Counting Sheep

This March, a large helicopter thrummed over the 13,000-foot-high pinnacles of Yosemite National Park’s Cathedral Range with an aluminum box dangling from a long cable. Inside the box were sheep—four Sierra Nevada bighorns waiting (patiently, in fact) to touch down in their new home, a rocky slope nubbled with glacial scree and lodgepole pines. “That’s the scariest part for me,” said California Department of Fish and Wildlife biologist Tom Stephenson, whose eyes were locked on the transport helicopter from a second helicopter. “Watching that box hang, and making sure it gets there safely.”

Biologists and the conservation community have been working for decades to restore populations of this endangered paragon of the Sierra Nevada’s rugged wilderness. Translocations—the transfer of rams and ewes from healthy herds to vacant habitat—have been their chief strategy to reestablish herds that once prospered along the Sierra crest. The March Yosemite drop, and another a few days later in Sequoia National Park, were the most complex translocations conducted in the Sierras so far. They are also close to the last—the sheep, scientists predict, can mostly take it from here. Stephenson, who leads the recovery effort, said that the new managed herds are thriving. Within five years their numbers should support downlisting from endangered to threatened. Permanent delisting should follow.

“Over the decades, I’ve heard some pessimists say that we’ll never delist Sierra Nevada bighorn sheep,” said Kevin Hurley, conservation director of the Wild Sheep Foundation. “Well, we’re getting closer all the time.”

Historically, bighorn sheep were common in alpine areas from British Columbia to northern Mexico. There are three subspecies: Rocky Mountain bighorns along the Continental Divide, desert bighorns of the arid Southwest, and the most isolated subspecies, the Sierra Nevada bighorns. Two hundred years ago, as many as two million bighorn sheep lived in North America. But exploitation during westward expansion, as well as the installment of domestic sheep, which can host pathogens fatal to wild sheep, drastically reduced these numbers. By the 1950s, bighorn populations were patchy and thin—only 25,000 sheep remained.

Sierra Nevada bighorns were always the most vulnerable because they are the most geographically isolated subspecies with the smallest population. Genetically distinct from the other types, these sheep, with their unique wide-flaring horns, evolved to thrive in the Sierras’ contiguous high-elevation crest, primarily on the drier eastern slopes. About 1,000 bighorn sheep occupied this mountain range before European settlement. By 1995, scientists could find only 100 Sierra Nevada bighorns in the whole range.

Thanks to a sustained conservation effort among state and federal agencies, hunters, tribal commissions, and other entities, bighorn populations across North America have more than tripled from their 1950s low. That’s largely a result of the relocation of 21,000 sheep since 1922. While the practice didn’t really pick up until the 1940s and ’50s, on average, that’s nearly 20 airlifted sheep a month, said Hurley.

The recovery story for Sierra Nevada sheep is even more remarkable, with populations rebounding to six times their 1995 numbers. The range’s national parks—Yosemite, Kings Canyon, and Sequoia—have been critical to this success because of their nearly intact habitat. But although sheep translocations have been occurring in the Sierras since the 1980s, the targeted sites have straddled only the parks’ eastern fringes.

This year’s operations delivered sheep deep within the interiors of Sequoia and Yosemite. The Cathedral Range site was not even considered an option at first; biologists thought it too high, too snowy, and too forested for a herd to survive there year-round. Wildlife managers began taking it seriously after they analyzed data from sheep GPS collars and realized they were underestimating where these hardy climbers could thrive. Even more significant, evidence showed that sheep had been in the Cathedral Range previously. In 1933, the wizened body of a ram, by then a few hundred years old, thawed out of glacier ice in the region. And last fall, when Yosemite wildlife biologist Sarah Stock and her family were backpacking in the Cathedral Range, she discovered an old ram skull. “We started to understand that there’s more of a diversity of habitats that the sheep could use,” said Stephenson.

In September 2012, Stephenson, Stock, and colleagues hiked to the Cathedral Range’s high cliffs to evaluate it as a possible release site. They saw very few deer tracks, which was good news—mountain lions would be less likely to hunt there. The buckwheat and grasses on the cliffs were also a reassuring sight—that meant the sheep were likely to stay healthy through the winter. Most important, it was far from domestic sheep operations, unlike the eastern edge of the park. “When Cathedral seemed to check out on all those fronts, we just got giddy,” Stock said. Stephenson felt they couldn’t meet the delisting goals without it.

Getting the sheep to the two isolated park sites, though, required a web of logistics. For each translocation, biologists had to choose source herds with sufficient genetic diversity, then set up base camps near those animals. An operator in a small, nimble helicopter hovered 15 feet above selected sheep, then, like Spider-Man, fired a “net gun” that captured them. After checking their vitals and drawing blood for genetic evaluation—all while the sheep were blindfolded, which made them immediately docile—the team fastened tracking collars and loaded the animals into the aluminum boxes. A truck carted the boxes to a local airport, where the larger helicopter was waiting. After a 15-minute flight over Yosemite’s high, rugged peaks, the operator gently deposited the three boxes. The biologists opened the doors and watched the sheep bolt for the cliffs. “It was almost like a parent having to let go,” said Stock. “You put so much preparation into it and you just have to trust: OK they got this.”

All signs say that they do. Nearly all of the translocated ewes were pregnant, and biologists are expecting to find a bunch of lambs at the new park sites when they complete their latest survey. “We need at least 305 adult and yearling females to be able to downlist and we are up to 275 females now,” Stephenson said. “I have very high hopes.”