Aerial surveys conducted in November by the Utah Division of Wildlife Resources (DWR) in Zion and on Bureau of Land Management lands on the park's south border show a booming population.
"They counted more than 500," said Cassie Waters, a National Park Service wildlife biologist at Zion. "...We could have up to 800 within this habitat in and out of the park. That's a little scary when you think about density-related [disease] outbreaks."
Dustin Schaible, a bighorn sheep biologist with the DWR, believes the Zion herd is feeling the need to find new range.
"It seems we have achieved the density where they will start spilling out into areas with high contact possibilities with domestic sheep," Schaible said.
A variety of fatal diseases can be spread between wild and domestic sheep, which complicates management of bighorn across the country. Another factor in population control efforts is the lack of hunting within the national park. Controlled and limited hunting is allowed on the BLM lands south of Zion.
Sore mouth disease, which can sometimes be fatal to lambs, showed up at Zion in spring 2013.
"It seems to have worked its way through the population and it appears to have been isolated," Waters said. "If we did lose any lambs I'm not aware of them."
A case of pneumonia would be an entirely different thing. Utah wildlife officials and others across the West have elected to kill entire herds infected with pneumonia to prevent them from spreading it and wiping out bighorn in large areas.
To deal with the burgeoning bighorn, Zion National Park is working with Utah wildlife managers with a plan to trap and relocate sheep within the park. An Environmental Assessment is being performed as part of the National Environmental Policy Act.
Public comment on the plan is accepted during a scoping period and will be due by March 19. Another opportunity to provide public comment will come when the Environmental Assessment is complete.
Schaible hopes to remove animals from Zion next winter. The proposed method involves firing net guns from helicopters, then transporting animals to a staging area for medical checkups. Animals would be loaded into trailers for a journey to their new homes.
Similar operations over the past two winters on BLM land south of the Zion border removed 45 animals, but that was barely a dent in the population.
Schaible is working on a comprehensive management plan for the entire Zion herd, but believes it will end up with a goal of about 500 bighorn.
"The Zion herd is our biggest desert bighorn herd in Utah," Schaible said. "We are trying to protect it by thinning the numbers and hoping to avoid another disappearance."
Bighorn sheep, native to Utah's rugged canyon country, were spotted in Zion until the 1950s, when a combination of human factors likely led to their demise. For years, the only sightings in the park were the many petroglyphs portraying the animals.
In 1973, a dozen desert bighorn sheep were reintroduced to Zion from Lake Mead. They initially were kept in pens so they could acclimate to their new surroundings.
Their numbers grew to about 20 and they were released only to vanish.
Then, in the mid-1990s, Zion visitors started spotting the sheep again. Counts pegged their numbers at about 50. More than a decade later, reports of wild sheep came from south and east of Zion at Mount Carmel Junction, Hildale and Kanab.
An aerial count in fall 2008 confirmed that the transplanted bighorn had been thriving in and around the park. In two hours, biologists counted 75 sheep. The next year, DWR biologists conducted a full survey in Zion and lands to the south. Their findings: 114 bighorn outside the park and 115 inside.
How to comment on Zion bighorn sheep plan
Zion National Park officials are considering working with Utah wildlife biologists to reduce the number of desert bighorn sheep in the park by using net-gunning to capture the mammals for relocation.
Comments are due by March 19. Information about the plan and how to comment can be found at http://parkplanning.nps.gov/zion.
Comments also can be sent to: Superintendent, Attention: Bighorn Management EA, Zion National Park, Springdale, UT 84767