RUPERT, Idaho (KLIX) – Idaho Department Fish and Game staff had to euthanize 20 pronghorn antelope that were injured while attempting to cross the Snake River at Lake Walcott.

Some 500 pronghorn attempted to cross the river on Sunday, according to witness reports to Fish and Game.

“They said about 200 made it across, then a portion of the group spooked and ran onto an extremely slick section of ice, where they were slipping and falling,” the department says in a news release. “The remaining antelope in the herd turned back and returned to shore.”

Fish and Game staff investigated that afternoon and found 47 pronghorn stranded in the middle of the frozen river. Due the remote area and fading light, staffers decided to gather equipment and mount a rescue the following morning.

Fish and Game staff and state wildlife veterinarian Mark Drew were on the ice by 9 a.m. and used an airboat to reach the animals. They found 36 pronghorn remaining, but 10 of those had been killed by predators.

Crews were able to get six uninjured pronghorns into the airboat and transport and release them back on shore. Drew examined all the stranded animals and determined that the other 20 had suffered significant injuries due to slipping on the ice and dislocating hips and shoulders. Those animals were euthanized, and the meat was salvaged, processed and is being donated to people in need.

Lake Walcott is a traditional migration route for wintering pronghorns, according to Fish and Game, but this was an unusual event.

Regional Wildlife Manager Daryl Meints said: “I have never seen anything like it in my 26-year career."