The trouble begins with this: Antelope Island — isn’t.

An island, that is.

The Great Salt Lake’s largest island has long served as a big-game sanctuary as well as one of Utah’s most popular state parks. Chronic low lake levels, however, have turned Antelope — and other islands in the lake — into mere peninsulas, allowing bison and, potentially, bighorn sheep to reach the mainland.

Some also fear the reverse: that the big game’s domestic disease-carrying cousins could get to the island.

For several years, buffaloes have, at times, invaded the mainland. But the consequences for Antelope Island’s bighorn herd have been more dire, prompting state officials to call for 10 miles of fencing around the island’s south end.

Last year, a respiratory illness wiped out nearly all of the island’s 150 wild sheep and the Division of Wildlife Resources was forced to kill the 26 survivors to ensure the disease would not spread. How the sheep were exposed to the pathogen remains a mystery, but the fencing to be built this fall is intended to confine the bighorns to the island.

Officials suspect a ram may have wandered off Antelope, been exposed to domestic sheep or goats that carry the pathogen, then returned to the island, where it spread the disease to other bighorns.

Officials hope the fence will prevent that from happening, even though other speculative theories explain how the herd got sick last year, according to park manager Jeremy Shaw. He noted there are no known instances in which a domestic animal got onto the island.

Once the fence is up, DWR plans to reestablish the island’s bighorn herd by relocating 35 sheep from elsewhere in the state.

The eight-foot fence will be built into the lakebed about a quarter mile off the island’s historic shoreline, Shaw said, at a cost of $300,000 to $500,000.

The Great Salt Lake has been near historic lows for the past several years, thanks to long-term drought and decades of upstream water diversions. Recent research indicates that water diversions would have to be cut by 29% just to keep the level from dropping even more.

Even so, additional diversions of the Bear River, the terminal lake’s main tributary, are on the drawing board.

Today, more than 750 square miles of lakebed are exposed and several islands are connected to the the mainland, disrupting delicate ecosystems that depend on isolation. Feral pigs have migrated off the privately owned Fremont Island, showing up near Antelope Island. Coyotes now roam the North Arm’s Gunnison Island, home to one of the nation’s most important breeding colonies of beach-nesting pelicans.

Farmington Bay, framed by the 16-mile-long Antelope Island and Davis County to the east, is now completely dry, save for where the Jordan River inflow has cut a channel around the island’s northeast shore.

A long-abandoned causeway connects the southern tip with Salt Lake City. It’s the area around this causeway that forms the easiest passage for animals traveling between the island and the south shore.

Mule deer have migrated on and off the island’s north end for years, even when water levels are high. The movement of deer does not pose a problem, Shaw said, but that is not the case with bison and bighorn sheep.

The Department of Natural Resources long has been concerned with bison exiting the island’s south shore and wandering to the mainland east of Salt Lake City International Airport, according to agency spokesman Nathan Schwebach.

“It’s a tough spot,” he said. “Lake levels are low and with that come issues.”

Earlier this month, park officials shot a 9-year-old male bison roaming the lake’s south shore after it reached airport property.

"We're committed to relocating wildlife whenever possible. However, relocation is not always possible, especially where public safety is involved," Schwebach wrote in an email. "Unfortunately, when bison learn to leave the island, they tend to continually leave after being brought back."

For the past five years, bison periodically have reached the Audubon’s 1,514-acre Gillmor Sanctuary, located four miles from Antelope Island, according to Ella Sorensen, who manages the preserve that harbors shorebirds, waterfowl and other migratory birds.

“They [bison] spend most of their time on the western side of Gillmor. We are OK with them here. It has been very quiet,” she said. “They have wandered back and forth [between the island and the sanctuary]. The fence will put an end to bison coming off the island.”

It will also result in the return of bighorn sheep. In January, DWR plans to begin releasing Rocky Mountain bighorns on the island.

Restoring this herd to 125 individuals is part of DWR’s proposed statewide plan for Utah’s 11 herds of Rocky Mountain bighorns and seven desert bighorns. Currently, the state’s bighorn population is 4,170, less than half what wildlife officials desire.

“There has been some struggle with our bighorn sheep populations recently because of respiratory disease,” said DWR biologist Jace Taylor, who oversees the bighorn sheep and mountain goat program. “Part of our objective is to expand bighorn sheep populations where possible and to maintain the overall population in a sustainable and healthy way across Utah to provide quality opportunities for wildlife viewing and hunting.”