natural and cultural resources at Zion National Park.
Certain types of bacteria are commonly present in the respiratory tracts of bighorn sheep that, under non-stressful conditions, aren't a problem for the animals, Armstrong said. In environmentally harsh conditions or when herd numbers exceed a habitat's ability to sustain, the animals undergo stress when resources become scarce, allowing pathogens, such as pneumonia, to develop and spread.
According to DWR's statewide management plan for bighorn sheep, 88 percent of pneumonia-induced die-offs occur at or within three years of peak population estimates.
Zion's herd isn't there yet.
"Since this has been determined to be a healthy population, we want to keep it that way," Armstrong said. "By keeping densities lower, there won't be significant stress on the herd from competing for resources."
In areas like Zion National Park where hunting is not allowed, transplanting is a primary tool used by wildlife managers in managing big game populations. The division has used trap-and-transfer methods to manage parts of the desert bighorn herd on Bureau of Land Management areas south and east of the park over the last two years, Schaible said.
In the proposal to partner with the Utah Division of Wildlife Resources, park officials said transplanting would reduce bighorn population densities in the park and help restoration efforts in other areas.
"These bighorn would be transplanted to supplement an existing small herd or start a new population, but with the primary objective of protecting the source herd," the proposal states.
Prior to relocations, bighorn sheep undergo health examinations and are often treated with antibiotics and vaccines to avoid introducing disease to other herds, according to the division's statewide bighorn sheep management plan.
Schaible says transplants are a "calculated risk," but keeping relocations as close as possible to the source herd helps to minimize that risk.
If Zion's bighorn management proposal is approved, areas near the San Juan River, Nokai Dome and Pine Valley would be considered as possible locations for transplants, Schaible said.
The park's almost 3 million annual visitors need not worry about fewer opportunities to observe wildlife if desert bighorn are removed, Armstrong said.
"A periodic management plan to remove animals and place them elsewhere is not going to adversely affect wildlife observing for visitors," he said.
As a cooperative effort between Zion National Park and DWR, the management plan would be mutually funded. The majority of the funding, however, would come from the division's conservation permit program — a program that auctions hunting permits to members of sporting organizations and allocates the proceeds to wildlife conservation projects, Schaible said.
Monello said the proposal, as with all bighorn management plans, will consider the benefits and risks associated with the herd size before a management plan is implemented.
"No decisions have been made, and the plan will consider a variety of methods for managing disease," he said. "However, on the whole, trap-and-transplant programs with bighorn sheep have been highly successful over the last several decades at re-establishing bighorn throughout their range."
The National Park Service will conduct an environmental assessment of the proposed plan and is open to public comment until March 19.