More than 80 percent of the elk population in the county is gone. When asked for a reason, one expert gave 9NEWS a simple answer: "They're dead."

EAGLE COUNTY, Colo. — A Colorado photographer used his skill to gather anecdotal evidence to explore why there seemed to be a decline in the mule deer population in his neighborhood.

Rick Spitzer has lived in the Eagle Valley for 18 years, and he often explores the wild up close through his camera lens.

"I really appreciate the things that animals have to endure to live in the mountains, especially in the winter," Spitzer explained to 9NEWS.

In 2013, he thought he was seeing fewer mule deer in his Avon neighborhood, so he decided to run an experiment starting the next year.

"It wasn't a scientific study, but it was a good way to gather anecdotal evidence," said Spitzer, who spent 15 summers as a ranger at Rocky Mountain National Park before becoming a high school biology teacher.

He couldn't tag the animals, but he did develop some photography skills over the years.

“I could gather the details with my camera. When I shot the pictures, I was careful to try to get them looking left, looking right, looking straight at me, even behind them if they would turn away from me,” said Spitzer.

He said the shape and size of the antlers -- the number and pattern of points -- the scrapes, bumps and scars, all helped identify individual animals. He created a database, giving each animal a letter and a number, and filed photos away on his hard-drive.

In 2014, he tracked the deer every day and counted 60 bucks. His most recent tally showed a dramatic decline.

“I’ve only seen five mule deer bucks this year. You start with 60 and you end up with five… There’s something going on,” said Spitzer.

Others in the area have noticed the decline, especially with the local elk population. Bill Andree spent 38 years studying wildlife in the Eagle Valley as an officer with Colorado Parks and Wildlife. He’s now retired and still lives in the county.

“You know these wildlife aren’t out here for an adventure, an event, a race, or excitement, they’re out there trying to survive,” said Andree.

As a wildlife officer, it was his job to track, study and protect the animals of Eagle County. He said that hunting licenses were reduced by 75 percent after they noticed the decline, but the population did not respond as they had hoped.

He said they also evaluated the Eagle Valley elk herd the state calls Herd 45 for disease, but none was found.

Andree said he believes sprawling human activities in the valley are hurting the animals, especially the elk. He said the Eagle Valley herd went from about 1,500 to 1,800 animals in 2008, to what he said might be as low as 300 today.

"You might not think that's much. What if 80 percent of the people in Eagle County vanished, then what would you think?" Andree asked.

Several viewers voiced their concern to 9NEWS by email and social media posts, asking where the elk have gone.

“Where are they going? They’re dead. The elk are dead," said Andree. "I mean there isn’t any place for them to go.”

He said a shrinking winter habitat in Eagle County can no longer provide enough food or refuge for the elk, and there is no room left in areas that sustain other elk herds.

"It doesn't take a herd long to get to a point where some of them can't survive a winter or produce viable offspring," said Andree.

The photographer, Spitzer, said he stays optimistic that man and wildlife can coexist, but he knows that beautiful wildlife pics will be harder to come by as the human footprint in Eagle County continues to grow.

“It’s kind of depressing in some respects because it is changing the land, and the wildlife that’s on it,” said Spitzer.