Horses being gathered in the Tri-State Calico area. © BLM
However, the California-based group Protect Mustangs has challenged that conclusion, suggesting the mustang was euthanized for being old.
The Calico muster aims to gather about 1300 wild horses and 140 burros from the range over a 40 to 50-day period.
The bureau's plan is to release up to 350 of the horses back on the range, with 85 to 115 mares treated with a long-term contraceptive and the remainder adjusted for sex ratios, with the plan to release 60 per cent males.
However, just days into the gather operation, Protect Mustangs has highlighted the circumstances around the capture of an older palomino mare.
Wild horse advocate and photographer Cat Kindsfather, attended the first day of the roundup, near Gerlach, and saw and photographed the mare, which she named Old Gold.
Kindsfather said the horse was captured after being chased by the helicopter for more than one hour.
She said she witnessed the horse being slammed into the trap corral by other horses and believes she must have been hurt as a consequence.
Protect Mustangs noted the bureau euthanized the mare on November 21 and listed it as a "non-gather-related" death.
The group believes it is a roundup-related death and needs to be counted as such.
Its executive director, Anne Novak, said: "Congress hears that there is only a 1 per cent death rate at 'gathers'.
"We want transparency and accountability for all the deaths at roundups."
Novak said the death of a horse such as Old Gold, after being chased for miles, captured, slammed into a metal panel in the corral by other horses, then transported to another facilities is euthanized a day or two later, is c;early gather-related.
"If Old Gold had not been rounded up, I bet she would be alive today."
Kindsfather's photos document the last moments of freedom for Old Gold.
She also photographed the mare in the trap pen and the incident in which she was crushed against the fence.
"She made it all the way in after being chased by the helicopter but was exhausted," Kindsfather said.
"In the trap pen, the group of horses were scared by the wrangler using a whip with a plastic bag tied on to it.
"The horses panicked, ran into the mare and piled up against her - smashing her into the fence. In an already terrifying situation, he continued to use the whip to get the mare up on her feet."
Kerry Becklund, a wild horse advocate with Protect Mustangs, said: "Advocates need to see the mustangs' when they are brought in but we are kept away.
"It's clear from looking at the photos that the mare was in distress and the whips aggravated her fear. She did not deserve to be put down after surviving this ordeal - she deserved to live."
Kindsfather added: "I'm sure some kind-hearted person would have adopted the mare to give her a good life despite any lack of body condition.
"It's too soon to be doing another Calico roundup. The wild horses should remain free-our living gems in a treasured landscape."
Protect Mustangs is circulating a petition to stop the roundup.
The bureau says the muster is necessary as it estimates there are 1602 wild horses and 179 burros in the Calico complex. The appropriate management level for the herd areas in question is 572 to 952 horses and 39 to 65 burros.
As of November 26, there had been 308 horses gathered, and four deaths. Three of the deaths have been recorded as non-gather related. The first death, that of Old Gold, was listed as a mare over 20 years old euthanized due to an inability to maintain a 3 on the Henneke body score condition scale.
The next death, on November 22, a day later, was due to a non-gather-related chronic condition.
The only gather-related death to date, on November 24, was a death in holding due to torsion of the bowel (colic).
The fourth death, on November 25, was the result of a non-gather-related chronic and incurable disease. The horse was euthanized, the bureau said.